- Email: [email protected]
Calibration and standardization of Iatroscan (TLC-FID) using standards derived from crude oils
~
AdvancesinOrganicGeochemistry1993 Copyright© 1994ElsevierScienceLtd
Org. Geochern. Vol. 22, No. 3-5, pp. 835-862, 1994
Pergamon 0146-6380(94)00086-7
Printed in Great Britain.All rights reserved 0146-6380/94$7.00+ 0.00
Calibration and standardization of latroscan (TLC-FID) using standards derived from crude oils SUNIL BHARATI,GEIR ARNE ROSTUM and RITA LOBERG Geolab Nor, P.O. Box 5740 Fossegrenda, 7002 Trondheim, Norway Abstract--The TLC-FID technique, using the Iatroscan instrument, which constitutes separation and quantification of a complex mixture into various compound classes, has recently become a widely used and reliable method for characterisation of solvent extracts (particularly from reservoir rocks) and crude oils. However, there has been only limited reported research conducted aimed at calibration of the instrument and usage of standards which are relevant to the petroleum industry. This study is specifically aimed at identifying, separating, characterising, testing and developing standards, derived from natural crude oil(s), that are well suited and chromatographically pure for quantification purposes. A sample set comprising of over 30 crude oils from various basins of the world was used. The oils range widely in maturity, API gravity (8-46°), source rock type, degree of biodegradation, reservoir rock type and relative compositions. Whole oils, maltenes and asphaltenes were initially screened using Iatroscan. The process to identify suitable saturated hydrocarbons, aromatic hydrocarbons, polars and asphaltenes for the purpose of making the standard(s) included MPLC, gas chromatography, column chromatography and preparative thin-layer chromatography. Only about half the oils separate into 4 'clean' fractions (saturated and aromatic hydrocarbons, resins and asphaltenes) resulting in 4 well defined peaks. Analysis of the maltenes and asphaltenes shows that neither of the fractions is free of the other fraction. Pure fractions of saturated and aromatic hydrocarbons, resins and asphaltene were isolated using a combination of various analytical techniques and standards prepared. Compositional variations between the different fractions was established using GC, Py-GC and GC-MS. Several synthetic standards were prepared and tested for comparison and the implication of their use discussed. The Iatroscan TLC-FID instrument was calibrated using data from naturally derived fractions/standard mixtures and a mathematical model developed that could be used as the basis for calibration. Key words--Iatroscan TLC-FID, petroleum, calibration, response factors, analytical accuracy, model,
natural standards, synthetic standards, oil fractions
INTRODUCTION The T L C - F I D technique, using the Iatroscan instrument, is fast becoming a widely practised and reliable method for qualitative and quantitative characterization of solvent extracts (particularly from reservoir rocks) and crude oils. T L C - F I D generally constitutes separation of a complex mixture, such as EOM or crude oil into compound classes. Depending upon the chromatographic requirement and the sensitivity of the instrument, a natural organic mixture, such as a crude oil, is typically separated into 3, 4 or even 5 fractions, each fraction itself, however, being a complex mixture, though essentially all components of a fraction are of similar polarity and belong to the same compound class. Recently, there have been some publications highlighting the practicality and application of Iatroscan in the petroleum industry (Goutx et al., 1990; Karlsen and Larter, 1989, 1991; Poirier et al,, 1984; Ray et al., 1982; Selucky, 1983; Yamamoto, 1988). However, these studies have limited focus towards calibration of the instrument and standardization of elutionary procedures. Most
previous work involving the use of the T L C - F I D technique using Iatroscan have relied upon synthetic compounds as standards to accomplish quantification. There are even lesser reported studies specifically aimed at developing natural standards (standards derived from natural crude oils or extracts) which are well suited and chromatographically pure during TLC for the purpose of calibrating the Iatroscan T L C - F I D instrument. Poirier et al. (1984), for example, have used natural extract fractions to calculate the relative response and Parish and Ackman (1985) have addressed the problem of calibration. In addition, different laboratories have different preferences with regards to the choice of synthetic compounds that they employ as standards. All these factors result in poor and inaccurate inter-laboratory comparison of data. The present study is specifically aimed at identifying, separating, characterising, testing and developing standards, derived from natural crude oil(s), that are well suited, easily accessible and chromatographically pure, that can be applied to Iatroscan and which are relevant to the petroleum industry. The term 'natural 835
1-04 24)5 3-06 4-27 5-07 64)8 7-28 8-09 9-30 10-10 I 1 11 12-12 13-13 14-26 154)3
16-31
17-14 18-15 19-25 20-16 2 I- 17 22-29 23-18 24-19 25-20 26-21 27-22 28-23 29-24 313-01 31-02
I 2 3 4 5 6 7 8 9 10 I1 12 13 14 15
16
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Tsagaanels Jibaro/Maranon San Jacinto/Maranon Monterey/Santa Maria Helm/Broad Fourteens Shiviyacu/Maranon Malongo West/Congo Helder/Broad Fourteens Malongo South/Congo San Jacinto/Maranon Matzen/Vienna Ruhlemoor-Valendis Troll/Horda Platform Troll/Horda Platform Haltenbanken Malongo West/Congo Matzen/Vicnna Dorissa/Maranon Selmo-Batman Central Graben Shiviyacu/Maranon Malongo North/Congo Snorre/N. Viking Graben Eich/K6nigsgarten Challis/N. Browse Jabiru/N. Browse Aderklaa/Vienna Central Graben Midgard/Sklinnabanken Tirrawarra/Cooper Patroclus/Enomanga
Field/Basin U. Cretaceous/Sandstone U. Cretaceous/Sandstone Miocene/Shale L. Cretaceous/Sandstone U. Cretaceous/Sandstone L. Cretaceous/Sandstone L. Cretaceous/Sandstone L. Cretaceous/Sandstone U. Cretaceous/Sandstone Tertiary/Sandstone L. Cretaceous/Sandstone M-U. Jurassic/Sandstone M-U. Jurassic/Sandstone L. Jurassic/Sandstone L. Cretaceous/Sandstone Triassic/Dolomite U. Cretaceous/Sandstone U. Cretaceous/Carbonate U. Jurassic/Sandstone U. Cretaceous/Sandstone L. Cretaceous/Sandstone L. Jurassic/Sandstone L. Cretaceous/Sandstone Triassic/Sandstone M. Jurassic/Sandstone Tertiary/Sandstone U. Jurassic/Silty Sandstone M. Jurassic/Argi. Sandstone Permian/Sandstone Jurassic/Sandstone
Reservoir Age/Lithology
Table I. Details of oil samples used in the study (listed according to increasing A P I Gravity)
Mongolia, Asia Peru, S. America Peru, S. America U.S.A., N. America Netherlands, Europe Peru, S. America Angola, W. Africa Netherlands, Europe Angola, W. Africa Peru, S. America Austria, Europe Germany, Europe Norway, Europe Norway, Europe Norway, Europe Angola, W. Africa Austria, Europe Peru, S. America Turkey, Europe Norway, Europe Peru, S. America Angola, W. Africa Norway, Europe Germany, Europe Ausiralia At.strlaia Austria, Europe Norway, Europe Norway, Europe Australia Australia
Location
*Actual API Gravity data for these oils not available. Data quoted are relative visual estimates. Note: The serial numbers in column I are not the same as oil numbers referred to in the text and tables.
Sample Code
S. No.
L. Cretaceous/Lacustrine U. Jurassic/Marine L. Oligoccne/Marine U. Jurassic/Marine U. Jurassic/Marine U. Jurassic/Marine U. Jurassic/Marine U. Jurassic/Marine Permian/Fluvial Deltaic Permian/Fluvial Deltaic
U. Cretaceous/Marine U. Jurassic?/Hypersaline?
U. Jurassic/Marine L. Oligocene/Marine U. Jurassic/Marine U. Jurassic/Marine L. Jurassic/Marine L. Cretaceous/Lacustrine U. Jurassic/Marine
L. Cretaceous/Lacustrine L. Jurassic/Marine L. Cretaceous/Lacustrine
Miocene/Marine L. Jurassic/Marine
Source Age/Type
ca 14" 17.4 20.6' 20.8 22.1 23.4 23.7 24 24.5 26.8' 27.5' 28.4 29.9 30.2 31./6 ca 32 * 33.5 34 34.2 37.1 38.6 39.5 42.3 43.2 44.2 44.6 45 46
11.4
ca8 * 10.7
API Gravity
Z
Calibration and standardization of Iatroscan standards' includes the four principle fractions of crude oil/solvent extract viz. saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes. For this purpose, a sample set comprising 31 crude oils from various basins/fields (onshore and offshore) of the world (North America, South America, Europe, West Africa, Asia and Australia) was used (Table l, oils listed according to increasing API gravity). The oils range widely in (1) maturity (low maturity to high maturity, but not condensate stage), (2) API gravity (8°-46°), (3) source rock type (marine clastic, marine carbonate, lacustrine, saline), (4) degree of biodegradation (none to strong), (5) reservoir rock type (sandstone, siltstone and carbonate) and (6) relative compositions. The final objective is to successfully isolate one fraction each of saturated hydrocarbons, aromatic hydrocarbons, NSO compounds and asphaltenes, pure from the T L C - F I D point of view, and gravimetrically prepare standards for Iatroscan calibration. In addition, an attempt has been made to refine the existing procedure for analysis, which includes a mathematical model to determine effective response factors and their relationships, which can be easily and readily employed by other users. ANALYTICAL PROCEDURES
Whole oil samples and individual fractions were analysed by Iatroscan at several stages during the course of the study. The following procedure for Iatroscan analysis (using Iatroscan MK-5) was found to be most suitable for analyses of a variety of samples and therefore recommended for general use (see also Karlsen and Larter, 1989). In the case of oils, 4-5 mg of whole oil is accurately weighed into a 2 ml GC glass vial, 0.5 ml of solvent added and the vial sealed. After being dissolved in the solvent, oils can be analysed immediately. Prior to sample spotting, the chromarods (quartz SIII, silica gel powder coated) are first activated by passing through the FID flame (in the normal analysis mode--30 s/ scan). 2 pl sample is then spotted (using a 2 #1 syringe) using preferably an auto-spotter (SES 3202/IS-01) and continuously blowing the spot with nitrogen or air to accomplish immediate drying and prevent bandspreading. This results in sharper and narrower peaks. The development tanks containing the solvents are lined with chromatographic paper on the inside back and side walls, from top to bottom. The following elution procedure is used: 35 min in n-hexane (to separate saturated hydrocarbons), air dry for 2 min, 14 min in toluene (to separate aromatic hydrocarbons), air dry for 2 min, 4 min in dichlorometane: methanol, 93 : 7 v/v (to separate resins) and finally 90 s at 60°C. Asphaltenes are retained at the spotting point. If it is intended to separate mono- and diaromatic hydrocarbons from poly-aromatic hydrocarbons (Karlsen and Larter, 1991) then insert the step of elution using cyclo-hexane for 14 min before eluting with toluene for 8 rain. The samples are analysed and the data
837
collected and processed either using an integrator or, as in our case, using a data acquisition system such as Multichrom v2. Raw data is automatically converted into mg/g rock units and relative percentages (see later). Although no whole rocks were analysed in the present study, the additional procedure related to their analyses is included in Appendix 1 for the benefit of the readers. The oils were deasphaltened using n-pentane, except in the case of asphaltene solubility experiments. Four samples were selected for asphaltene solubility tests (oils 5, 10, 16 and 19, which cover a wide range of API gravity) and each of these oils were subjected to asphaltene precipitation using six different solvents viz. n-pentane (which is normally used), n-hexane, n-heptane, n-octane, n-nonane and n-decane. The purpose was to monitor the dependence, if any, of asphaltene composition on the solvent chain length, which also implies increasing boiling point of the solvent. In addition, our intention was to investigate and review the compositions of asphaltenes resulting from precipitation using solvents other than n-pentane, and to establish whether an asphaltene fraction can be obtained through conventional precipitation method which is the same or close to the asphaltene fraction defined by TLC-FID. Such a fraction would be expected to result in only one peak on analysis using the Iatroscan, following the normal elution procedure. Maltenes were separated into fractions (saturated and aromatic hydrocarbons and polars) using MPLC as described by Radke et al. (1980) (each of the oils was analysed with one parallel to increase the amount of resulting fractions). However, one significant deviation from the routine method was during collection of the fractions. As the purpose in the present study was to obtain pure fractions using MPLC and not quantification, the initial and final cuts (about 10% each) of each of the fractions (saturated and aromatic hydrocarbons and resins) were discarded (which are normally collected) by closely monitoring the RI detector in the case of saturated hydrocarbons and u.v. detector in the case of aromatic hydrocarbons. This was done to ensure that minimal amounts of unwanted components are present in a fraction. Selected saturated and aromatic hydrocarbon fractions were characterised by gas chromatography, while selected polar and asphaltene fractions were characterised by pyrolysis-gas chromatography. The purity of the two hydrocarbon fractions was tested using gas chromatography-mass spectrometry, while the composition of the selected resin and asphaltene fractions was established using pyrolysis-gas chromatographymass spectrometry. The procedures followed were according to the Norwegian Industry Guide (1993). However, the selection of ions for GC-MS was made keeping in mind the primary objective of major contaminant identification. As the analysis was carried out in high resolution mode, the number of ions was restricted to 10. However, in separate runs, a few ions
838
SUN1L BHARATI et al.
characteristic for biomarkers were also detected. The ions specified during G C - M S are listed in Appendix 2. Similarly, the ions for P y 4 3 C - M S were selected in such a way so as to indicate the presence of saturated hydrocarbons, aromatic hydrocarbons (mono-, diand poly-aromatic) and the sulphur bearing thiophenes. Additionally, the selected masses would also indicate the type of hydrocarbons that have the maximum tendency to be retained in the polar fraction. No significance was placed on masses specific to commonly known biomarkers as these are not expected to occur in these fractions, although m/z 163, 231 and 253 were included to get a general picture of their presence. The list of the masses specified during Py~GC-MS is presented in Appendix 3. In addition, whole oil samples and selected resin fractions were analysed using preparative thin-layer chromatography (PTLC), the main objective of this being to purify the resin fractions by separating out aromatic hydrocarbons from the resins. The oils and resin fractions from MPLC were first dissolved in dichloromethane and spotted along a straight line (as a band) on a ready-made TLC plate (Merck DC-A1 foil coated with silica gel 60, gel thickness 0.2 mm,
silica with F254 fluorescence activator). The elution procedure employed was the same as that of Iatroscan. The zone separation was checked using a u.v. detector at 254 nm wavelength (cf. Plate 1) and portions with different fractions scrapped using a spatula. The 4 silica gel fractions were then extracted using D C M : M e O H (93:7 v/v), weighed and analysed by Iatroscan. RESULTS AND DISCUSSION
Composition of oils, asphaltenes and maltenes Preliminary screening of all oils using Iatroscan (Table 2) shows that the composition of the oils is highly variable, as is their total hydrocarbon (saturated and aromatic) content. The relative percentage of saturated hydrocarbons varies from about 3 to 82%, aromatic hydrocarbons vary from about 13 to 54%, resins vary from about 2 to 46% and asphaltenes vary from about 0.1 to 26% (Table 2). Nevertheless, hydrocarbons are dominant in all the oils, varying from about 76 to 98%, except for one oil which has a total hydrocarbon content of only 42% relative to resins and asphaltenes. The quality of
10em
Plate. 1. On left are the results of TLC development of various fractions from MPLC (saturated hydrocarbons--SAT, aromatic hydrocarbons--ARO, resins--NSO and asphaltene--ASP and whole oil--EOM). On right is the TLC development of the MPLC resin fraction, which splits into 2 zones--resins and aromatics. Zone 1 marks the line of sample spotting, where the asphaltenes are also retained, Zone 2 is resins, Zone 3 is aromatic hydrocarbons and Zone 4 is saturated hydrocarbons which cannot be detected by fluorescence.
839
Calibration and standardization of Iatroscan
Table 3. Relative percentages of separated fractions by latroscan from asphaltenes
Table 2. Relative percentages of separated fractions by latrnscan from whole oils
Oil No.
Sat HC %
Aro HC %
Resins %
Asph %
Total HC %
Total Polars %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
72.8 82.2 77.3 62.3 19.7 23.7 34.1 34.2 44.6 46.4 49.8 33.4 53.0 64.3 58.3 56.2 56.5 62.5 70.0 61.3 78.5 78.3 59.3 76.3 47.5 57.0 2.9 56.8 42.6 66.4 50.1
25.3 13.9 21.1 19.3 39.7 38.8 54.2 42.8 45.2 40.5 42.3 44.3 42.2 31.1 34.8 34.2 37.4 31.1 25.7 35.4 20.0 19.3 35.7 22.2 44.2 37.3 39.3 19.4 33.5 22.6 33.2
1.6 1.6 1.3 11.8 15.9 10.9 8.0 8.7 6.5 6.7 7.2 17.6 3.5 3.7 5.4 6.8 4.7 5.0 2.3 2.9 1.4 2.2 4.5 1.2 6.1 5.3 46.5 12.7 18.3 10.1 13.9
0.3 2.3 0.3 6.6 24.7 26.6 3.7 14.3 3.7 6.4 0.7 4.7 1.3 0.9 1.5 2.8 1.4 1.4 2.0 0.4 0.1 0.2 0.5 0.3 2.2 0.4 11.3 11.1 5.6 0.9 2.8
98,1 96.1 98.4 81.6 59.4 62,5 88,3 77,0 89.8 86.9 92.1 77.7 95.2 95.4 93A 90.4 93.9 93.6 95.7 96.7 98.5 92.6 95.0 98.5 91.7 94.3 42.2 76.2 76.1 89.0 83.3
1.9 3.9 1.6 18.4 40.6 37.5 11.7 23.0 10.2 13.1 7.9 22.3 4.8 4.6 6.9 9.6 6.1 6.4 4.3 3.3 1.5 2.4 5.0 1.5 8.3 5.7 57.8 23.8 23.9 11.0 16.7
Sat HC %
Aro HC %
Resins %
Asph %
Total HC %
Total Polars %
40.0 -15.4 11.5 -5. I --
--------
50.0 64.7 28.8 17.4 71.4 4.9 32.4
10.0 35.3 55.8 71.1 28.6 90.0 67.6
40.0 -15.4 11.5 -5.1 --
60.0 I00.0 84.6 88.5 100.0 94.9 100.0 100.0 100.0 100.0 100.0 74.9 100.0 100.0 100.0 93.6 100.0 64.5 100.0 30.3 67.6 46.7 83.0 100.0 81.5 100.0 100.0
Oil No. 1 2 3 4 5 6 7 8
--
9 10
--
---
11
--
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
14.8 ---6.4 -25.9 -53.3 32.4 53.7 17.0 -8.9 ---
individual
fractions
distinct groups (35 and chromatography (saturated
hydrocarbons,
resins
asphaltenes)
and
aromatic giving
hydrocarbons,
4 prominent
37.8 32.8 49.5 34.0 49.2 45.0 37.4 19.9 42.7 41.2 27.3 26.6 56.8 35.2 59.1 -30.6 45.5 --
in these
---25.1 ---6.4 -35.5 -69.7 32.4 53.7 17.0 -18.5 ---
oils to
separate
and 4 min for non-
(10 cm).
Deasphalting
of oils shows
that except for a few
oils, most oils contain asphaltenes
between
by weight.
or a continuum
hydrocarbon between
fractions
[Fig.
l(b)]
the two non-hydrocarbons
[ F i g . l ( c ) ] f r a c t i o n s is p r e s e n t . T h i s is a t t r i b u t a b l e the composition
stantial
However,
portion
in the range 1-4%
in case of several oils, a sub-
of the asphaltene
within the filter and was impossible
to
of these oils and the inability of the
by repeated
washing
remained
I
cm a
0
10
cm
b
0
10
even
using either dicholoromethane
!
!
locked
to remove
$
10
into
and the restricted length available for
[Fig. l(a)], except in a few cases where a continuum the two
--
62.2 67.2 50.5 40.9 50.8 55.0 62.6 73.7 57.3 23.3 72.7 3.7 10.8 11.1 23.9 100.0 50.9 54.5 100.0
within the given time for separation
separation/elution
peaks
100.0
14 m i n f o r h y d r o c a r b o n s
hydrocarbons),
is g e n e r a l l y g o o d w i t h t h e 4 f r a c t i o n s
--
---10.3 -----9.6 -16.4 ----9.6 ---
cm
0
O
Fig. 1. T L C - F I D c h r o m a t o g r a m s o f w h o l e oils. (a) A typical oil s a m p l e w i t h 4 p r o m i n e n t p e a k s r e p r e s e n t i n g 4 m a j o r c o m p o n e n t s viz. s a t u r a t e d h y d r o c a r b o n s (SAT), a r o m a t i c h y d r o c a r b o n s ( A R O ) , resins ( P O L ) a n d a s p h a l t e n e s ( A S P H ) . (b) A n oil s a m p l e w i t h a c o n t i n u u m b e t w e e n the 2 h y d r o c a r b o n f r a c t i o n s , S A T a n d A R O . (c) A n oil s a m p l e w i t h a c o n t i n u u m b e t w e e n the 2 n o n - h y d r o c a r b o n f r a c t i o n s , POL and ASPH.
840
SUNIL BHARATI
I
10
I
I
I
on a
I , , [
I
010
I
cm
0
et al.
i
10
i
i
era
b
i
0
i , 10
c
i
, em
"f 0
d
Fig. 2. TLC-FID chromatograms of asphaltenes. (a) An asphaltene fraction containing significant amounts of hydrocarbons (SAT and ARO). (b) A typical asphaltene fraction consisting mainly of resins (POL) and asphaltenes (ASPH). (c) Chromatogram of an asphaltene with least amount of other components. (d) A typical asphaltene from low API gravity oils, exhibiting a strong continuum between the 2 non-hydrocarbon components, POL and ASPH. or dichloromethane: methanol (93 : 7 v/v). This loss is even more significant for oils rich in asphaltenes (more than 10% by weight) such as oils 4, 5, 6, 8, 10 and 27. The analysis of asphaltenes using Iatroscan (Table 3) shows that none of the pentane precipitated asphaltene fractions result in pure "asphaltene" fractions from the T L C - F I D point of view, as also documented earlier (Karlsen and Larter, 1991). Several of the asphaltene fractions analysed (such as from oils 1, 2, 3, 20, 21, 22, 23 and 24) give a misleading picture (Table 3) due to very low absolute concentrations of all the components and therefore their data are considered to be insignificant/unreliable. However, in several others, the asphaltene precipitated by n-pentane from oils such as 4, 6, 12, 16, 18 and 25, continue to retain substantial amounts of hydrocarbons, and their chromatograms show up to 4 distinct peaks [e.g. Fig. 2(a)]. It seems though that saturated hydrocarbons have a greater tendency of being retained in the pentane precipitated asphaltenes, relative to aromatic hydrocarbons (Table 3). In most of the samples, however, the pentane precipitated asphaltene splits into mainly resins and asphaltenes, as shown in Fig. 2(b). In none of the samples did the pentane precipitated asphaltene fraction result in only one (asphaltene) peak [example with the most prominent asphaltene peak relative to resins is shown in Fig. 2(c)]. Iatroscan separation of asphaltenes from nearly all of the low API gravity oils, which are also generally the richest in asphaltenes (Table 3), results in a continuum between the resins and asphaltenes [e.g. Fig. 2(d)]. This is indicative of either compositional similarity between the resins and asphaltenes of these oils or inability of the components to separate, as no overloading of samples occurred. Had the pentane precipitated asphaltene fraction resulted in only one peak on analysis using Iatroscan,
then it would mean that the fraction resulting from conventional pentane precipitation, which we commonly refer to as asphaltene, is compositionally the same as the asphaitene defined by the Iatroscan technique. Apparently, asphaltenes precipitated by n-pentane are compositionally different to the fraction defined as asphaltene by Iatroscan. The pentane precipitated asphaltene nearly always consists of two principle fractions--resins and asphaltenes (Table 3). It must also be noted that accurate integration of the resin and asphaltene peaks from Iatroscan is not possible in the case of some samples due to the Table 4. Relative percentages of separated fractions by latroscan from maltenes Sat HC
Aro HC
Resins
Asph
Oil No.
Total HC
Total Polars
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
77.6 87.6 78.7 54.7 24.7 28.9 30.9 40.0 42.5 49.6 54.2 34.0 58.1 69.9 61.2 58.8 58.2 62.7 74.4 73.8 82.2 83.5 64.5 71.0 52.9 82.6 --
20.4 10.1 19.9 9.3 48.8 57.9 54.8 49.9 45.3 41.5 35.1 43.0 32.6 22.9 31.5 28.4 33.5 28.4 20.9 20.6 15.5 12.0 30.0 16.5 37.0 13.9 37.9
!.7 1.8 1.3 16.4 16.4 12.9 13.4 9.5 11.6 8.4 10.4 19.5 8.8 6.7 6.7 10.4 7.7 8.3 4.2 5.1 2.0 3.4 4.4 3.5 7.8 2.9 59.6
0.3 0.5 0.1 19.6 19.6 0.3 0.9 0.6 0.6 0.5 0.3 3.5 0.5 0.5 0.6 2.4 0.6 0.6 0.5 0.5 0.3 1.1 1.1 9.0 2.3 0.6 2.5
98.0 97.7 98.6 64.0 64.0 86.8 85.7 89.9 87.8 91.1 89.3 77.0 90.7 92.8 92.7 87.2 91.7 91.1 95.3 94.4 97.7 95.5 94.5 78.5 89.9 96.5 37.9
2.0 2.3 1.4 36.0 36.0 13.2 14.3 10.1 12.2 8.9 10.7 23.0 9.3 7.2 7.3 12.8 8.3 8.9 4.7 5.6 2.3 4.5 5.5 12.5 10.1 3.5 62.1
Calibration and standardization of latroscan
841
I i
I0
cm
0
10
cm
0
10
cm
•
L
0
10
em
0
a b c d Fig. 3. TLC-FID chromatograms of maltenes. (a) A typical maltene fraction, rich in hydrocarbon components SAT and ARO. (b) A maltene fraction with a dominant saturated hydrocarbon peak (SAT). (c) A maltene fraction with a dominant aromatic hydrocarbon peak (ARO). (d) A maltene fraction with significant amounts of non-hydrocarbons present (POL and ASPH).
continuum, and the data therefore can be quite erroneous. Based on the results and the above considerations, asphaltenes from oils 4, 5, 7, 8, 10, 16 and 19 are thought to have the best potential for use as standards. Iatroscan analysis of maltenes (Table 4) show that hydrocarbons are the most abundant species, typically between 90 and 98% and generally over 77% of the total maltene fraction [Fig. 3(a)], except for one sample (sample 27). The composition of the hydrocarbons however, is highly variable with saturated hydrocarbons occurring in the range of about 25-81% [Fig. 3(b) shows a sample rich in saturated hydrocarbons] with one exception (no saturated hydrocarbons detected in sample 27) and aromatic hydrocarbons ranging about 10-57% of the total content [Fig. 3(c) shows a sample rich in aromatic hydrocarbons]. Resins or polar compounds are in most cases minor components (generally 1-8%) but occur in larger amounts in a few samples (10-16%) [Fig. 3(d) shows a sample rich in resins]. One exception is sample 27 which contains about 59% resins. Asphaltenes, which ideally should not be detected in maltenes, occur in all maltenes although their percentage is small--ranging about 0.1-3%, with 3 exceptions (oils 4, 5 and 24).
Asphaltene solubility test Table 5 shows the results of the asphaltene solubility experiment and the amount of asphaltene yields by using 6 different solvents. Apparently, there is little change in the relative yields of asphaltenes from each of the 4 oils when different solvents are used; this is also true for maltenes [Fig. 4(a-d)]. Data for n-decane is incomplete/erroneous due to difficulties encountered during solvent evaporation, these being attributable to very high boiling point of n-decane
(174°C) and consequently limited capacity of the rota-vapour to remove the solvent. From the qualitative point of view, both the asphaltenes and maltenes of each oil are compositionally very similar, resulting in nearly identical T L C FID chromatograms for a given oil, for all solvents
Table 5. Asphaltene solubility experiment: gravimetric weights o f resulting maltenes and asphaltenes using different solvents Sample
Oil (mg)
Asp (rag)
Malt (mg)
Asp (%)
Malt (%)
Loss (%)
Oil Oil Oil Oil
5 10 16 19
47.3 45.8 45.5 47.7
8.1 4.1 1.0 0.9
Pentane 33.8 36.8 37.4 34.0
17.12 8.95 2.2 1.89
71.46 80.35 82.20 71.28
11.42 10.70 15.60 26.83
Oil Oil Oil Oil
5 10 16 19
48.1 41.9 51.5 49.5
9.5 3.3 0.3 0.8
Hexane 34.1 19.75 35.9 7.88 43.5 0.58 37.5 1.62
70.89 85.68 84.47 75.76
9.36 6.44 14.95 22.62
Oil Oil Oil Oil
5 10 16 19
55.8 46.6 59.0 49.6
11.4 3.7 0.5 0.7
Heptane 40.2 39.1 50.0 13.9"
20.43 7.94 0.85 1.41
72.04 83.91 84.75 --
7.53 8.15 14.40 --
Oil Oil Oil Oil
5 10 16 19
41.7 61.0 51.2 48.7
8.1 4.5 0.3 1.1
Octane 21.5 19.42 50.4 7.38 41.0 0.59 14.9" 2.26
51.56 82.62 80.08 --
29.02 10.0 19.33 --
Oil Oil Oil Oil
5 10 16 19
61.2 45.2 53.5 48.6
12.5 29.8 3.6 0.5
Nonane 37.5 20.42 35.8¢ -42.1 6.73 32.9 1.03
61.27 79.20 78.69 67.70
18.31 -14.58 31.27
Oil Oil Oil Oil
5 10 16 19
51.4 46.5 47.3 48.8
t08.3t 36.6t 2.2 6.3
Oecane 143.5t -267.5t -95.3t 4.65 98.5t 12.91
-----
-----
*Loss during separating asphaltene from oil. #Impossible to evaporate the solvent completely.
842
SUNIL BHARATI el
(a)
al.
(c)
SAMPLE 5
SAMPLE
16
100 -
100 F
, . ~
80 ~ . . . . .
~
80
~x
• ....
60 -
60 Asphaltene 40 I -
• Maltene
40 -
/ 20 ~-~.-..-.----- • - ' - - - " ' - - - • " " " - - - - - - • 0
>-
I 6
5
v "O
J 7
(b) 100 --
I 8
20 -
"• I 9
I
10
0 --------i
7
6
8
9
10
9
10
(d) SAMPLE
10
80 ,w-
80
60 --
60
40 --
40
20 --
20
0
I 6
5
7
8
SAMPLE
100
9
10
19
-
5
6
7
8
C a r b o n a t o m s in s o l v e n t ( n - a l k a n e )
Fig. 4. Relative percentages of asphaltene and maltene yields for 4 different oils (samples 5, 10, 16 and 19) on deasphaltening using n-pentane, n-hexane, n-heptane, n-octane, n-nonane and n-decane.
material (Table 6), even after the solvent removal procedure [Fig. 5(b)]. This effect is also noticed in the case of asphaltenes, as asphaltenes resulting from n-pentane up to n-nonane precipitation are nearly free of any saturated hydrocarbons [Fig. 5(c)] while
except n-decane. Maltenes resulting from n-pentane up to n-nonane contain mainly hydrocarbons, but also noticeable amounts of resins and asphaltenes [Fig. 5(a)]. In the case of n-decane, significant amount of the solvent is apparently retained with the sample
10
cm a
0
10
cm b
0
10
cm c
0
10
cm
0
d
Fig. 5. latroscan analysis of maltene and asphaltene fractions resulting from deasphaltening of oils using various solvents. Typical chromatograms showing (a) representative maltene of n-pentane to n-nonane precipitation, (b) maltene of n-decane precipitation, (c) representative asphaltene of n-pentane to n-nonane precipitation and (d) asphaltene of n-decane precipitation.
843
Calibration and standardization of latroscan Table 6, Asphaltene solubility experiment: results of T L C - F I D analyses of maltene and asphaltene fractions---relative percentages of separated fractions Sample
Maltenes Aro% Pol%
Sat%
Asp%
Oil Oil Oil Oil
5 10 16 19
24.8 46.7 58.8 71.6
62.7 49.9 33.0 24.5
I 1.9 3.3 7.5 3.8
Pentane 0.6 0.1 0.7 0.1
Oil Oil Oil Oil
5 10 16 19
24.8 44.9 55.9 69.0
63.4 49.5 35.4 27.1
11.3 5.2 7.7 3.7
Hexane 0.6 0.4 1.0 0.2
Oil Oil Oil Oil
5 10 16 19
24.7 47.2 55.2 74.8
65.8 48.2 36.3 20.2
9.2 4.4 7.6 4.8
Heptane 0.4 0.2 0.9 0.2
14.6 4.8 8.1 6.5
Octane 0.3 0.2 0.8 0.2
14.6 5.3 9.5 4.2
Nonane 0.4 0.4 1.0 0.3
3.8 2.1 2.6 1.1
Oecane 0.3 0.2 0.5 0.1
Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil
5 10 16 19
27.7 41.6 56.0 73.0
5 10 16 19
57.4 53.4 35.1 20.3
21.7 45.1 58.6 72.5
5 10 16 19
63.3 49.1 30.9 23.0
68.2 71.6 87.5 86.1
27.6 26.1 9.4 12.7
the asphaltene fraction resulting from n-decane has a conspicuous saturated hydrocarbon peak [Fig. 5(d)]. Apparently, no significant compositional differences are obtained by using solvents other than that normally used (n-pentane), except for n-decane. In any case, this exercise was not helpful in obtaining a "purer" asphaltene fraction from the T L C - F I D point of view. Conventional M P L C separation
Based on the results obtained from whole oil and maltene analyses, 7 samples (oils 1, 5, 7, 16, 19,
Sat%
m
Asphaltenes Aro% Pol%
m
m
m
m
_
_
m
m
m
4.0 ---
-
10.5 ---
-
m
_
_
m
_
_
m
51.5 70.8 78.3 73.9
-8.7 9.4 --
Asp%
26.6 45.6 52.9 50.6
73.4 54.4 47. I 49.4
27.4 37.1 68.6 54. I
75.8 62.9 31.4 45.9
24.2 32.7 78.4 66.1
75.8 67.3 21.6 33.9
28.5 40.2 82.5 68.1
57.0 59.8 17.5 31.9
32.8 45.1 75.0 55.8
67.2 54.9 25.0 44.2
15.7 10.5 11.2 16.0
32.8 10.0 11.1 10.1
22 and 30) were selected for fresh deasphalting and separation of maltenes into fractions using MPLC (Radke et al., 1980). The selection was made on the basis that at least about two "clean and pure" (from T L C - F I D point of view) fractions each of saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes would be obtained from these oils. The compositions of the oils and the relative percentages of the various fractions are shown in Table 7, although one must bear in mind that the data of Table 7 are not completely representative of the true composition, due to discarding of the initial and final
Table 7. Composition and relative percentages of chromatographic fractions from MPLC Sample
Oil (mg)
Sat (mg)
Aro (rag)
Pol (mg)
Asp (mg)
Total (mg)
Loss* (mg)
Loss* (%)
Sat (%)
Aro (%)
Pol (%)
Asp (%)
Oil 1 la
94.6 101.8
34.9 39.9
18.0 13.8
0.9 1.1
13.0 7.0
66.8 61.8
27.8 40.0
29.4 39.3
52.2 64.5
27.0 22.4
1.3 1.8
19.5 11.3
Oil 5 5a
91.8 104.9
15.6 21.5
19.8 23.4
13.5 15.5
22.0 25.1
70.9 85.5
20.9 19.4
22.8 18.5
22.1 25.1
27.9 27.4
19.0 18.1
31.0 29.4
Oil 7 7a
107.2 109,3
38.1 32.4
23.9 27.9
15.0 14.6
9.8 11.4
86.8 86.3
20.4 23.0
19.0 21.0
43.9 37.6
27.5 32.3
17.3 16.9
11.3 13.2
Oil 16 16a
93.4 105.3
41.9 47.3
17.9 19.7
9.8 14.1
1.5 2.1
71.1 83.2
22.3 22.1
23.9 21.0
58.9 56.8
25.2 23.8
13.8 16.9
2.1 2.5
Oil 19 19a
95.7 97.2
52.4 54.5
13.4 13.4
3.6 3.0
1.8 3.6
71.2 74.5
24.5 22.7
25.6 23.4
73.6 73.2
18.8 18.0
5.1 4.0
2.5 4.8
Oil 22 22a
96.3 98.1
49.8 49.8
7.8 7.8
1.8 2.1
1.4 1.4
60.8 61.1
35.5 37.0
36.9 37.7
81.9 81.5
12.8 12.8
3.0 3.4
2.3 2.3
Oi130 30a
111.5 126.7
52.5 59.4
18.9 26.1
15.6 19.1
3.1 3.7
90.1 108.2
21.4 18.45
19.2 14.6
58.3 54.9
21.0 24.1
17.3 17.6
3.4 3.4
*The loss indicated includes the inherent loss related to the MPLC technique (estimated 5-10%) +loss due to discardation of 10% each of the initial and final cuts of the saturated and aromatic hydrocarbon and resin fractions (cf. analytical procedures).
SUNIL BHARATI et al.
844
Table 8. Resultsof TLC-FID analysis of chromatographicfractions from MPLC: relativepercentagesof separated fractions Sat
Aro
Pol
Asp
(%)
(%)
(%)
(%)
99.0 94.0 67.4 99.7 100.0 100.0 81.8
0.5 5.3 32.3 ---15,9
0.5 0.7 0.3 0.3 --2.3
--------
Oil 1 Oil 5 Oil 7
7.1 ---
91.8 98.0 98.3
1.1 1.9 1.7
-0.1 --
Oil Oil Oil Oil
16 19 22 30
---1.1
97,6 99.5 99,2 96.9
2,4 0.5 0.8 2.0
-----
Oil Oil Oil Oil
1 5 7 16
-----
63.9 68.2 70.3 61.7
36,1 31.8 29.7 38.3
-----
O i l 19 Oil 22 Oil 30
----
64.3 48.4 66.7
35.7 51.6 33,3
----
Oil I Oil 5
40.1 --
52.9 --
6.7 --
0.3 100.0
Oil Oil Oil Oil Oil
15.7 --76.2 --
19.6 --16.0 31.6
51.8 54.2 56.2 7,5 --
12.9 45.8 43.8 0.3 68.4
MPLC fraction
Sample
Saturated hydrocarbons
Oil Oil Oil Oil Oil Oil Oil
Aromatic hydrocarbons
Resins
Asphaltenes
1 5 7 16 19 22 30
7 16 19 22 30
clean and contain insignificant or no aromatic hydrocarbons and/or resins as impurities [an example shown in Fig. 6(a)]. In the case of oils 16, 19 and 22, where 99-100% of the material is reported as saturated hydrocarbons, there seems to be a substantial portion of saturated hydrocarbons which tends to elute in the aromatic hydrocarbon region, the latter being "dragged" forward as shown in Fig. 6(b). The dragged material is suspected to be mainly saturated, but weakly aromatic in nature, possibly involving compounds such as alkyl benzenes with long alkyl chains. In some of the saturated hydrocarbon fractions, however, there is a distinct separation into saturated and supposedly "weakly aromatic" saturated hydrocarbon portions (oils 7 and 30) [Fig. 6(c)]. Aromatic hydrocarbon fractions from MPLC are relatively free of saturated hydrocarbons, except for oils 1 and 30 where minor amounts of saturated hydrocarbons can be observed (Table 8). However, all the aromatic hydrocarbon fractions obtained by MPLC contain minor amounts of resins (0.5-2%) [Fig. 7(a)]. In the case of resins obtained by MPLC, all the fractions split into two distinct peaks during Iatroscan analysis, with one eluting as an aromatic hydrocarbon peak and the other as the resin peak [Fig. 7(b)]. As this bimodality occurred in all fractions and the qualitative results for all the resin fractions are very similar, it is believed that this is either due to the separation procedure adopted during MPLC, where the elution of aromatic hydrocarbons may not be complete, or that some of the aromatic hydrocarbon and resin species are closely related with respect to their structure and polarity. If the latter is true, then clearly the small differences in their physical properties are detected by T L C - F I D elution, but apparently not by the MPLC elution procedure. The results for the asphaltene fractions are of variable quality (Table 8). In most cases, the contamination is rather significant in form of minor amounts
cuts of each fraction (cf. analytical procedure for MPLC). Nevertheless, some general and relative differences in the oil compositions are apparent. Oils 1, 7, 16, 19, 22 and 30 are relatively rich in hydrocarbons; oils 5, 7, 16 and 30 are rich in resins; while oils 1, 5 and 7 are rich in asphaltenes. The purity of all the resulting fractions (7 samples each of saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene fractions) was established using Iatroscan, the results of which are presented in Table 8. Except for the saturated fractions from oils 5, 7 and 30, the other saturated hydrocarbon fractions are rather a:
J
1
i I0
cm a
0
I0
cm b
0
I0
cm
0
¢
Fig. 6. latroscan analysis of saturated hydrocarbon fractions obtained by MPLC separation. (a) A pure saturated hydrocarbon fraction. (b) Saturated hydrocarbons slightly contaminated by aromatic hydrocarbons. (c) Saturated hydrocarbons contaminated by "weakly aromatic" compounds.
Calibration and standardization of Iatroscan
845
1 I0
cm
0
10
a
cm b
0
10
cm
0
c
Fig. 7. Examples showing TLC-FID chromatograms of (a) a pure aromatic hydrocarbon, (b) a typical resin and (c) a relatively pure asphaltene fraction. of hydrocarbons or resins. However, asphaltene fractions from oils 5, 19 and 30 are rich in asphaltenes, as also noted earlier, the latter two being contaminated by resins. One asphaltene however, apparently gives good results (oil 5) and as seen in Fig. 7(c), no hydrocarbons or resins seem to be present as contaminants.
Preparative thin-layer chromatography (PTLC) As noted above, all the resin fractions resulting from MPLC separation split into two peaks on Iatroscan analysis, one peak indicative of aromatic hydrocarbons and the other of resins. The selected resin fractions for preparative TLC were from oils 5 and 16, and each resin fraction resulted in 2 fractions (aromatic hydrocarbons or N S O - A R O and "pure" resins or NSO-NSO). The resulting pure resin fraction (NSO-NSO) is characterised by a dominant resin peak and a minor asphaltene peak [Fig. 8(a)].
I
10
cm It
0
10
I
I
I
cm
I
1
0
b
Fig. 8. Iatroscan analysis of the separated resin fractions using preparative-TLC. (a) The pure resin fraction (NSO-NSO) and (b) an impure resin fraction (NSO-ARO). OG 2 2 / 3 - ~
More importantly, the principle resin peak does not split into 2 components and the resulting fraction is considered to be clean and uncontaminated. The second fraction (NSO-ARO), however, also contains a dominant component, apparently resins but which should to be of a different composition, and some hydrocarbons [Fig. 8(b)]. Nevertheless, this exercise suggests that the resin fraction resulting from MPLC and which splits into two distinct components on Iatroscan analysis, can be further separated into two fractions on a preparative scale, as shown in Plate 1. For comparison, the results of TLC separation of all the fractions and whole oil using a TLC plate are also shown in Plate 1. Subsequent separation of all oils using preparative TLC plates and the same elution procedure as employed in Iatroscan, shows that the four principle fractions separate well and the u.v. detector (254 and 356 nm) showed three distinct zones containing aromatic hydrocarbons, resins and asphaltenes (saturated hydrocarbons do not fluoresce) (Plate 1). Analysis of the four principle fractions from MPLC showed that the saturated hydrocarbons separated into two zones--one of major saturated hydrocarbons and one of minor aromatic hydrocarbons. The aromatic hydrocarbons separate into two fractions-one major aromatic hydrocarbons and one minor resins, and perhaps one minor saturated hydrocarbon zone, but this is difficult to establish as saturated hydrocarbons are not possible to detect on a u.v. detector. The resin fraction clearly separates into 2 equally dominant fractions--aromatic hydrocarbons and resins. Finally, the asphaltenes separate into mainly asphaltenes and minor resins, aromatic hydrocarbons and probably minor amounts of saturated hydrocarbons.
Composition and purity of the separated fractions Gas chromatography of the 5 saturated hydrocarbon fractions (from oils 1, 5, 16, 19 and 30) which
846
SUNILBHARATIet al.
°/
/
t,E:Ou 9£0u ..~ -..=¢ 0GOU
I 0£Ou'~ ~Ou ;¢Ou
C=
--
:~ES E
V- N
-
~NW.AHa
/.~3u
g=Ou'~
~N~£SI~d
LL0u
-i
:
e~01
tn
:tNV.LAHd
eI0u ~N~£SI~d
iI0u
g~o!
o
g~Ou
o I'tO!
~10!
Q
, , , I , , ,
T
o
o
o
N
o
oo
=o
=o
o
, , m
(Am) ~l!suelul -I
, J ,
, , ,
, , ,
,
e~ 0
,~
o
(AW)/~l!suelul
./
/
e
~
0
N ~£Ou
N 0~Ou -
~
eg[_ 3
i
E=
gtou L~0U
~NV~,kHd~, ~NVISI~d
3 NV.I.SIEId e £0
3
u
N
~
~
S~Ou -%
(ALU) /~:llsueluI
0
(AtU) ~,!suelul
o
oo
03
03
Calibration and standardization o f latroscan
847
/
Y
0
d~H. dl~'~ c
E
v
o
~
c~
_
E
N~a~'t NlflO'9' t
M~lO-.,'~+g'~ NIq
NMo'g' l
NWO-L'~"g'Z
Hdg
Ni~l-I
I
N ~
Nt~l-~
0
E o 0 o
0
..r
i
I
0
,
,
,
I
i
i
i
I
,
,
,
I
,
,
,
I
i
i
,
I
,
o
~
(ALU) ~l!suelul
(^uJ)/~llsuelul
E <
i
.o
F
C~
o
p
~
2
g
2
i=
o
dl~l'~
die4-6 diN'(:
i-:
o NI~O-9'I.
o
o
o
(Am) fIjsue),u I
o
o
NINO-Z'Z+9'~
" "
o
(Am)/;~!suelul
848
S U N 1 L B H A R A T I et
were thought to have the best potential of being used as standards indicates that there are significant differences in the composition and maturity of these oils. Oil i [Fig. 9(a)] is relatively rich in lighter n-alkane components, probably due to its slightly higher maturity; oil 5 [Fig. 9(b)] is moderate to strongly biodegraded; oil 16 [Fig. 9(c)] is unusually rich in isoprenoids relative to the n-alkanes (probably to do with an unusual source); oil 19 [Fig. 9(d)] is relatively rich in heavier n-alkane components with n-Cl5 being the most abundant alkane; and oil 30 is mildly biodegraded. The isoprenoid content in oil 19 is also higher, indicating a lower maturity, though not as high a content as in oil 16. Gas chromatography of the 4 aromatic hydrocarbon fractions which were thought to have best potential of being used as standards also indicates significant compositional differences. Oil 7 [Fig. 10(a)] is biodegraded and is depleted in diaromatic hydrocarbons; oil 16 [Fig. 10(b)] is very rich in diaromaticand depleted in triaromatic hydrocarbons; while oils 19 [Fig. 10(c)] and 22 [Fig. 10(d)] are relatively rich in diaromatic, but also have significant triaromatic hydrocarbon moieties. In addition, oil 19 is more mature than oil 22 (indicative from the higher 2methylnaphthalene/1-methylnaphthalene ratio in oil 19). FPD traces of the 4 aromatic hydrocarbon fractions show that only oils 7 and 16 contain traces of dibenzothiophene, with an apparent difference in maturity between the two. It is important to understand as to which of the aromatic hydrocarbon species have the greatest tendency to be retained as contaminants in the saturated hydrocarbon fraction and vice-versa. This semi-quantitative and qualitative assessment was achieved by analysing selected saturated and aromatic hydrocarbon fractions using the GC-MS technique. The objective of semi-quantitative assessment and contaminant identification was especially significant in light of the conventional methods adopted for separating fractions (MPLC). Therefore the purest/least contaminated saturated and aromatic hydrocarbon fractions amongst all the oils considered in the study, were chosen for this exercise. The saturated hydrocarbon fraction is rich in nalkanes, but the cyclo-alkane moieties are evidently also abundant (m/z 83 and 97), although long chains of alkylbenzenes are also detected in m/z 83 [Fig. 1l(a)]. In addition, isoprenoids are also present in significant proportions. However, based on m/z 91 and 106, alkylbenzenes are not present in large amounts, contrary to expectations as alkylbenzenes with long chains were thought to be the most likely contaminants in the saturated hydrocarbon fraction. These observations are apparent in Fig. 1l(a), where the absolute intensities of the 10 major ions are plotted. The figure shows the maximum intensity in a fragmentogram and the general (most common) intensity in that fragmentogram. This semi-quantitative approach indicates the relative abundances of the
al. (a) 16
-
14
-
12
-
Saturated Hydrocarbons Intensity
lO -
[ ] General
g
a-
--~
6
-,
4
-
// //
2
-
7,//
//
7,
// /J
// t,
0
type
[ ] Maximum
n_=l
L
77 83 91
(b) 12r--
97
99 106 134 142 163 183
Aromatic Hydrocarbons 77
10
-
¢/~ r-
.o
~/ //
s-
--
/~ //
0
-]173~ r~--,,,,.,~ I ~ ,.~_~ 77 83 91 97 99 106 134 142 163 183 m/z
Fig. II. Summarized results of GC-MS analysis of the hydrocarbon fractions. Relative maximum intensities of 10 major ions in (a) saturated hydrocarbon fraction and (b) aromatic hydrocarbon fraction. various types of molecules present in the fraction. Apparently, Cl-benzene, the basic aromatic ring structure with a methyl group, seems to be the most abundant aromatic hydrocarbon species present. The aromatic hydrocarbon fraction on the other hand is dominated by short-chain methylated benzenes, but relatively smaller amounts of some C4alkylbenzene are also present. Amongst the saturated hydrocarbon species, n-alkanes and isoprenoids seem to be nearly equally distributed [Fig. I l(b)], in addition to methyl cyclic alkanes (m/z 97). Unlike the saturated hydrocarbon fraction, which contains practically no dibenzothiophenes, the aromatic hydrocarbon fraction retains some dibenzothiophene moieties, this being observed in the m/z 198 fragmentogram. On comparing Figs l l(a) and l l(b), it seems that the aromatic hydrocarbons have a greater tendency to retain saturated hydrocarbon species than vice-versa. However, the limitations of this observation must be borne in mind, as the conclusion is based on the analysis of only two samples. The 2 asphaltenes selected for making the standards differ significantly in the composition of their thermal extracts. While oil 5 is strongly biodegraded [Fig. 12(a)], oil 19 has a bimodal distribution of n-alkanes [Fig. 12(b)]. Their pyrolysates, however, are quite similar in composition [Figs 12(c) and (d)]
~'.
Intensity (mY) o
~]
.~...~
'
I
'
'
'
o I
'
'
'
m m n s f f y ~mv]
c) I
'
'
'
o
I
'
'
'
I
'
'
'
o I
°
o~:
!"-
m
~,:,_nC I 0 fl~
i ~,~
~,
nC15 ~
o~
PRISTANE
nC17
I I
c) o
}
), 03
"0
e..
,~s
|=________.__
!
_
o~
mlensffy I.mv) (x 1U'{I) o
o
o
E
(b (D
(n
/ n l e n s i ~ ( m v ] ~x 1U1;I) o
o
o
ol
E ~LrNE
u -<
0
b o -'
c)
3
o
(D
~
o~ -
C17
G18
0~ "10 ~r m m (D
o~ cao
C~
OV~
ub'a~uJlUl ju uu!luz!pJ~puul~
pu~
=uH-a~l:l~ J
t5
10
25
25
20
20
30
30
$
40
45
50
40
45
50
Time (minutes)
35
Time (minutes)
35
55
55
60
60
65
65
a
70
70
10
lo
15
is
20
2-o
2s
t
25
30
30
4s
50
1
40
4s
50
°
Time (minutes)
as
0
40
Time (minutes)
35
TIC of Asphaltenes
ss
55
sO
60
6s
d
es
C
r0
ro
Fig. 13. Results of PY--GC-MS analysis of resins and asphaltenes. Total Ion Chromatograms (TIC) of (a) resin from oil 16, (b) resin from oil 22, (c) asphaltene from oil 5 and (d) asphaltene from oil 16. Peak identification: V--toluene; V--n-hydrocarbon doublets; O--alkylcyclohexanes; n--methylphenanthrene; O--alkylnaphthalenes; ~--nCl8 doublet; ,>~--internal standard.
15
10
t $
TIC of Resins
>.
t-" =0 ,i;>
_z
C
851
Calibration and standardization of latroscan
and are indicative of a marine source. Py-GC of Ion Chromatograms (TIC) of the two asphaltene fractions are shown in Figs 13(c) and (d). resins did not give any conclusive results. P y - G C - M S of selected resin and asphaltene Identification, isolation and purification of selected fractions allowed to assess the presence of any hydrofractions for standard carbons, if any, that might have been retained in these Based on the findings this far, 4 oils covering fractions after MPLC separation. The resin fraction of oil 16 contains only traces of n-hydrocarbons, as practically the entire API gravity range, were selected is indicative by the generally imperceptible homology for re-separation into fractions. Preliminary analyses in m/z 97 and 99, although m/z 99 does exhibit indicated that one or more of these oils could be presence of some n-alkanes. Alkylbenzenes are successfully used to isolate pure fractions, which could be further used to calibrate Iatroscan via present, but are believed to occur in small amounts. Alkylnaphthalenes are more prominent relative to prepared standards. Four parallels of each of the four alkyl-phenanthrenes. Dibenzothiophenes and other oils, oil 5, oil 16, oil 19 and oil 22, were subjected to substituted derivatives are also present. No significant MPLC in order to get large amounts of each fraction biomarkers were recorded. Compared to the resin from each oil. At this stage, we had 4 fractions × 4 from oil 16, the resin fraction from oil 22 is much parallels for each oil. Each of the four fractions more depleted in most of the hydrocarbon species, from these oils were later mixed and re-run through although the latter contains a stronger n-alkane the MPLC system to purify and clean the fractions. homology, as observed in m/z 99, perhaps indicative This ultimately resulted in one large portion each of saturated hydrocarbon, aromatic hydrocarbon, resin of the presence of more saturated hydrocarbons. Most alkylbenzenes are of low molecular weight and asphaltene fractions for each of the 4 oils. On and are not interpreted to be significant. The results analyzing these fractions using the Iatroscan, it was found that the aromatic fraction of all the oils was for other ions are similar, but these compounds are present in low concentrations. The Total Ion contaminated by substantial amounts of resin species, Chromatograms (TIC) of the two resin fractions are a problem which had also been observed earlier. The 4 aromatic hydrocarbon fractions were run through shown in Figs 13(a) and (b). The 2 asphaltenes seem to be considerably different MPLC twice more to remove the resins. The typical T L C - F I D chromatograms of the final in composition. While oil 5 asphaltene is rich in shortand medium-length alkyl chains, oil 16 asphaltene pure fractions (saturate and aromatic hydrocarbons, is depleted in these moieties (m/z 97 and 99). Oil 5 resins and asphaltenes) used in Iatroscan calibration asphaltene is also relatively rich in alkylbenzenes. and the standard are shown in Figs 14(a-d). Clearly, Alkylnaphthalenes, however, seem to be equally the fractions are uncontaminated with only traces of prominent in both the asphaltenes, in contrast to unwanted components. However, as the four saturated alkylphenanthrenes which are more conspicuous in hydrocarbon and aromatic hydrocarbon fractions the oil 5 asphaltene. Dibenzothiophenes are depleted differed significantly from each other with respect to in oil 16 asphaltene relative to oil 5 asphaltene. These density and composition, individual response factors results perhaps indicate that oil 16 may be generally were calculated to see if the response factors were richer in branched/cyclo hydrocarbons. The Total affected by these parameters. Each of the fractions
10
cm
a
0
10
cm
b
0
10
cm ¢
0
10
cm
0
d
Fig. 14. TLC-FID chromatograms of "pure" isolated fractions obtained by a combination of repeated MPLC and preparative-TLC. (a) Saturated hydrocarbons, (b) aromatic hydrocarbons, (c) resins and (d) asphaltenes.
852
SUN]]. (a)
Saturated
BHARATIet
al.
(c)
HC
Resins
7 --
5-
4 -I
/
•
t-
3 -
~"
2
E O
-=
1
}-
0.5
"5 ~
5
OIL TYPE [] Heavy • Medium 1 A Medium 2 • Light I 1.0
I 1.5
(b)
O I L TYPE [] Heavy • Medium 1
--
I 2.0
I 2.5
I 3.0
3 3.0
I 3.5
(d)
A r o m a t i c HC
I 4.0
Asphaltenes
14--
ffl cO
[]
13--
if}
rr
12--
5 --
~ . ~ ~ ' ~ ~ ~"
OIL TYPE Heavy • Medium 1 zx Medium 2 • Light
4
--
3 0.5
z~
I 1.0
I 1.5
I 2.0
I 2.5
I 5.0
-/
11 - -
o
..
I 4.5
OIL TYPE [] Heavy • Medium 1 zx Medium 2
I
10 -
•
9-I 3.0
8 0.5
I 1.0
I 1.5
I 2.0
I 2.5
S a m p l e a m o u n t (pl)
Fig. 15. Dependence of calculated response factors of Iatroscan on oil's composition and density. Plot of response factor versus sample amount from 4 different oils for (a) saturated hydrocarbons, (b) aromatic hydrocarbons, (c) resins and (d) asphaltenes.
of the four oils (one heavy--API gravity around 10°, two medium--API gravity around 30 ° and one light - - A P I gravity more than 40 °) were analysed by Iatroscan 3-5 times, using different sample amounts each time (0.5-5/~1 of the same solution), and response factors calculated in each case using the equation shown below. Response factor (Re) = Peak area (A)/Sample amount ( M ) The results are shown in Figs 15(a~i). Dietz (1967) has shown earlier that relative sensitivity for the flame ionization detector (FID) is quite different for hydrocarbons and non-hydrocarbons. While most straight chain, branched and cyclo alkanes have similar FID sensitivities (close to 1.0), aromatic hydrocarbons tend to be very slightly lower (0.98-1.02). However, non-hydrocarbons (resins and asphaltenes), which have heteroatoms such as N, S and O, have much lower FID sensitivities and the values vary appreciably. Alcohols, for example, vary from 0.23 to 0.85; acids, from 0.01 to 0.65 etc. (Dietz, 1967). The variation in the sensitivity value depends mainly on type of compound, number of heteroatoms, molecular weight etc. (Dietz, 1967; Drushel, 1983).
As seen in Fig. 15(a), the response factors of the four saturated hydrocarbon fractions vary significantly (from about 2000 to 4500). Closer examination of the results reveals that the variation is not random, but is controlled by at least two factors. Saturated hydrocarbons from the heavy oil have the greatest response factor, while those from a light oil have the least. Compositionally, the saturated hydrocarbons from the heavy oil are strongly biodegraded, those from the first medium oil (plotted as solid squares) are very rich in isoprenoids, those of the second medium oil are rich in medium to heavy range n-alkanes and lastly those from the light oil are rich in low molecular weight n-alkanes (cf. Fig. 9). In case of the aromatic hydrocarbons [Fig. 15(b)], the variation in response factor is from about 4000 for the lightest oil to about 6500 for the heaviest oil. With regards to the compositional variation between the four aromatic hydrocarbons, the heavy oil is biodegraded, as noted above, the first medium oil (plotted as solid squares) is richer in alkylnaphthalenes relative to triaromatics, while the second medium oil and the light oil are relatively rich in triaromatics (cf. Fig. 10). In case of the resins and asphaltenes, the trend is opposite to those of the two hydrocarbon groups
Calibration and standardization of Iatroscan
853
Response Factor(Thousands) _..A..- -o 12
Jk--'"
ught o . J
Medium011
I Heavy011 I
X
..........
~ ........... . ~ - a- i - -i' ......... - - - - i -i....... .....
B
SATHC ~AROHC 10
20
24
-g-NSO
30
35
,~ASP 40
I 50
API Gravity 1") Fig. 16. Relationship of oil density (API gravity) and Iatroscan detector response (Response factor) for the 4 principle fractions of oil viz. saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes. The API gravity is inversely proportional to detector response in case of the 2 hydrocarbon fractions (as indicated by the negative slope of the 2 solid linear regression trends) and directly proportional in case of the 2 non-hydrocarbon fractions (as indicated by the positive slope of the 2 dashed linear regression trends).
[Figs 15(c) and (d)]. Unlike the hydrocarbons, the response factors of the heavy oil fractions are lower than those of their lighter counterparts. The significance of this reversal is not exactly clear, but it is suspected that it may be related to lower carbon and hydrogen contents/greater ionization efficiency of the components in the heavier oil's resin and asphaltene fractions. Calibration of Iatrosean standard
and testing of prepared
Clearly, the present results indicate that there cannot be one unique response factor for a given fraction to calibrate Iatroscan. Parrish and Ackman (1985) had also observed variations in the Iatroscan's FID response, on analysing marine lipids. It is controlled to a large extent by the oil's API gravity (which can be expressed mathematically) and the gross composition of the fraction (which is a subjective aspect and cannot be expressed mathematically). In Fig. 16, the API gravity of the 4 oils is plotted against the average response factor of each of the 4 fractions of each of the 4 oils. The variation of the response factors in case of each of the four fractions is more apparent here. While the calculated response factors decrease with increasing API gravity for the two hydrocarbons, it increases for the two nonhydrocarbon fractions. Moreover, there is a remarkable similarity between the two hydrocarbon fractions as regards to their response factor progression with increasing API gravity (the two linear regression trends--solid lines are parallel to each other). Similarly, the linear regression trends of the two nonhydrocarbon fractions (the two dotted lines) progress
nearly parallel to each other, although unlike the hydrocarbons their response factors increase with increasing API gravity. As response factors cannot be calculated for every unit of the API gravity, and there is no unique response factor for a given fraction either, the whole suite of oils has been divided into three principle types for simplicity--heavy (API gravity 10-24°), medium (API gravity 24-35 °) and light (API gravity 35 ° and above). The American Petroleum Institute defines an oil as heavy if it has an API gravity of 20 ° or lower. However, in our case (Fig. 16), the response factor of the polar fraction is lower than that of the saturated hydrocarbon fraction until about 24°, but then crosses over and is subsequently greater than the saturated hydrocarbons. This point was therefore taken as the boundary between the heavy and medium oils. It is not known, however, whether this fact is of any significance or not. Considering the results plotted in Fig. 16, it is assumed that if each of the 4 fractions have 3 different response factors according to the type of oil (one each for heavy, medium and light oils), calibration of Iatroscan will be reasonably good. The mathematical model developed to derive these factors is exemplified below. The slopes of the two hydrocarbon trends are negative while those of the two non-hydrocarbons are positive. Ideally (typically for single synthetic compounds and theoretically for a mixture of similar compounds), R~=A/M
(1)
where Rc = response factor, A = peak area, M = sample amount.
SUNIL BHARATIet al.
854
| $
tl
10
-I 0
cm
I
I~ 10
em
b
A
0
"" 10
cm
0
¢
Fig. 17. TLC-FID chromatograms of prepared standards. (a) A natural standard based on a medium oil (Standard A). (b) A natural standard based on a light oil (Standard B). (c) A synthetic standard comprising several synthetic components (GEO3).
But in the present case,
where C = f ( a ) , a being the API gravity of the oil. This is similar to the linear regression equation
The correction constants can be obtained for each fraction through linear regression, using the slope of the trend and the applicable cr in equations (5), (6), (7) and (8). An example for saturated hydrocarbons is shown below:
y = a + bx
CI(SAT)= --53.3 × 17 = --906
Rc = A / M + C,
(2)
(3)
where 17 is the assumed representath~e of the a Range [tO, 241.
where a is a constant and b is the slope. .'. equation (2) can be written as
Rc=A/M+b
xa
(4)
The derived equations for the 4 components in Fig. 16 are:
Saturated hydrocarbons.
C2CSAT)= -- 53.3 × 29 = -- 1546
where 29 is the assumed representative of the cr Range [24, 35]. C3(SAT)= -- 53.3 × 42 = -- 2238
Re(sAT) = 5292 + (--53.3 x a)
(5)
Aromatic hydrocarbons. Rc(ARO)= 7328 + (--47.3 X a)
(6)
Rc(eoL) = 5393 + (34.6 x a)
(7)
Polars.
Asphaltenes. RclAsp) = 9781 + (66.3 × a)
(8)
Equation (4) above can be considered to be the basic "Correction Equation". This can theoretically have infinite number of forms for every given a, but for simplicity we will consider 3 principle forms suitable for heavy oils, medium oils and light oils. The 3 equations can be written as,
R c H = A / M +CI,
10<~a <~24
(9)
RcM=A/M+Cz,
24<~-N<35
(10)
RcL=A/M+C3,
35
(11)
~<50
where C~, 6"2, C3 are "Correction Constants".
where 42 is the assumed representative of the a Range [35, 50]. Using the correction constants, the response factor (Re) of a given fraction for each oil type can be calculated using equations (5), (6), (7) and (8), as shown below for saturated hydrocarbons as an example [the equation employed in this case is equation (5)]. RcH = 5292 -- 906 = 4386 RcM = 5292 -- 1546 = 3746 RcL = 5292 -- 2238 = 3054 Similarly, the other correction constants and response factors can be calculated for the other fractions and each type of oil. These were calculated and are shown in Table 9. According to the table below, we have a total of 4 fractions x 3 oil types i.e. 12 response factors for use and calibration. It must be noted, however, that these are response factors of the F I D used to analyze the samples in the present study, and they will not be necessarily the same for other users. However, the
855
Calibration and standardization of latroscan Table 9. (a) Calculated correction constants (C) Sat Ct C2 C3
-906 - 1546 -2238
Aro -804 - 1371 - 1987
Pol
Asp
588 1003 1453
1127 1923 2785
(a)
Standard A
80--
60 - -
Fraction o SAT x ARO
/
• POL (b) Calculated response factors (R c)
A ASP
Sat
Aro
Pol
Asp
Heavy oil (Rcs) Medium oil (RcM) Light oil (RcL)
4386 3746 3054
6524 5957 5341
5981 6396 6846
10908 11704 12566
Average (Balk) (R¢s)
3729
5941
6408
11726
, 0
~
20--
o -~"v
relationship of the response factors to each other is more likely to remain the same, if the same standard is used to calibrate the instrument. Therefore, for other users, the 2 tables below (Table 10) could be used to extrapolate the response factors from the given standard, which is based on a medium oil. The relationships in Table 10(a) below are derived directly from the data in Table 9(b). A closer examination of these relationships reveals that in relation to the medium oil values, both the hydrocarbon fractions of heavy and light oils have a rather constant relationship (1.17 to 1.1 times and 0.82 to 0.9 times respectively) and both the non-hydrocarbon fractions of heavy and light oils also have a constant relationship (0.93 times 1.07 times respectively). This further strengthens the interpretation that these variations are not random, but are controlled by distinct factor(s). The user may utilise either Table 10(a) or 10(b) to extrapolate response factors from the given medium oil based standard, Standard A. Two standards have been prepared using topped oils, for calibrating Iatroscan and subsequent use. The strength of the solutions in both cases was aimed in the range 10-15 mg oil/ml solvent, as it has been found through experience that this is the ideal range to ensure adequate individual component amounts, good separation and to avoid overloading. The solvent chosen to dissolve the standards was toluene instead of commonly used dichloromethane, due to toluene's greater stability (higher boiling point) under ambient conditions. Table 10. (a) Relationship of response factors (Re) of a fraction with respect to medium oil
Heavy oil (RcH) Medium oil (R¢M) Light oil (ReL)
Sat
Aro
Pol
Asp
1.17p p 0.82p
1.1q q 0.9q
0.93r r 1.07r
0.93s s 1.07s
p, q , r and s are the user's calculated response factors based on prepared Standard A. (b) Relationship of response factors (Re) of a type of oil w.r.t. saturated hydrocarbons
Saturated HC Aromatic HC Polars Asphaltenes
Heavy oil
Medium oil
Light oil
(RcH)
(RcM)
(RcL)
x 1.49x 1.36x 2.49x
y 1.59y 1.71y 3.12y
z 1.75z 2.24z 4.11 z
x is the user's calculated response factor for saturated hydrocarbon fraction based on prepared Standard A.
0.5
[-
T
I
I
I
I
1.0
1.5
2.0
2.5
3.0
(b) ~
80 --
<
60 --
Standard B
[]
[]
2.0
2.5
40 --
20
0.5
1.0
1.5
3.0
Sample Amount (lad Fig. 18. C a l i b r a t i o n curves f o r the 2 n a t u r a l s t a n d a r d s , A a n d B. (a) P l o t o f s a m p l e a m o u n t versus p e a k a r e a o f the 4 i n d i v i d u a l c o m p o n e n t s o f S t a n d a r d A. (b) P l o t o f s a m p l e a m o u n t versus p e a k a r e a o f the 4 i n d i v i d u a l c o m p o n e n t s o f S t a n d a r d B. N o t e the l i n e a r i t y o f p e a k - a r e a progression w i t h i n c r e a s i n g s a m p l e a m o u n t in case o f e a c h c o m p o n e n t in b o t h the s t a n d a r d s .
Standard A is primarily made from a medium oil and is rich in polars and asphaltenes. In contrast, Standard B is primarily from a light oil and contains minor polars and practically no asphaltenes. The chromatograms of these two standards are shown in Figs 17(a-b). These two standards were analysed by Iatroscan using 5 parallels, with different sample amounts each time. The sample amount varied from 0.5 to 3#1 in each case. Calibration curves were drawn and are shown in Figs 18(a-b), where it can be observed that the peak area progression is rather Table 11. Composition and quantitative data for the two prepared standards
Fraction
Peak area uVs
Corrected Response area factor uVs [cf. Table9(b)]
Saturated HC Aromatic HC Polars Asphaltenes
45792 20847 26794 10645
Standard A 48332 20727 26177 10599
3746 5957 6396 11704
12.90 (60.37) 3.48 (16.29) 4.09(19.14) 0.90 (4.2)
Saturated HC Aromatic HC Polars Asphaltenes
42018 10266 4284 236
Standard B 45537 10022 5100 312
3054 5341 6846 12566
14.91 (84.96) 1.88(10.71) 0.74 (4.22) 0.02 (0. I 1)
Amount ~ul (%)
19.0
18. Stearamide
19. Berberine sulphate
18.2
17. Stearyl alcohol
9.8
15. I-benzothiophene 20.1
12.7
16. Stearic acid
20.2
31.8
NSO1
13. Thenoic acid
100
11.3
5.1
16.4
27.1
8.3
AROI
14. Dibenzothiophene
50.4
12. Phenanthrene
16.5
10. 2-methylnaphthalene
49.6
7.5
9. Biphenylene
I1. 1,3-dimethylanthracene
24.1
3.8
8. Naphthalene
5.5
5. Hexatriacontane
12.1
19.7
4. Tfiacontane
39.8
26.9
3. Hexacosane
7. 1,4-dicyclohexylbenzene
41.4
1. Hexadecane
2. Docosane
MODII POLYI POLY2
6. 2,3-dimethylnaphthalene
SAT1
Compound
23.7
29.5
46.8
NSO2
71.4
28.6
NSO3
55.5
17.8
26.7
NSO4
49
12.8
22.5
15.7
NSO5
20.1
21.7
19.6
38.5
NSO6
Standard Name
33.4
13.4
14.5
13.1
25.6
NSO7
33.3
33.1
33.6
NSO8
100
NSO9
100 100
NSOI0 NSOII
25
75
25
75
25
75
NSOI2 NSOI3 NSOI4
Table 12. List of compounds used to prepare various synthetic standards. The figures indicate the percentage of the compound used in a standard
I00
ASPI
2 g~
-e
z r~
Calibration and standardization of Iatroscan consistent in the case of all the four principle fract i o n s - s a t u r a t e d hydrocarbons, aromatic hydrocarbons, polars and asphaltenes. Corrected peak areas, based on linear regression, were used to calculate the fraction amounts. The response factors used to derive the individual component amounts are derived from Table 9(b). The case for 2 #i sample amounts for both the standards is shown below in Table 11 and taken as correct, as this is the sample amount (sample dissolved in solvent) that is applied on the chromarods.
Synthetic standards Table 12 shows the details of the synthetic standards (total 21 basic standards) prepared in the present study. One basic criterion that was followed while making these standards was that in each standard, the distribution of components should be as close as possible to the distribution commonly encountered in the natural samples. A total of 19 compounds, most of which occur naturally in solvent extracts and/or crude oils, was chosen with objective of obtaining response factors as close as possible to natural mixtures. The first stage in synthetic standard analysis included mainly establishing the correct combination of various compounds which are compatible with each other in a solution. This was therefore a more qualitative objective. The second stage consisted of making a mixture of all the compatible compounds, with at least one compound representative of each of the principle fraction type, and calibrating Iatroscan and deriving the response factors. The qualitative assessment and compound compatibility findings are described below for each fraction type.
857
Saturated hydrocarbons. Squalane is the most commonly used synthetic compound for the purpose of calibration and quantification of saturated hydrocarbons. We, however, chose to use a mixture of 5 different n-alkanes ranging widely in the carbon chain-length (nCl6 to nC36) to prepare the saturated hydrocarbon standard, SAT1 (Table 12). The T L C FID chromatogram is shown in Fig. 19(a). Iatroscan analysis of SATI gave very good results with one dominant peak. This was therefore the only standard made for saturated hydrocarbons. Aromatic hydrocarbons. The standard for monoand diaromatic hydrocarbons (MODII), which was a mixture of 5 compounds (Table 12), resulted in one sharp peak on Iatroscan analysis [Fig. 19(b)]. In addition, a combination of MODI1 components and a poly-aromatic hydrocarbon (phenanthrene) (ARO1) resulted in two well defined peaks on Iatroscan analysis [Fig. 19(c)]. Asphaltene. The criteria for selecting a compound which could serve as a natural asphaltene equivalent were simple: (1) the molecule should be large, (2) the molecule should be polar, (3) the molecule should contain nitrogen, sulphur and oxygen. Berberine sulphate (MW = 822, formula = C40H42N20~5 S) was selected and the standard ASP1 and ASP2 (a much diluted version of ASPI) analysed by Iatroscan. The results were good with one dominant peak at the spotting point [Fig. 19(d)]. Resins. Making the right synthetic standard for resins was the most difficult task in the entire synthetic standard study. Up to 14 different mixtures (Table 12) had to be made to reach the right composition of the resin standard. Initially, the criteria was to include at least one compound containing
E
r-tO
cm
a
0
==,, lO
,% cm
b
OlO
cm C
0
10
, cm
, 0
d
Fig. 19. TLC-FID chromatograms of synthetic standards. (a) Synthetic standard SAT1, comprising 5 n-alkanes with varying carbon chain-length (C16-C36). (b) Synthetic standard MODII, comprising 5 different mono- and diaromatic hydrocarbon components. (c) Synthetic standard AROI, comprising MODI1 and a poly-aromatic hydrocarbon (phenanthrene). (d) Synthetic standard ASP1, comprising a single compound, berberine sulphate, which is thought to be comparable to a natural asphaltene molecule with regards to polarity and elution characteristics during chromatography.
858
SUNIL BHARATIet al.
|
.~~ 10
10
cm a
0
cm
0
e
10
10
i ~
."I
cm b
0
cm
0
f
IO
10
l~',
cm c
0
¢m
0
g
10
, cm
d
10
cm
0
h
Fig. 20. Results of latroscan analysis of the various synthetic resin standards. TLC-FID chromatograms of (a) NSO 1, comprising 6 different components, including 2 acids, an alcohol, an amide and 2 thiophenes. (b) NSO2, comprising 3 components- thenoic acid and 2 thiophenes. (c) NSO5, comprising ASP1 and C18 acid, alcohol and amide. (d) NSO6, comprising thenoic acid and C 18--acid, alcohol and amide. (e) NSO7, comprising NSO6 and ASP1. (f) NSO8, comprising only the 3 C 18 components--acid, alcohol and amide. (g) NSO11, comprising only C18 amide and (h) NSOI4, comprising C18 amide and berberine sulphate (ASP1).
nitrogen, sulphur and oxygen and at the same time, maintaining the individual molecular weight low. NSOI standard was made, but on analysis, the mixture split into three peaks with one peak eluting as an aromatic hydrocarbon [Fig. 20(a)]. NSO2 (made to identify the incompatible component) split into two distinct components [Fig. 20(b)] and therefore abandoned. NSO3, NSO4 and NSO5, which consisted of similar compounds mixed with berberine sulphate, and which were expected to give 2 peaks each, gave variable results. While NSO3 resulted in two peaks, thenoic acid did not elute as much as it should have; NSO4 gave only one peak; and NSO5 seemed to have worked well with the combined CI8acid, alcohol and amide peak separating very well from the berberine sulphate peak [Fig. 20(c)]. This led us to believe that perhaps the two benzothiophenes are the problematic components. To ascertain this,
NSO6 and NSO7 were analysed, but contrary to expectations NSO6 resulted in two peaks [Fig. 20(d)] and NSO7 in three peaks [Fig. 20(e)]. This clearly indicated that the combination of stearic acid, stearyl alcohol and stearamide is also not compatible. This was evident from NSO8 analysis [Fig. 20(f)], where three components gave three peaks. NSO9, NSOI0 and NSO1 i were analysed to identify the incompatible component. All the three gave reasonably good single peaks, but stearamide [Fig. 20(g)] seemed to be most consistent in its retention time. A combination of these three components with berberine sulphate (NSO12, NSO13 and NSOI4) confirmed that stearamide was most compatible with the asphaltene component, berberine sulphate [Fig. 20(h)]. This concluded the qualitative and compatibility assessment of the selected synthetic compounds and the final standard mixture GEO3, which comprised
859
Calibration and standardization of Iatroscan Table 13. Calibration results of the Synthetic Standard GEO3 (based on 10 parallels t~/"2#1 ) Fraction
Mean Area
Area SD
Area (%)
Amount (%)
Amount (/~g)
Rcs
Saturated Hydrocarbons Mono + Diaromatic Hydrocarbons Poly-aromatic Hydrocarbons Resin Asphaltene
14727 4842 3917 8035 6961
962 399 282 688 531
37.51 12.72 9.98 21.13 18.79
41.8 24.5 9.3 14.7 9.6
4.32 2.532 0.96 1.52 0.996
3409 1912 4080 5286 6989
SATI, AROI, NSOI1 and ASP2 was analysed. The resulting T L C - F I D chromatogram is shown in Fig. 17(c). Clearly, the 13 compounds used in GEO3 are compatible and stable in each other's presence, with one peak each of saturated hydrocarbons, mono+diaromatic hydrocarbons, polyaromatic hydrocarbons, resins and asphaltene. The new GEO3, which was subsequently made using fresh chemicals with the purpose of calibrating Iatroscan and deriving response factors, comprised of the following: saturated hydrocarbons 40%, aromatic hydrocarbons 35%, resin component 15% and asphaltene component 10%. Ten parallels of 2 #1 spotting were analysed to establish reproducibility and obtain optimum peak areas. Table 13 above summarizes the findings. The implications of the calculated response factors from synthetic standards (R~s) are discussed in the next section.
Validity of oil type dependent calculated response factors The calculated response factors as shown in Table 9(b) apparently indicate potentially large differences in the resulting quantitative data when these factors are used. It is intended in this section to demonstrate the actual differences that will be obtained if only a single set of bulk response factors are used instead of calculated response factors with oil type dependence. The test is performed for heavy and light oils' response factors, keeping the average bulk response factors constant. In the first case, we assume that Standards A and B are heavy oils and use the corrected areas to calculate the fraction amounts using the heavy oil response factors i.e. RcH, and this will give Amount 1 for both the standards. Next we
use the bulk response factors (Rca) and calculate the fraction amounts (Amount 2). The fraction amounts resulting by employing RoB in Case 1 should be quite close to the actual values of the standards calculated using RcM, and which are shown in Table 11. This is because RoB are closest to Rosa. Next, the response factors for light oil (R~L) are used and the fraction amounts calculated (Amount 3). Finally, the response factors derived from the GEO3 synthetic standard (Rcs from Table 13) are used and the individual fraction amounts calculated (Amount 4). The results are presented below in Table 14. From Table 14 the following conclusion can be made. The first general effect of using a single set of bulk response factors is that the quantity of fractions in heavy oils are overestimated and those in light oils are underestimated. This means, for example, that the concentration of a heavy oil solution is reported as 21.43 and 13.69 mg/ml instead of 19.55 and 11.89 mg/ ml respectively, an overestimate of about 10-12%. Likewise, the light oil solution which should have a concentration of 24.37 and 16.32 mg/ml is reported as 21.43 and 13.69 mg/ml respectively, an underestimate of about 12%. In addition, it must be borne in mind that even the figures quoted in the above table are likely to be slightly erroneous (say about + 5%) and therefore the true error due to use of bulk response factors could be in the range of about 18%, which is undoubtedly a high percentage. However, the relative percentages are less affected and tend to remain rather constant, except in the case of saturated hydrocarbons where the variation is about + 9 % for Standard A, which is relatively depleted in saturated hydrocarbons, and + 3 % for Standard B, which is rich in saturated hydrocarbons.
Table 14. Comparison of quantitative data obtained by using different response factors (R¢., Rca, RcL and R~s) and Standards A & B peak data Fraction
Area mVs
RcH
Amount 1 ul (%)
Sat. HC Aro. HC Polars Asphalt.
48332 20727 26177 10599
4386 6524 5981 10908
11.02 (56) 3.18 (16) 4.38 (22) 0.97 (5)
Total Sat. HC Aro. HC Polars Asphalt. Total
19.55 (99) 45537 10022 5100 312
4386 6524 5981 10908
10.38 (81) 1.54 (12) 0.85 (7) 0.03 (0) 12.80 (100)
R~a
Amount 2 /~1 (%)
Case I: Standard A data 3729 12.96 (60) 5941 3.49 (16) 6408 4.08 (19) 11726 0.90 (4)
R~L
Amount 3 /tl (%)
R~s
Amount 4 #1 (%)
3054 5341 6846 12566
15.83 (65) 3.88(16) 3.82 (16) 0.84 (3)
3409 2996* 5286 6989
14.18 (51) 6.92(25) 4.95 (18) 1.52 (6)
21.43 (99) Case 2: Standard B data 3729 12.21 (83) 5941 1.69(11) 6408 0.80 (5) 11726 0.02 (0)
14.72 (99)
24.37 (100) 3054 5341 6846 12566
14.91 (85) 1.88 ( l l ) 0.74 (4) 0.02 (0) 17.55 (100)
Note: Response factor types: R~H = heavy oil, R~a = bulk (average), RcL = light oil and R~s = synthetic standard. *Average of mono + diaromatic and poly-aromatic hydrocarbons.
27.57 (100) 3409 2996* 5286 6989
13.36 (75) 3.34 (19) 0.96 (5) 0.04 (0) 17.70 (100)
860
SUNIL BHARATI et al.
Oils with high proportions of polars are also likely to be adversely affected with the variation being about _+8%. Assuming that most oils would fall in the category of medium oils, the use of bulk response factors will most commonly not bring about any significant change in the quantitative data. On the other hand, the use of bulk response factors, would result in an underestimation of saturated hydrocarbons in heavyand hydrocarbon-depleted oils which could be crucial. Similarly, an overestimation of saturated hydrocarbons in hydrocarbon depleted light oils is also critical, though this case is less likely to occur. With regard to the use of bulk response factors in the analysis of solvent extracts from reservoir rocks, the degree of inaccuracy should be minimal as the majority of the extracts would in any case fall in the category of medium oils. This is, however, not equally applicable to extracts from source rocks, as source rocks are much more likely to be rich in high molecular weight non-hydrocarbon species, such extracts being comparable to low API gravity oils. In general, therefore, the use of bulk response factors for rock extracts could be substantially erroneous, since for rock extracts it is not only the relative percentage data that is important, but also the net yield data. Yields obtained by using response factors derived from synthetic standards (Rcs) are even more erroneous than those obtained by using bulk response factors. The net saturated hydrocarbon yield is grossly overestimated for heavy and medium oils (28 and 9% respectively), while slightly underestimated for light oil. Aromatic hydrocarbons are most adversely affected by using R~s. The net yield is almost doubled, irrespective of the type of oil, and this is the principle reason for highly erroneous relative percentages. Resins are overestimated for all types of oils, the medium and low API gravity samples being the most affected. Asphaltenes are also highly overestimated (up to 80% for light oils), despite employing a large molecule such as berberine sulphate as an asphaltene standard. Apparently, the response of berberine sulphate is closer to the natural aromatic hydrocarbon fraction from a light oil. All these factors contribute to the highly inaccurate relative percentages for a given extract or oil sample, regardless of its composition and API gravity, if synthetic standards response factors are used for calibration and quantification. SUMMARY AND CONCLUSIONS
Of the 31 oils world wide that were used in the study, only a few were found suitable for the purpose of preparing standards and calibrating the Iatroscan T L C - F I D instrument. For the remaining oils, the primary reason for not qualifying as Iatroscan Standard precursors was their inability to separate into 4 distinct and clean fractions, which were chromatographically pure from the T L C - F I D point of view.
This inability is believed to be primarily due to their inherent composition. However, a few oils covering a wide API gravity range were found to be suitable and were later used to calibrate Iatroscan and prepare standards. As expected, the most problematic fractions were the non-hydrocarbons, due to mainly two reasons. One reason was the limited amounts of these fractions obtained from the oils, and second being the difficulty in obtaining pure, uncontaminated fractions. This problem was eventually minimised through repeated separation and parallel analyses. A major finding of this study is that the response factors of the separated fractions from oil are not unique for all oils, but are instead greatly dependent on the composition of the fraction and the API gravity of the oil. It will therefore be highly erroneous to use a single response factor for, say saturated hydrocarbons, without taking into account the API gravity of the oil. For the sake of simplicity, a set of 3 response factors for each of the 4 fractions, have been derived which are suitable for heavy oils, medium oils and light oils, these being defined as Rcn, RcM and RcL •
Examples of two standards, A (rich in polars and asphaltenes) and B (lean in polars and asphaltenes nearly absent), have been prepared through this study for use in the petroleum industry. A mathematical model has been developed to derive response factors and their relationships to the given standard have been described. These can be readily employed by different Iatroscan users. The use of these standards and the proposed mathematical model will ensure realistic inter-laboratory data comparison and standardization of the Iatroscan technique in the petroleum industry. Based on the results obtained in the present study and bearing in mind the various limitations/problems encountered, we recommend the following: 1. Further work must be performed to better understand the dependability of response factors on API gravity and composition of the oil. More data points are required in this paper's Figs 16 and 17, to make the proposed model better and subsequently derive more reliable and correct response factors. This will require further MPLC separations of oils, GC of hydrocarbon fractions and Iatroscan analyses. 2. The concentration of the solution that will be used to spot samples on the chromarods must be 10-15mg oil/ml solvent, to ensure good chromatography and avoid overloading. In the case of whole rocks, SI data from Rock-Eval should be used to determine the rock weight, but if nothing is known then 3g should be dissolved in 5 ml solvent. 3. An amount of 2 p l solvent containing the sample should be spotted on the rod as a general practice, preferably using an auto-spotter, to avoid band-spreading and to ensure good
Calibration and standardization of Iatroscan
4.
5.
6.
7.
chromatography. Blowing the spotting point continuously using nitrogen has been found to be very effective in quickly removing the solvent, and should be routinely practised. As n-pentane precipitated asphaltene fraction is compositionally not at all comparable to the fraction hitherto referred to as asphaltene in the T L C - F I D technique, we propose the use of the terms Resin 1 for resins (synonymous to the resin or NSO fraction from MPLC) and Resin 2 for the fraction hitherto referred to as asphaltene in the T L C - F I D technique. Resin 2 will therefore essentially comprise of heavier, high molecular weight and large molecules of N S O compounds, relative to Resin 1. Adoption of the terms Resin 1 and Resin 2 will also minimize confusion when the quantitative data from M P L C is compared to quantitative data from Iatroscan. Synthetic compounds should not be used as standards for quantification purposes, as response factors derived from them result in highly erroneous data for real samples. Natural standards (such as the prepared Standards A and B of the present study) should be used for calibration and subsequently should be routinely analysed (say once a fortnight or after every 100 analyses) and the individual response factors updated accordingly, as the response/sensitivity of any F I D alters with time and usage. Bulk response factors must not be used as these are likely to give erroneous results. Different response factors for heavy oils, medium oils and light oils must be used instead, which can be easily calculated and applied by the users.
Acknowledgements--Many thanks to the following oil companies (arranged in alphabetical order) for oil samples: BEB Erdgas und Erd61 GmbH, BHP Petroleum Pty. Ltd, Chevron/Cabinda Gulf Oil Company Limited, Mobil Turkey, Norsk Hydro, OMV Aktiengesellschaft, Occidental Petroleum Corporation of Peru, SANTOS Ltd, Saga Petroleum a/s, Unocal Netherlands B.V., Geolab Nor; and Norsk Hydro, Saga Petroleum a/s, and Statoil for financial support. F. Behar and D. Karlsen are thanked for critical comments and improving the quality of the manuscript.
REFERENCES
Dietz W. A. (1967) Response factors for gas chromatographic analyses. J. Gas Chromatogr. 5, 68 71. Drushel H. V. (1983) Needs of the chromatographer-detectors. J. Chromatogr. Sci. 21, 375-384. Goutx M., Gerin C. and Bertrand J. C. (1990) An application of Iatroscan thin-layer chromatography with flame ionization detection--lipid classes of microorganisms as biomarkers in the marine environment. In Advances in Organic Geochemistry 1989 (Edited by Durand B. and Behar F.), pp. 1231-1237. Pergamon Press, Oxford. Karlsen D. and Larter S. (1989) A rapid correlation method for petroleum population mapping within individual petroleum reservoirs: applications to petroleum reservoir description. In Correlation in Hydrocarbon Exploration, OG 22/3-5---HH
861
pp. 77-85. Norwegian Petroleum Society, Graham & Trotman, London. Karlsen D. and Larter S. (1991) Analysis of petroleum fractions by TLC-FID--applieations to petroleum reservoir description. Org. Geochem. 17, 603-617. Norwegian Industry Guide to Organic Geochemical Analyses (1993) 3rd edition. Jointly published by Statoil, Norsk Hydro, Saga Petroleum, Geolab Nor, IKU and Norwegian Petroleum Directorate. Poirier M. A., Rahimi P. and Ahmed S. M. (1984) Quantitative analysis of coal derived liquids residues by TLC with flame ionisation detection. J. Chromatogr. Sci. 22, 116-119. Radke M., Willsch H. and Welte D. H. (1980) Preparative hydrocarbon group type determination by automated medium pressure liquid chromatography. Anal. Chem. 56, 2538-2546. Ray J. E., Oliver K. M. and Wainwright J. C. (1982) The application of the Iatroscan TLC technique to the analysis of fossil fuels. In Petroanalysis 81, IP Symposium, London, 361-388. Heyden and Son, London. Selucky M. L. (1983) Quantitative analysis of coal derived liquids by thin layer chromatography with flame ionisation detection. Anal. Chem. 55, 141-143. Yamamoto Y. (1988) Analysis of heavy oils by thin-layer chromatography with flame ionisation detection. Sekiyu Gakkaishi 31, 351-362.
APPENDIX
1
Whole rock (cuttings, core-chips, side wall cores) is crushed using either a mill or a pestle and mortar to fine particle size. If the sample is a shale, then the particle size should be 65 ,am or finer. If the sample is a loosely packed sandstone, then it is sufficient that all the individual sand grains are free and finer crushing does not improve the results. About 2 g of whole rock is accurately weighed in a 8 ml glass vial with screw cap (cap with a Teflon lining inside), and 2.0 ml of solvent (dichloromethane: methanol, 93 : 7 v/v) is added and the vial sealed immediately. The sample is allowed to be extracted for 48-72 h (preferably 72 h if the sample is a shale), with vigorous agitation 4-5 times a day. If the rock is very rich in EOM (the colour is dark brown), then an additional 1 ml solvent can be added after one day. After this stage, the procedure is the same as in the case of oils.
APPENDIX
2
List of Ions Specified During GC-MS of Saturated and Aromatic Hydrocarbons 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1I. 12. 13. 14.
m/z 77 83 91 97 99 106 134 142 163 183 177 191 198 217
15.
231
16
253
Compound --benzene ---cyclo-alkanes --Cl-alkylbenzenes --n-alkenes/methylcyclic-hexanes --n -alkanes --alkylbenzenes ~4-alkylbenzenes --methylnaphthalenes --terpanes --isoprenoids ~emethylated triterpanes --triterpanes ~ibenzothiophenes --steranes --triaromatic steranes --mono-aromatic steranes
SUNIL BHARAT1et al.
862
APPENDIX
3
List of Ions Specified During P Y - G C - M S of Resins and Asphaltenes m/z I. 2. 3. 4. 5.
77 91 97 99 106
Compound --benzene --Cl-benzene (toluene) --n-alkenes --n-alkanes --alkylbenzenes
6. 7. 8. 9. 10. I I. 12. 13. 14.
134 142 156 163 170 192 198 206 212
15.
231
16.
253
---C4-alkylbenzenes --methylnaphthalenes --C2-naphthalcnes --terpanes --C3-napht halenes --methylphenanthrenes ---dibenzothiophenes ---C2-phenanthrenes --C2-dibenzothiophenes --triaromatic steranes --mono-aromatic steranes
AdvancesinOrganicGeochemistry1993 Copyright© 1994ElsevierScienceLtd
Org. Geochern. Vol. 22, No. 3-5, pp. 835-862, 1994
Pergamon 0146-6380(94)00086-7
Printed in Great Britain.All rights reserved 0146-6380/94$7.00+ 0.00
Calibration and standardization of latroscan (TLC-FID) using standards derived from crude oils SUNIL BHARATI,GEIR ARNE ROSTUM and RITA LOBERG Geolab Nor, P.O. Box 5740 Fossegrenda, 7002 Trondheim, Norway Abstract--The TLC-FID technique, using the Iatroscan instrument, which constitutes separation and quantification of a complex mixture into various compound classes, has recently become a widely used and reliable method for characterisation of solvent extracts (particularly from reservoir rocks) and crude oils. However, there has been only limited reported research conducted aimed at calibration of the instrument and usage of standards which are relevant to the petroleum industry. This study is specifically aimed at identifying, separating, characterising, testing and developing standards, derived from natural crude oil(s), that are well suited and chromatographically pure for quantification purposes. A sample set comprising of over 30 crude oils from various basins of the world was used. The oils range widely in maturity, API gravity (8-46°), source rock type, degree of biodegradation, reservoir rock type and relative compositions. Whole oils, maltenes and asphaltenes were initially screened using Iatroscan. The process to identify suitable saturated hydrocarbons, aromatic hydrocarbons, polars and asphaltenes for the purpose of making the standard(s) included MPLC, gas chromatography, column chromatography and preparative thin-layer chromatography. Only about half the oils separate into 4 'clean' fractions (saturated and aromatic hydrocarbons, resins and asphaltenes) resulting in 4 well defined peaks. Analysis of the maltenes and asphaltenes shows that neither of the fractions is free of the other fraction. Pure fractions of saturated and aromatic hydrocarbons, resins and asphaltene were isolated using a combination of various analytical techniques and standards prepared. Compositional variations between the different fractions was established using GC, Py-GC and GC-MS. Several synthetic standards were prepared and tested for comparison and the implication of their use discussed. The Iatroscan TLC-FID instrument was calibrated using data from naturally derived fractions/standard mixtures and a mathematical model developed that could be used as the basis for calibration. Key words--Iatroscan TLC-FID, petroleum, calibration, response factors, analytical accuracy, model,
natural standards, synthetic standards, oil fractions
INTRODUCTION The T L C - F I D technique, using the Iatroscan instrument, is fast becoming a widely practised and reliable method for qualitative and quantitative characterization of solvent extracts (particularly from reservoir rocks) and crude oils. T L C - F I D generally constitutes separation of a complex mixture, such as EOM or crude oil into compound classes. Depending upon the chromatographic requirement and the sensitivity of the instrument, a natural organic mixture, such as a crude oil, is typically separated into 3, 4 or even 5 fractions, each fraction itself, however, being a complex mixture, though essentially all components of a fraction are of similar polarity and belong to the same compound class. Recently, there have been some publications highlighting the practicality and application of Iatroscan in the petroleum industry (Goutx et al., 1990; Karlsen and Larter, 1989, 1991; Poirier et al,, 1984; Ray et al., 1982; Selucky, 1983; Yamamoto, 1988). However, these studies have limited focus towards calibration of the instrument and standardization of elutionary procedures. Most
previous work involving the use of the T L C - F I D technique using Iatroscan have relied upon synthetic compounds as standards to accomplish quantification. There are even lesser reported studies specifically aimed at developing natural standards (standards derived from natural crude oils or extracts) which are well suited and chromatographically pure during TLC for the purpose of calibrating the Iatroscan T L C - F I D instrument. Poirier et al. (1984), for example, have used natural extract fractions to calculate the relative response and Parish and Ackman (1985) have addressed the problem of calibration. In addition, different laboratories have different preferences with regards to the choice of synthetic compounds that they employ as standards. All these factors result in poor and inaccurate inter-laboratory comparison of data. The present study is specifically aimed at identifying, separating, characterising, testing and developing standards, derived from natural crude oil(s), that are well suited, easily accessible and chromatographically pure, that can be applied to Iatroscan and which are relevant to the petroleum industry. The term 'natural 835
1-04 24)5 3-06 4-27 5-07 64)8 7-28 8-09 9-30 10-10 I 1 11 12-12 13-13 14-26 154)3
16-31
17-14 18-15 19-25 20-16 2 I- 17 22-29 23-18 24-19 25-20 26-21 27-22 28-23 29-24 313-01 31-02
I 2 3 4 5 6 7 8 9 10 I1 12 13 14 15
16
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Tsagaanels Jibaro/Maranon San Jacinto/Maranon Monterey/Santa Maria Helm/Broad Fourteens Shiviyacu/Maranon Malongo West/Congo Helder/Broad Fourteens Malongo South/Congo San Jacinto/Maranon Matzen/Vienna Ruhlemoor-Valendis Troll/Horda Platform Troll/Horda Platform Haltenbanken Malongo West/Congo Matzen/Vicnna Dorissa/Maranon Selmo-Batman Central Graben Shiviyacu/Maranon Malongo North/Congo Snorre/N. Viking Graben Eich/K6nigsgarten Challis/N. Browse Jabiru/N. Browse Aderklaa/Vienna Central Graben Midgard/Sklinnabanken Tirrawarra/Cooper Patroclus/Enomanga
Field/Basin U. Cretaceous/Sandstone U. Cretaceous/Sandstone Miocene/Shale L. Cretaceous/Sandstone U. Cretaceous/Sandstone L. Cretaceous/Sandstone L. Cretaceous/Sandstone L. Cretaceous/Sandstone U. Cretaceous/Sandstone Tertiary/Sandstone L. Cretaceous/Sandstone M-U. Jurassic/Sandstone M-U. Jurassic/Sandstone L. Jurassic/Sandstone L. Cretaceous/Sandstone Triassic/Dolomite U. Cretaceous/Sandstone U. Cretaceous/Carbonate U. Jurassic/Sandstone U. Cretaceous/Sandstone L. Cretaceous/Sandstone L. Jurassic/Sandstone L. Cretaceous/Sandstone Triassic/Sandstone M. Jurassic/Sandstone Tertiary/Sandstone U. Jurassic/Silty Sandstone M. Jurassic/Argi. Sandstone Permian/Sandstone Jurassic/Sandstone
Reservoir Age/Lithology
Table I. Details of oil samples used in the study (listed according to increasing A P I Gravity)
Mongolia, Asia Peru, S. America Peru, S. America U.S.A., N. America Netherlands, Europe Peru, S. America Angola, W. Africa Netherlands, Europe Angola, W. Africa Peru, S. America Austria, Europe Germany, Europe Norway, Europe Norway, Europe Norway, Europe Angola, W. Africa Austria, Europe Peru, S. America Turkey, Europe Norway, Europe Peru, S. America Angola, W. Africa Norway, Europe Germany, Europe Ausiralia At.strlaia Austria, Europe Norway, Europe Norway, Europe Australia Australia
Location
*Actual API Gravity data for these oils not available. Data quoted are relative visual estimates. Note: The serial numbers in column I are not the same as oil numbers referred to in the text and tables.
Sample Code
S. No.
L. Cretaceous/Lacustrine U. Jurassic/Marine L. Oligoccne/Marine U. Jurassic/Marine U. Jurassic/Marine U. Jurassic/Marine U. Jurassic/Marine U. Jurassic/Marine Permian/Fluvial Deltaic Permian/Fluvial Deltaic
U. Cretaceous/Marine U. Jurassic?/Hypersaline?
U. Jurassic/Marine L. Oligocene/Marine U. Jurassic/Marine U. Jurassic/Marine L. Jurassic/Marine L. Cretaceous/Lacustrine U. Jurassic/Marine
L. Cretaceous/Lacustrine L. Jurassic/Marine L. Cretaceous/Lacustrine
Miocene/Marine L. Jurassic/Marine
Source Age/Type
ca 14" 17.4 20.6' 20.8 22.1 23.4 23.7 24 24.5 26.8' 27.5' 28.4 29.9 30.2 31./6 ca 32 * 33.5 34 34.2 37.1 38.6 39.5 42.3 43.2 44.2 44.6 45 46
11.4
ca8 * 10.7
API Gravity
Z
Calibration and standardization of Iatroscan standards' includes the four principle fractions of crude oil/solvent extract viz. saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes. For this purpose, a sample set comprising 31 crude oils from various basins/fields (onshore and offshore) of the world (North America, South America, Europe, West Africa, Asia and Australia) was used (Table l, oils listed according to increasing API gravity). The oils range widely in (1) maturity (low maturity to high maturity, but not condensate stage), (2) API gravity (8°-46°), (3) source rock type (marine clastic, marine carbonate, lacustrine, saline), (4) degree of biodegradation (none to strong), (5) reservoir rock type (sandstone, siltstone and carbonate) and (6) relative compositions. The final objective is to successfully isolate one fraction each of saturated hydrocarbons, aromatic hydrocarbons, NSO compounds and asphaltenes, pure from the T L C - F I D point of view, and gravimetrically prepare standards for Iatroscan calibration. In addition, an attempt has been made to refine the existing procedure for analysis, which includes a mathematical model to determine effective response factors and their relationships, which can be easily and readily employed by other users. ANALYTICAL PROCEDURES
Whole oil samples and individual fractions were analysed by Iatroscan at several stages during the course of the study. The following procedure for Iatroscan analysis (using Iatroscan MK-5) was found to be most suitable for analyses of a variety of samples and therefore recommended for general use (see also Karlsen and Larter, 1989). In the case of oils, 4-5 mg of whole oil is accurately weighed into a 2 ml GC glass vial, 0.5 ml of solvent added and the vial sealed. After being dissolved in the solvent, oils can be analysed immediately. Prior to sample spotting, the chromarods (quartz SIII, silica gel powder coated) are first activated by passing through the FID flame (in the normal analysis mode--30 s/ scan). 2 pl sample is then spotted (using a 2 #1 syringe) using preferably an auto-spotter (SES 3202/IS-01) and continuously blowing the spot with nitrogen or air to accomplish immediate drying and prevent bandspreading. This results in sharper and narrower peaks. The development tanks containing the solvents are lined with chromatographic paper on the inside back and side walls, from top to bottom. The following elution procedure is used: 35 min in n-hexane (to separate saturated hydrocarbons), air dry for 2 min, 14 min in toluene (to separate aromatic hydrocarbons), air dry for 2 min, 4 min in dichlorometane: methanol, 93 : 7 v/v (to separate resins) and finally 90 s at 60°C. Asphaltenes are retained at the spotting point. If it is intended to separate mono- and diaromatic hydrocarbons from poly-aromatic hydrocarbons (Karlsen and Larter, 1991) then insert the step of elution using cyclo-hexane for 14 min before eluting with toluene for 8 rain. The samples are analysed and the data
837
collected and processed either using an integrator or, as in our case, using a data acquisition system such as Multichrom v2. Raw data is automatically converted into mg/g rock units and relative percentages (see later). Although no whole rocks were analysed in the present study, the additional procedure related to their analyses is included in Appendix 1 for the benefit of the readers. The oils were deasphaltened using n-pentane, except in the case of asphaltene solubility experiments. Four samples were selected for asphaltene solubility tests (oils 5, 10, 16 and 19, which cover a wide range of API gravity) and each of these oils were subjected to asphaltene precipitation using six different solvents viz. n-pentane (which is normally used), n-hexane, n-heptane, n-octane, n-nonane and n-decane. The purpose was to monitor the dependence, if any, of asphaltene composition on the solvent chain length, which also implies increasing boiling point of the solvent. In addition, our intention was to investigate and review the compositions of asphaltenes resulting from precipitation using solvents other than n-pentane, and to establish whether an asphaltene fraction can be obtained through conventional precipitation method which is the same or close to the asphaltene fraction defined by TLC-FID. Such a fraction would be expected to result in only one peak on analysis using the Iatroscan, following the normal elution procedure. Maltenes were separated into fractions (saturated and aromatic hydrocarbons and polars) using MPLC as described by Radke et al. (1980) (each of the oils was analysed with one parallel to increase the amount of resulting fractions). However, one significant deviation from the routine method was during collection of the fractions. As the purpose in the present study was to obtain pure fractions using MPLC and not quantification, the initial and final cuts (about 10% each) of each of the fractions (saturated and aromatic hydrocarbons and resins) were discarded (which are normally collected) by closely monitoring the RI detector in the case of saturated hydrocarbons and u.v. detector in the case of aromatic hydrocarbons. This was done to ensure that minimal amounts of unwanted components are present in a fraction. Selected saturated and aromatic hydrocarbon fractions were characterised by gas chromatography, while selected polar and asphaltene fractions were characterised by pyrolysis-gas chromatography. The purity of the two hydrocarbon fractions was tested using gas chromatography-mass spectrometry, while the composition of the selected resin and asphaltene fractions was established using pyrolysis-gas chromatographymass spectrometry. The procedures followed were according to the Norwegian Industry Guide (1993). However, the selection of ions for GC-MS was made keeping in mind the primary objective of major contaminant identification. As the analysis was carried out in high resolution mode, the number of ions was restricted to 10. However, in separate runs, a few ions
838
SUN1L BHARATI et al.
characteristic for biomarkers were also detected. The ions specified during G C - M S are listed in Appendix 2. Similarly, the ions for P y 4 3 C - M S were selected in such a way so as to indicate the presence of saturated hydrocarbons, aromatic hydrocarbons (mono-, diand poly-aromatic) and the sulphur bearing thiophenes. Additionally, the selected masses would also indicate the type of hydrocarbons that have the maximum tendency to be retained in the polar fraction. No significance was placed on masses specific to commonly known biomarkers as these are not expected to occur in these fractions, although m/z 163, 231 and 253 were included to get a general picture of their presence. The list of the masses specified during Py~GC-MS is presented in Appendix 3. In addition, whole oil samples and selected resin fractions were analysed using preparative thin-layer chromatography (PTLC), the main objective of this being to purify the resin fractions by separating out aromatic hydrocarbons from the resins. The oils and resin fractions from MPLC were first dissolved in dichloromethane and spotted along a straight line (as a band) on a ready-made TLC plate (Merck DC-A1 foil coated with silica gel 60, gel thickness 0.2 mm,
silica with F254 fluorescence activator). The elution procedure employed was the same as that of Iatroscan. The zone separation was checked using a u.v. detector at 254 nm wavelength (cf. Plate 1) and portions with different fractions scrapped using a spatula. The 4 silica gel fractions were then extracted using D C M : M e O H (93:7 v/v), weighed and analysed by Iatroscan. RESULTS AND DISCUSSION
Composition of oils, asphaltenes and maltenes Preliminary screening of all oils using Iatroscan (Table 2) shows that the composition of the oils is highly variable, as is their total hydrocarbon (saturated and aromatic) content. The relative percentage of saturated hydrocarbons varies from about 3 to 82%, aromatic hydrocarbons vary from about 13 to 54%, resins vary from about 2 to 46% and asphaltenes vary from about 0.1 to 26% (Table 2). Nevertheless, hydrocarbons are dominant in all the oils, varying from about 76 to 98%, except for one oil which has a total hydrocarbon content of only 42% relative to resins and asphaltenes. The quality of
10em
Plate. 1. On left are the results of TLC development of various fractions from MPLC (saturated hydrocarbons--SAT, aromatic hydrocarbons--ARO, resins--NSO and asphaltene--ASP and whole oil--EOM). On right is the TLC development of the MPLC resin fraction, which splits into 2 zones--resins and aromatics. Zone 1 marks the line of sample spotting, where the asphaltenes are also retained, Zone 2 is resins, Zone 3 is aromatic hydrocarbons and Zone 4 is saturated hydrocarbons which cannot be detected by fluorescence.
839
Calibration and standardization of Iatroscan
Table 3. Relative percentages of separated fractions by latroscan from asphaltenes
Table 2. Relative percentages of separated fractions by latrnscan from whole oils
Oil No.
Sat HC %
Aro HC %
Resins %
Asph %
Total HC %
Total Polars %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
72.8 82.2 77.3 62.3 19.7 23.7 34.1 34.2 44.6 46.4 49.8 33.4 53.0 64.3 58.3 56.2 56.5 62.5 70.0 61.3 78.5 78.3 59.3 76.3 47.5 57.0 2.9 56.8 42.6 66.4 50.1
25.3 13.9 21.1 19.3 39.7 38.8 54.2 42.8 45.2 40.5 42.3 44.3 42.2 31.1 34.8 34.2 37.4 31.1 25.7 35.4 20.0 19.3 35.7 22.2 44.2 37.3 39.3 19.4 33.5 22.6 33.2
1.6 1.6 1.3 11.8 15.9 10.9 8.0 8.7 6.5 6.7 7.2 17.6 3.5 3.7 5.4 6.8 4.7 5.0 2.3 2.9 1.4 2.2 4.5 1.2 6.1 5.3 46.5 12.7 18.3 10.1 13.9
0.3 2.3 0.3 6.6 24.7 26.6 3.7 14.3 3.7 6.4 0.7 4.7 1.3 0.9 1.5 2.8 1.4 1.4 2.0 0.4 0.1 0.2 0.5 0.3 2.2 0.4 11.3 11.1 5.6 0.9 2.8
98,1 96.1 98.4 81.6 59.4 62,5 88,3 77,0 89.8 86.9 92.1 77.7 95.2 95.4 93A 90.4 93.9 93.6 95.7 96.7 98.5 92.6 95.0 98.5 91.7 94.3 42.2 76.2 76.1 89.0 83.3
1.9 3.9 1.6 18.4 40.6 37.5 11.7 23.0 10.2 13.1 7.9 22.3 4.8 4.6 6.9 9.6 6.1 6.4 4.3 3.3 1.5 2.4 5.0 1.5 8.3 5.7 57.8 23.8 23.9 11.0 16.7
Sat HC %
Aro HC %
Resins %
Asph %
Total HC %
Total Polars %
40.0 -15.4 11.5 -5. I --
--------
50.0 64.7 28.8 17.4 71.4 4.9 32.4
10.0 35.3 55.8 71.1 28.6 90.0 67.6
40.0 -15.4 11.5 -5.1 --
60.0 I00.0 84.6 88.5 100.0 94.9 100.0 100.0 100.0 100.0 100.0 74.9 100.0 100.0 100.0 93.6 100.0 64.5 100.0 30.3 67.6 46.7 83.0 100.0 81.5 100.0 100.0
Oil No. 1 2 3 4 5 6 7 8
--
9 10
--
---
11
--
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
14.8 ---6.4 -25.9 -53.3 32.4 53.7 17.0 -8.9 ---
individual
fractions
distinct groups (35 and chromatography (saturated
hydrocarbons,
resins
asphaltenes)
and
aromatic giving
hydrocarbons,
4 prominent
37.8 32.8 49.5 34.0 49.2 45.0 37.4 19.9 42.7 41.2 27.3 26.6 56.8 35.2 59.1 -30.6 45.5 --
in these
---25.1 ---6.4 -35.5 -69.7 32.4 53.7 17.0 -18.5 ---
oils to
separate
and 4 min for non-
(10 cm).
Deasphalting
of oils shows
that except for a few
oils, most oils contain asphaltenes
between
by weight.
or a continuum
hydrocarbon between
fractions
[Fig.
l(b)]
the two non-hydrocarbons
[ F i g . l ( c ) ] f r a c t i o n s is p r e s e n t . T h i s is a t t r i b u t a b l e the composition
stantial
However,
portion
in the range 1-4%
in case of several oils, a sub-
of the asphaltene
within the filter and was impossible
to
of these oils and the inability of the
by repeated
washing
remained
I
cm a
0
10
cm
b
0
10
even
using either dicholoromethane
!
!
locked
to remove
$
10
into
and the restricted length available for
[Fig. l(a)], except in a few cases where a continuum the two
--
62.2 67.2 50.5 40.9 50.8 55.0 62.6 73.7 57.3 23.3 72.7 3.7 10.8 11.1 23.9 100.0 50.9 54.5 100.0
within the given time for separation
separation/elution
peaks
100.0
14 m i n f o r h y d r o c a r b o n s
hydrocarbons),
is g e n e r a l l y g o o d w i t h t h e 4 f r a c t i o n s
--
---10.3 -----9.6 -16.4 ----9.6 ---
cm
0
O
Fig. 1. T L C - F I D c h r o m a t o g r a m s o f w h o l e oils. (a) A typical oil s a m p l e w i t h 4 p r o m i n e n t p e a k s r e p r e s e n t i n g 4 m a j o r c o m p o n e n t s viz. s a t u r a t e d h y d r o c a r b o n s (SAT), a r o m a t i c h y d r o c a r b o n s ( A R O ) , resins ( P O L ) a n d a s p h a l t e n e s ( A S P H ) . (b) A n oil s a m p l e w i t h a c o n t i n u u m b e t w e e n the 2 h y d r o c a r b o n f r a c t i o n s , S A T a n d A R O . (c) A n oil s a m p l e w i t h a c o n t i n u u m b e t w e e n the 2 n o n - h y d r o c a r b o n f r a c t i o n s , POL and ASPH.
840
SUNIL BHARATI
I
10
I
I
I
on a
I , , [
I
010
I
cm
0
et al.
i
10
i
i
era
b
i
0
i , 10
c
i
, em
"f 0
d
Fig. 2. TLC-FID chromatograms of asphaltenes. (a) An asphaltene fraction containing significant amounts of hydrocarbons (SAT and ARO). (b) A typical asphaltene fraction consisting mainly of resins (POL) and asphaltenes (ASPH). (c) Chromatogram of an asphaltene with least amount of other components. (d) A typical asphaltene from low API gravity oils, exhibiting a strong continuum between the 2 non-hydrocarbon components, POL and ASPH. or dichloromethane: methanol (93 : 7 v/v). This loss is even more significant for oils rich in asphaltenes (more than 10% by weight) such as oils 4, 5, 6, 8, 10 and 27. The analysis of asphaltenes using Iatroscan (Table 3) shows that none of the pentane precipitated asphaltene fractions result in pure "asphaltene" fractions from the T L C - F I D point of view, as also documented earlier (Karlsen and Larter, 1991). Several of the asphaltene fractions analysed (such as from oils 1, 2, 3, 20, 21, 22, 23 and 24) give a misleading picture (Table 3) due to very low absolute concentrations of all the components and therefore their data are considered to be insignificant/unreliable. However, in several others, the asphaltene precipitated by n-pentane from oils such as 4, 6, 12, 16, 18 and 25, continue to retain substantial amounts of hydrocarbons, and their chromatograms show up to 4 distinct peaks [e.g. Fig. 2(a)]. It seems though that saturated hydrocarbons have a greater tendency of being retained in the pentane precipitated asphaltenes, relative to aromatic hydrocarbons (Table 3). In most of the samples, however, the pentane precipitated asphaltene splits into mainly resins and asphaltenes, as shown in Fig. 2(b). In none of the samples did the pentane precipitated asphaltene fraction result in only one (asphaltene) peak [example with the most prominent asphaltene peak relative to resins is shown in Fig. 2(c)]. Iatroscan separation of asphaltenes from nearly all of the low API gravity oils, which are also generally the richest in asphaltenes (Table 3), results in a continuum between the resins and asphaltenes [e.g. Fig. 2(d)]. This is indicative of either compositional similarity between the resins and asphaltenes of these oils or inability of the components to separate, as no overloading of samples occurred. Had the pentane precipitated asphaltene fraction resulted in only one peak on analysis using Iatroscan,
then it would mean that the fraction resulting from conventional pentane precipitation, which we commonly refer to as asphaltene, is compositionally the same as the asphaitene defined by the Iatroscan technique. Apparently, asphaltenes precipitated by n-pentane are compositionally different to the fraction defined as asphaltene by Iatroscan. The pentane precipitated asphaltene nearly always consists of two principle fractions--resins and asphaltenes (Table 3). It must also be noted that accurate integration of the resin and asphaltene peaks from Iatroscan is not possible in the case of some samples due to the Table 4. Relative percentages of separated fractions by latroscan from maltenes Sat HC
Aro HC
Resins
Asph
Oil No.
Total HC
Total Polars
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
77.6 87.6 78.7 54.7 24.7 28.9 30.9 40.0 42.5 49.6 54.2 34.0 58.1 69.9 61.2 58.8 58.2 62.7 74.4 73.8 82.2 83.5 64.5 71.0 52.9 82.6 --
20.4 10.1 19.9 9.3 48.8 57.9 54.8 49.9 45.3 41.5 35.1 43.0 32.6 22.9 31.5 28.4 33.5 28.4 20.9 20.6 15.5 12.0 30.0 16.5 37.0 13.9 37.9
!.7 1.8 1.3 16.4 16.4 12.9 13.4 9.5 11.6 8.4 10.4 19.5 8.8 6.7 6.7 10.4 7.7 8.3 4.2 5.1 2.0 3.4 4.4 3.5 7.8 2.9 59.6
0.3 0.5 0.1 19.6 19.6 0.3 0.9 0.6 0.6 0.5 0.3 3.5 0.5 0.5 0.6 2.4 0.6 0.6 0.5 0.5 0.3 1.1 1.1 9.0 2.3 0.6 2.5
98.0 97.7 98.6 64.0 64.0 86.8 85.7 89.9 87.8 91.1 89.3 77.0 90.7 92.8 92.7 87.2 91.7 91.1 95.3 94.4 97.7 95.5 94.5 78.5 89.9 96.5 37.9
2.0 2.3 1.4 36.0 36.0 13.2 14.3 10.1 12.2 8.9 10.7 23.0 9.3 7.2 7.3 12.8 8.3 8.9 4.7 5.6 2.3 4.5 5.5 12.5 10.1 3.5 62.1
Calibration and standardization of latroscan
841
I i
I0
cm
0
10
cm
0
10
cm
•
L
0
10
em
0
a b c d Fig. 3. TLC-FID chromatograms of maltenes. (a) A typical maltene fraction, rich in hydrocarbon components SAT and ARO. (b) A maltene fraction with a dominant saturated hydrocarbon peak (SAT). (c) A maltene fraction with a dominant aromatic hydrocarbon peak (ARO). (d) A maltene fraction with significant amounts of non-hydrocarbons present (POL and ASPH).
continuum, and the data therefore can be quite erroneous. Based on the results and the above considerations, asphaltenes from oils 4, 5, 7, 8, 10, 16 and 19 are thought to have the best potential for use as standards. Iatroscan analysis of maltenes (Table 4) show that hydrocarbons are the most abundant species, typically between 90 and 98% and generally over 77% of the total maltene fraction [Fig. 3(a)], except for one sample (sample 27). The composition of the hydrocarbons however, is highly variable with saturated hydrocarbons occurring in the range of about 25-81% [Fig. 3(b) shows a sample rich in saturated hydrocarbons] with one exception (no saturated hydrocarbons detected in sample 27) and aromatic hydrocarbons ranging about 10-57% of the total content [Fig. 3(c) shows a sample rich in aromatic hydrocarbons]. Resins or polar compounds are in most cases minor components (generally 1-8%) but occur in larger amounts in a few samples (10-16%) [Fig. 3(d) shows a sample rich in resins]. One exception is sample 27 which contains about 59% resins. Asphaltenes, which ideally should not be detected in maltenes, occur in all maltenes although their percentage is small--ranging about 0.1-3%, with 3 exceptions (oils 4, 5 and 24).
Asphaltene solubility test Table 5 shows the results of the asphaltene solubility experiment and the amount of asphaltene yields by using 6 different solvents. Apparently, there is little change in the relative yields of asphaltenes from each of the 4 oils when different solvents are used; this is also true for maltenes [Fig. 4(a-d)]. Data for n-decane is incomplete/erroneous due to difficulties encountered during solvent evaporation, these being attributable to very high boiling point of n-decane
(174°C) and consequently limited capacity of the rota-vapour to remove the solvent. From the qualitative point of view, both the asphaltenes and maltenes of each oil are compositionally very similar, resulting in nearly identical T L C FID chromatograms for a given oil, for all solvents
Table 5. Asphaltene solubility experiment: gravimetric weights o f resulting maltenes and asphaltenes using different solvents Sample
Oil (mg)
Asp (rag)
Malt (mg)
Asp (%)
Malt (%)
Loss (%)
Oil Oil Oil Oil
5 10 16 19
47.3 45.8 45.5 47.7
8.1 4.1 1.0 0.9
Pentane 33.8 36.8 37.4 34.0
17.12 8.95 2.2 1.89
71.46 80.35 82.20 71.28
11.42 10.70 15.60 26.83
Oil Oil Oil Oil
5 10 16 19
48.1 41.9 51.5 49.5
9.5 3.3 0.3 0.8
Hexane 34.1 19.75 35.9 7.88 43.5 0.58 37.5 1.62
70.89 85.68 84.47 75.76
9.36 6.44 14.95 22.62
Oil Oil Oil Oil
5 10 16 19
55.8 46.6 59.0 49.6
11.4 3.7 0.5 0.7
Heptane 40.2 39.1 50.0 13.9"
20.43 7.94 0.85 1.41
72.04 83.91 84.75 --
7.53 8.15 14.40 --
Oil Oil Oil Oil
5 10 16 19
41.7 61.0 51.2 48.7
8.1 4.5 0.3 1.1
Octane 21.5 19.42 50.4 7.38 41.0 0.59 14.9" 2.26
51.56 82.62 80.08 --
29.02 10.0 19.33 --
Oil Oil Oil Oil
5 10 16 19
61.2 45.2 53.5 48.6
12.5 29.8 3.6 0.5
Nonane 37.5 20.42 35.8¢ -42.1 6.73 32.9 1.03
61.27 79.20 78.69 67.70
18.31 -14.58 31.27
Oil Oil Oil Oil
5 10 16 19
51.4 46.5 47.3 48.8
t08.3t 36.6t 2.2 6.3
Oecane 143.5t -267.5t -95.3t 4.65 98.5t 12.91
-----
-----
*Loss during separating asphaltene from oil. #Impossible to evaporate the solvent completely.
842
SUNIL BHARATI el
(a)
al.
(c)
SAMPLE 5
SAMPLE
16
100 -
100 F
, . ~
80 ~ . . . . .
~
80
~x
• ....
60 -
60 Asphaltene 40 I -
• Maltene
40 -
/ 20 ~-~.-..-.----- • - ' - - - " ' - - - • " " " - - - - - - • 0
>-
I 6
5
v "O
J 7
(b) 100 --
I 8
20 -
"• I 9
I
10
0 --------i
7
6
8
9
10
9
10
(d) SAMPLE
10
80 ,w-
80
60 --
60
40 --
40
20 --
20
0
I 6
5
7
8
SAMPLE
100
9
10
19
-
5
6
7
8
C a r b o n a t o m s in s o l v e n t ( n - a l k a n e )
Fig. 4. Relative percentages of asphaltene and maltene yields for 4 different oils (samples 5, 10, 16 and 19) on deasphaltening using n-pentane, n-hexane, n-heptane, n-octane, n-nonane and n-decane.
material (Table 6), even after the solvent removal procedure [Fig. 5(b)]. This effect is also noticed in the case of asphaltenes, as asphaltenes resulting from n-pentane up to n-nonane precipitation are nearly free of any saturated hydrocarbons [Fig. 5(c)] while
except n-decane. Maltenes resulting from n-pentane up to n-nonane contain mainly hydrocarbons, but also noticeable amounts of resins and asphaltenes [Fig. 5(a)]. In the case of n-decane, significant amount of the solvent is apparently retained with the sample
10
cm a
0
10
cm b
0
10
cm c
0
10
cm
0
d
Fig. 5. latroscan analysis of maltene and asphaltene fractions resulting from deasphaltening of oils using various solvents. Typical chromatograms showing (a) representative maltene of n-pentane to n-nonane precipitation, (b) maltene of n-decane precipitation, (c) representative asphaltene of n-pentane to n-nonane precipitation and (d) asphaltene of n-decane precipitation.
843
Calibration and standardization of latroscan Table 6, Asphaltene solubility experiment: results of T L C - F I D analyses of maltene and asphaltene fractions---relative percentages of separated fractions Sample
Maltenes Aro% Pol%
Sat%
Asp%
Oil Oil Oil Oil
5 10 16 19
24.8 46.7 58.8 71.6
62.7 49.9 33.0 24.5
I 1.9 3.3 7.5 3.8
Pentane 0.6 0.1 0.7 0.1
Oil Oil Oil Oil
5 10 16 19
24.8 44.9 55.9 69.0
63.4 49.5 35.4 27.1
11.3 5.2 7.7 3.7
Hexane 0.6 0.4 1.0 0.2
Oil Oil Oil Oil
5 10 16 19
24.7 47.2 55.2 74.8
65.8 48.2 36.3 20.2
9.2 4.4 7.6 4.8
Heptane 0.4 0.2 0.9 0.2
14.6 4.8 8.1 6.5
Octane 0.3 0.2 0.8 0.2
14.6 5.3 9.5 4.2
Nonane 0.4 0.4 1.0 0.3
3.8 2.1 2.6 1.1
Oecane 0.3 0.2 0.5 0.1
Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil
5 10 16 19
27.7 41.6 56.0 73.0
5 10 16 19
57.4 53.4 35.1 20.3
21.7 45.1 58.6 72.5
5 10 16 19
63.3 49.1 30.9 23.0
68.2 71.6 87.5 86.1
27.6 26.1 9.4 12.7
the asphaltene fraction resulting from n-decane has a conspicuous saturated hydrocarbon peak [Fig. 5(d)]. Apparently, no significant compositional differences are obtained by using solvents other than that normally used (n-pentane), except for n-decane. In any case, this exercise was not helpful in obtaining a "purer" asphaltene fraction from the T L C - F I D point of view. Conventional M P L C separation
Based on the results obtained from whole oil and maltene analyses, 7 samples (oils 1, 5, 7, 16, 19,
Sat%
m
Asphaltenes Aro% Pol%
m
m
m
m
_
_
m
m
m
4.0 ---
-
10.5 ---
-
m
_
_
m
_
_
m
51.5 70.8 78.3 73.9
-8.7 9.4 --
Asp%
26.6 45.6 52.9 50.6
73.4 54.4 47. I 49.4
27.4 37.1 68.6 54. I
75.8 62.9 31.4 45.9
24.2 32.7 78.4 66.1
75.8 67.3 21.6 33.9
28.5 40.2 82.5 68.1
57.0 59.8 17.5 31.9
32.8 45.1 75.0 55.8
67.2 54.9 25.0 44.2
15.7 10.5 11.2 16.0
32.8 10.0 11.1 10.1
22 and 30) were selected for fresh deasphalting and separation of maltenes into fractions using MPLC (Radke et al., 1980). The selection was made on the basis that at least about two "clean and pure" (from T L C - F I D point of view) fractions each of saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes would be obtained from these oils. The compositions of the oils and the relative percentages of the various fractions are shown in Table 7, although one must bear in mind that the data of Table 7 are not completely representative of the true composition, due to discarding of the initial and final
Table 7. Composition and relative percentages of chromatographic fractions from MPLC Sample
Oil (mg)
Sat (mg)
Aro (rag)
Pol (mg)
Asp (mg)
Total (mg)
Loss* (mg)
Loss* (%)
Sat (%)
Aro (%)
Pol (%)
Asp (%)
Oil 1 la
94.6 101.8
34.9 39.9
18.0 13.8
0.9 1.1
13.0 7.0
66.8 61.8
27.8 40.0
29.4 39.3
52.2 64.5
27.0 22.4
1.3 1.8
19.5 11.3
Oil 5 5a
91.8 104.9
15.6 21.5
19.8 23.4
13.5 15.5
22.0 25.1
70.9 85.5
20.9 19.4
22.8 18.5
22.1 25.1
27.9 27.4
19.0 18.1
31.0 29.4
Oil 7 7a
107.2 109,3
38.1 32.4
23.9 27.9
15.0 14.6
9.8 11.4
86.8 86.3
20.4 23.0
19.0 21.0
43.9 37.6
27.5 32.3
17.3 16.9
11.3 13.2
Oil 16 16a
93.4 105.3
41.9 47.3
17.9 19.7
9.8 14.1
1.5 2.1
71.1 83.2
22.3 22.1
23.9 21.0
58.9 56.8
25.2 23.8
13.8 16.9
2.1 2.5
Oil 19 19a
95.7 97.2
52.4 54.5
13.4 13.4
3.6 3.0
1.8 3.6
71.2 74.5
24.5 22.7
25.6 23.4
73.6 73.2
18.8 18.0
5.1 4.0
2.5 4.8
Oil 22 22a
96.3 98.1
49.8 49.8
7.8 7.8
1.8 2.1
1.4 1.4
60.8 61.1
35.5 37.0
36.9 37.7
81.9 81.5
12.8 12.8
3.0 3.4
2.3 2.3
Oi130 30a
111.5 126.7
52.5 59.4
18.9 26.1
15.6 19.1
3.1 3.7
90.1 108.2
21.4 18.45
19.2 14.6
58.3 54.9
21.0 24.1
17.3 17.6
3.4 3.4
*The loss indicated includes the inherent loss related to the MPLC technique (estimated 5-10%) +loss due to discardation of 10% each of the initial and final cuts of the saturated and aromatic hydrocarbon and resin fractions (cf. analytical procedures).
SUNIL BHARATI et al.
844
Table 8. Resultsof TLC-FID analysis of chromatographicfractions from MPLC: relativepercentagesof separated fractions Sat
Aro
Pol
Asp
(%)
(%)
(%)
(%)
99.0 94.0 67.4 99.7 100.0 100.0 81.8
0.5 5.3 32.3 ---15,9
0.5 0.7 0.3 0.3 --2.3
--------
Oil 1 Oil 5 Oil 7
7.1 ---
91.8 98.0 98.3
1.1 1.9 1.7
-0.1 --
Oil Oil Oil Oil
16 19 22 30
---1.1
97,6 99.5 99,2 96.9
2,4 0.5 0.8 2.0
-----
Oil Oil Oil Oil
1 5 7 16
-----
63.9 68.2 70.3 61.7
36,1 31.8 29.7 38.3
-----
O i l 19 Oil 22 Oil 30
----
64.3 48.4 66.7
35.7 51.6 33,3
----
Oil I Oil 5
40.1 --
52.9 --
6.7 --
0.3 100.0
Oil Oil Oil Oil Oil
15.7 --76.2 --
19.6 --16.0 31.6
51.8 54.2 56.2 7,5 --
12.9 45.8 43.8 0.3 68.4
MPLC fraction
Sample
Saturated hydrocarbons
Oil Oil Oil Oil Oil Oil Oil
Aromatic hydrocarbons
Resins
Asphaltenes
1 5 7 16 19 22 30
7 16 19 22 30
clean and contain insignificant or no aromatic hydrocarbons and/or resins as impurities [an example shown in Fig. 6(a)]. In the case of oils 16, 19 and 22, where 99-100% of the material is reported as saturated hydrocarbons, there seems to be a substantial portion of saturated hydrocarbons which tends to elute in the aromatic hydrocarbon region, the latter being "dragged" forward as shown in Fig. 6(b). The dragged material is suspected to be mainly saturated, but weakly aromatic in nature, possibly involving compounds such as alkyl benzenes with long alkyl chains. In some of the saturated hydrocarbon fractions, however, there is a distinct separation into saturated and supposedly "weakly aromatic" saturated hydrocarbon portions (oils 7 and 30) [Fig. 6(c)]. Aromatic hydrocarbon fractions from MPLC are relatively free of saturated hydrocarbons, except for oils 1 and 30 where minor amounts of saturated hydrocarbons can be observed (Table 8). However, all the aromatic hydrocarbon fractions obtained by MPLC contain minor amounts of resins (0.5-2%) [Fig. 7(a)]. In the case of resins obtained by MPLC, all the fractions split into two distinct peaks during Iatroscan analysis, with one eluting as an aromatic hydrocarbon peak and the other as the resin peak [Fig. 7(b)]. As this bimodality occurred in all fractions and the qualitative results for all the resin fractions are very similar, it is believed that this is either due to the separation procedure adopted during MPLC, where the elution of aromatic hydrocarbons may not be complete, or that some of the aromatic hydrocarbon and resin species are closely related with respect to their structure and polarity. If the latter is true, then clearly the small differences in their physical properties are detected by T L C - F I D elution, but apparently not by the MPLC elution procedure. The results for the asphaltene fractions are of variable quality (Table 8). In most cases, the contamination is rather significant in form of minor amounts
cuts of each fraction (cf. analytical procedure for MPLC). Nevertheless, some general and relative differences in the oil compositions are apparent. Oils 1, 7, 16, 19, 22 and 30 are relatively rich in hydrocarbons; oils 5, 7, 16 and 30 are rich in resins; while oils 1, 5 and 7 are rich in asphaltenes. The purity of all the resulting fractions (7 samples each of saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene fractions) was established using Iatroscan, the results of which are presented in Table 8. Except for the saturated fractions from oils 5, 7 and 30, the other saturated hydrocarbon fractions are rather a:
J
1
i I0
cm a
0
I0
cm b
0
I0
cm
0
¢
Fig. 6. latroscan analysis of saturated hydrocarbon fractions obtained by MPLC separation. (a) A pure saturated hydrocarbon fraction. (b) Saturated hydrocarbons slightly contaminated by aromatic hydrocarbons. (c) Saturated hydrocarbons contaminated by "weakly aromatic" compounds.
Calibration and standardization of Iatroscan
845
1 I0
cm
0
10
a
cm b
0
10
cm
0
c
Fig. 7. Examples showing TLC-FID chromatograms of (a) a pure aromatic hydrocarbon, (b) a typical resin and (c) a relatively pure asphaltene fraction. of hydrocarbons or resins. However, asphaltene fractions from oils 5, 19 and 30 are rich in asphaltenes, as also noted earlier, the latter two being contaminated by resins. One asphaltene however, apparently gives good results (oil 5) and as seen in Fig. 7(c), no hydrocarbons or resins seem to be present as contaminants.
Preparative thin-layer chromatography (PTLC) As noted above, all the resin fractions resulting from MPLC separation split into two peaks on Iatroscan analysis, one peak indicative of aromatic hydrocarbons and the other of resins. The selected resin fractions for preparative TLC were from oils 5 and 16, and each resin fraction resulted in 2 fractions (aromatic hydrocarbons or N S O - A R O and "pure" resins or NSO-NSO). The resulting pure resin fraction (NSO-NSO) is characterised by a dominant resin peak and a minor asphaltene peak [Fig. 8(a)].
I
10
cm It
0
10
I
I
I
cm
I
1
0
b
Fig. 8. Iatroscan analysis of the separated resin fractions using preparative-TLC. (a) The pure resin fraction (NSO-NSO) and (b) an impure resin fraction (NSO-ARO). OG 2 2 / 3 - ~
More importantly, the principle resin peak does not split into 2 components and the resulting fraction is considered to be clean and uncontaminated. The second fraction (NSO-ARO), however, also contains a dominant component, apparently resins but which should to be of a different composition, and some hydrocarbons [Fig. 8(b)]. Nevertheless, this exercise suggests that the resin fraction resulting from MPLC and which splits into two distinct components on Iatroscan analysis, can be further separated into two fractions on a preparative scale, as shown in Plate 1. For comparison, the results of TLC separation of all the fractions and whole oil using a TLC plate are also shown in Plate 1. Subsequent separation of all oils using preparative TLC plates and the same elution procedure as employed in Iatroscan, shows that the four principle fractions separate well and the u.v. detector (254 and 356 nm) showed three distinct zones containing aromatic hydrocarbons, resins and asphaltenes (saturated hydrocarbons do not fluoresce) (Plate 1). Analysis of the four principle fractions from MPLC showed that the saturated hydrocarbons separated into two zones--one of major saturated hydrocarbons and one of minor aromatic hydrocarbons. The aromatic hydrocarbons separate into two fractions-one major aromatic hydrocarbons and one minor resins, and perhaps one minor saturated hydrocarbon zone, but this is difficult to establish as saturated hydrocarbons are not possible to detect on a u.v. detector. The resin fraction clearly separates into 2 equally dominant fractions--aromatic hydrocarbons and resins. Finally, the asphaltenes separate into mainly asphaltenes and minor resins, aromatic hydrocarbons and probably minor amounts of saturated hydrocarbons.
Composition and purity of the separated fractions Gas chromatography of the 5 saturated hydrocarbon fractions (from oils 1, 5, 16, 19 and 30) which
846
SUNILBHARATIet al.
°/
/
t,E:Ou 9£0u ..~ -..=¢ 0GOU
I 0£Ou'~ ~Ou ;¢Ou
C=
--
:~ES E
V- N
-
~NW.AHa
/.~3u
g=Ou'~
~N~£SI~d
LL0u
-i
:
e~01
tn
:tNV.LAHd
eI0u ~N~£SI~d
iI0u
g~o!
o
g~Ou
o I'tO!
~10!
Q
, , , I , , ,
T
o
o
o
N
o
oo
=o
=o
o
, , m
(Am) ~l!suelul -I
, J ,
, , ,
, , ,
,
e~ 0
,~
o
(AW)/~l!suelul
./
/
e
~
0
N ~£Ou
N 0~Ou -
~
eg[_ 3
i
E=
gtou L~0U
~NV~,kHd~, ~NVISI~d
3 NV.I.SIEId e £0
3
u
N
~
~
S~Ou -%
(ALU) /~:llsueluI
0
(AtU) ~,!suelul
o
oo
03
03
Calibration and standardization o f latroscan
847
/
Y
0
d~H. dl~'~ c
E
v
o
~
c~
_
E
N~a~'t NlflO'9' t
M~lO-.,'~+g'~ NIq
NMo'g' l
NWO-L'~"g'Z
Hdg
Ni~l-I
I
N ~
Nt~l-~
0
E o 0 o
0
..r
i
I
0
,
,
,
I
i
i
i
I
,
,
,
I
,
,
,
I
i
i
,
I
,
o
~
(ALU) ~l!suelul
(^uJ)/~llsuelul
E <
i
.o
F
C~
o
p
~
2
g
2
i=
o
dl~l'~
die4-6 diN'(:
i-:
o NI~O-9'I.
o
o
o
(Am) fIjsue),u I
o
o
NINO-Z'Z+9'~
" "
o
(Am)/;~!suelul
848
S U N 1 L B H A R A T I et
were thought to have the best potential of being used as standards indicates that there are significant differences in the composition and maturity of these oils. Oil i [Fig. 9(a)] is relatively rich in lighter n-alkane components, probably due to its slightly higher maturity; oil 5 [Fig. 9(b)] is moderate to strongly biodegraded; oil 16 [Fig. 9(c)] is unusually rich in isoprenoids relative to the n-alkanes (probably to do with an unusual source); oil 19 [Fig. 9(d)] is relatively rich in heavier n-alkane components with n-Cl5 being the most abundant alkane; and oil 30 is mildly biodegraded. The isoprenoid content in oil 19 is also higher, indicating a lower maturity, though not as high a content as in oil 16. Gas chromatography of the 4 aromatic hydrocarbon fractions which were thought to have best potential of being used as standards also indicates significant compositional differences. Oil 7 [Fig. 10(a)] is biodegraded and is depleted in diaromatic hydrocarbons; oil 16 [Fig. 10(b)] is very rich in diaromaticand depleted in triaromatic hydrocarbons; while oils 19 [Fig. 10(c)] and 22 [Fig. 10(d)] are relatively rich in diaromatic, but also have significant triaromatic hydrocarbon moieties. In addition, oil 19 is more mature than oil 22 (indicative from the higher 2methylnaphthalene/1-methylnaphthalene ratio in oil 19). FPD traces of the 4 aromatic hydrocarbon fractions show that only oils 7 and 16 contain traces of dibenzothiophene, with an apparent difference in maturity between the two. It is important to understand as to which of the aromatic hydrocarbon species have the greatest tendency to be retained as contaminants in the saturated hydrocarbon fraction and vice-versa. This semi-quantitative and qualitative assessment was achieved by analysing selected saturated and aromatic hydrocarbon fractions using the GC-MS technique. The objective of semi-quantitative assessment and contaminant identification was especially significant in light of the conventional methods adopted for separating fractions (MPLC). Therefore the purest/least contaminated saturated and aromatic hydrocarbon fractions amongst all the oils considered in the study, were chosen for this exercise. The saturated hydrocarbon fraction is rich in nalkanes, but the cyclo-alkane moieties are evidently also abundant (m/z 83 and 97), although long chains of alkylbenzenes are also detected in m/z 83 [Fig. 1l(a)]. In addition, isoprenoids are also present in significant proportions. However, based on m/z 91 and 106, alkylbenzenes are not present in large amounts, contrary to expectations as alkylbenzenes with long chains were thought to be the most likely contaminants in the saturated hydrocarbon fraction. These observations are apparent in Fig. 1l(a), where the absolute intensities of the 10 major ions are plotted. The figure shows the maximum intensity in a fragmentogram and the general (most common) intensity in that fragmentogram. This semi-quantitative approach indicates the relative abundances of the
al. (a) 16
-
14
-
12
-
Saturated Hydrocarbons Intensity
lO -
[ ] General
g
a-
--~
6
-,
4
-
// //
2
-
7,//
//
7,
// /J
// t,
0
type
[ ] Maximum
n_=l
L
77 83 91
(b) 12r--
97
99 106 134 142 163 183
Aromatic Hydrocarbons 77
10
-
¢/~ r-
.o
~/ //
s-
--
/~ //
0
-]173~ r~--,,,,.,~ I ~ ,.~_~ 77 83 91 97 99 106 134 142 163 183 m/z
Fig. II. Summarized results of GC-MS analysis of the hydrocarbon fractions. Relative maximum intensities of 10 major ions in (a) saturated hydrocarbon fraction and (b) aromatic hydrocarbon fraction. various types of molecules present in the fraction. Apparently, Cl-benzene, the basic aromatic ring structure with a methyl group, seems to be the most abundant aromatic hydrocarbon species present. The aromatic hydrocarbon fraction on the other hand is dominated by short-chain methylated benzenes, but relatively smaller amounts of some C4alkylbenzene are also present. Amongst the saturated hydrocarbon species, n-alkanes and isoprenoids seem to be nearly equally distributed [Fig. I l(b)], in addition to methyl cyclic alkanes (m/z 97). Unlike the saturated hydrocarbon fraction, which contains practically no dibenzothiophenes, the aromatic hydrocarbon fraction retains some dibenzothiophene moieties, this being observed in the m/z 198 fragmentogram. On comparing Figs l l(a) and l l(b), it seems that the aromatic hydrocarbons have a greater tendency to retain saturated hydrocarbon species than vice-versa. However, the limitations of this observation must be borne in mind, as the conclusion is based on the analysis of only two samples. The 2 asphaltenes selected for making the standards differ significantly in the composition of their thermal extracts. While oil 5 is strongly biodegraded [Fig. 12(a)], oil 19 has a bimodal distribution of n-alkanes [Fig. 12(b)]. Their pyrolysates, however, are quite similar in composition [Figs 12(c) and (d)]
~'.
Intensity (mY) o
~]
.~...~
'
I
'
'
'
o I
'
'
'
m m n s f f y ~mv]
c) I
'
'
'
o
I
'
'
'
I
'
'
'
o I
°
o~:
!"-
m
~,:,_nC I 0 fl~
i ~,~
~,
nC15 ~
o~
PRISTANE
nC17
I I
c) o
}
), 03
"0
e..
,~s
|=________.__
!
_
o~
mlensffy I.mv) (x 1U'{I) o
o
o
E
(b (D
(n
/ n l e n s i ~ ( m v ] ~x 1U1;I) o
o
o
ol
E ~LrNE
u -<
0
b o -'
c)
3
o
(D
~
o~ -
C17
G18
0~ "10 ~r m m (D
o~ cao
C~
OV~
ub'a~uJlUl ju uu!luz!pJ~puul~
pu~
=uH-a~l:l~ J
t5
10
25
25
20
20
30
30
$
40
45
50
40
45
50
Time (minutes)
35
Time (minutes)
35
55
55
60
60
65
65
a
70
70
10
lo
15
is
20
2-o
2s
t
25
30
30
4s
50
1
40
4s
50
°
Time (minutes)
as
0
40
Time (minutes)
35
TIC of Asphaltenes
ss
55
sO
60
6s
d
es
C
r0
ro
Fig. 13. Results of PY--GC-MS analysis of resins and asphaltenes. Total Ion Chromatograms (TIC) of (a) resin from oil 16, (b) resin from oil 22, (c) asphaltene from oil 5 and (d) asphaltene from oil 16. Peak identification: V--toluene; V--n-hydrocarbon doublets; O--alkylcyclohexanes; n--methylphenanthrene; O--alkylnaphthalenes; ~--nCl8 doublet; ,>~--internal standard.
15
10
t $
TIC of Resins
>.
t-" =0 ,i;>
_z
C
851
Calibration and standardization of latroscan
and are indicative of a marine source. Py-GC of Ion Chromatograms (TIC) of the two asphaltene fractions are shown in Figs 13(c) and (d). resins did not give any conclusive results. P y - G C - M S of selected resin and asphaltene Identification, isolation and purification of selected fractions allowed to assess the presence of any hydrofractions for standard carbons, if any, that might have been retained in these Based on the findings this far, 4 oils covering fractions after MPLC separation. The resin fraction of oil 16 contains only traces of n-hydrocarbons, as practically the entire API gravity range, were selected is indicative by the generally imperceptible homology for re-separation into fractions. Preliminary analyses in m/z 97 and 99, although m/z 99 does exhibit indicated that one or more of these oils could be presence of some n-alkanes. Alkylbenzenes are successfully used to isolate pure fractions, which could be further used to calibrate Iatroscan via present, but are believed to occur in small amounts. Alkylnaphthalenes are more prominent relative to prepared standards. Four parallels of each of the four alkyl-phenanthrenes. Dibenzothiophenes and other oils, oil 5, oil 16, oil 19 and oil 22, were subjected to substituted derivatives are also present. No significant MPLC in order to get large amounts of each fraction biomarkers were recorded. Compared to the resin from each oil. At this stage, we had 4 fractions × 4 from oil 16, the resin fraction from oil 22 is much parallels for each oil. Each of the four fractions more depleted in most of the hydrocarbon species, from these oils were later mixed and re-run through although the latter contains a stronger n-alkane the MPLC system to purify and clean the fractions. homology, as observed in m/z 99, perhaps indicative This ultimately resulted in one large portion each of saturated hydrocarbon, aromatic hydrocarbon, resin of the presence of more saturated hydrocarbons. Most alkylbenzenes are of low molecular weight and asphaltene fractions for each of the 4 oils. On and are not interpreted to be significant. The results analyzing these fractions using the Iatroscan, it was found that the aromatic fraction of all the oils was for other ions are similar, but these compounds are present in low concentrations. The Total Ion contaminated by substantial amounts of resin species, Chromatograms (TIC) of the two resin fractions are a problem which had also been observed earlier. The 4 aromatic hydrocarbon fractions were run through shown in Figs 13(a) and (b). The 2 asphaltenes seem to be considerably different MPLC twice more to remove the resins. The typical T L C - F I D chromatograms of the final in composition. While oil 5 asphaltene is rich in shortand medium-length alkyl chains, oil 16 asphaltene pure fractions (saturate and aromatic hydrocarbons, is depleted in these moieties (m/z 97 and 99). Oil 5 resins and asphaltenes) used in Iatroscan calibration asphaltene is also relatively rich in alkylbenzenes. and the standard are shown in Figs 14(a-d). Clearly, Alkylnaphthalenes, however, seem to be equally the fractions are uncontaminated with only traces of prominent in both the asphaltenes, in contrast to unwanted components. However, as the four saturated alkylphenanthrenes which are more conspicuous in hydrocarbon and aromatic hydrocarbon fractions the oil 5 asphaltene. Dibenzothiophenes are depleted differed significantly from each other with respect to in oil 16 asphaltene relative to oil 5 asphaltene. These density and composition, individual response factors results perhaps indicate that oil 16 may be generally were calculated to see if the response factors were richer in branched/cyclo hydrocarbons. The Total affected by these parameters. Each of the fractions
10
cm
a
0
10
cm
b
0
10
cm ¢
0
10
cm
0
d
Fig. 14. TLC-FID chromatograms of "pure" isolated fractions obtained by a combination of repeated MPLC and preparative-TLC. (a) Saturated hydrocarbons, (b) aromatic hydrocarbons, (c) resins and (d) asphaltenes.
852
SUN]]. (a)
Saturated
BHARATIet
al.
(c)
HC
Resins
7 --
5-
4 -I
/
•
t-
3 -
~"
2
E O
-=
1
}-
0.5
"5 ~
5
OIL TYPE [] Heavy • Medium 1 A Medium 2 • Light I 1.0
I 1.5
(b)
O I L TYPE [] Heavy • Medium 1
--
I 2.0
I 2.5
I 3.0
3 3.0
I 3.5
(d)
A r o m a t i c HC
I 4.0
Asphaltenes
14--
ffl cO
[]
13--
if}
rr
12--
5 --
~ . ~ ~ ' ~ ~ ~"
OIL TYPE Heavy • Medium 1 zx Medium 2 • Light
4
--
3 0.5
z~
I 1.0
I 1.5
I 2.0
I 2.5
I 5.0
-/
11 - -
o
..
I 4.5
OIL TYPE [] Heavy • Medium 1 zx Medium 2
I
10 -
•
9-I 3.0
8 0.5
I 1.0
I 1.5
I 2.0
I 2.5
S a m p l e a m o u n t (pl)
Fig. 15. Dependence of calculated response factors of Iatroscan on oil's composition and density. Plot of response factor versus sample amount from 4 different oils for (a) saturated hydrocarbons, (b) aromatic hydrocarbons, (c) resins and (d) asphaltenes.
of the four oils (one heavy--API gravity around 10°, two medium--API gravity around 30 ° and one light - - A P I gravity more than 40 °) were analysed by Iatroscan 3-5 times, using different sample amounts each time (0.5-5/~1 of the same solution), and response factors calculated in each case using the equation shown below. Response factor (Re) = Peak area (A)/Sample amount ( M ) The results are shown in Figs 15(a~i). Dietz (1967) has shown earlier that relative sensitivity for the flame ionization detector (FID) is quite different for hydrocarbons and non-hydrocarbons. While most straight chain, branched and cyclo alkanes have similar FID sensitivities (close to 1.0), aromatic hydrocarbons tend to be very slightly lower (0.98-1.02). However, non-hydrocarbons (resins and asphaltenes), which have heteroatoms such as N, S and O, have much lower FID sensitivities and the values vary appreciably. Alcohols, for example, vary from 0.23 to 0.85; acids, from 0.01 to 0.65 etc. (Dietz, 1967). The variation in the sensitivity value depends mainly on type of compound, number of heteroatoms, molecular weight etc. (Dietz, 1967; Drushel, 1983).
As seen in Fig. 15(a), the response factors of the four saturated hydrocarbon fractions vary significantly (from about 2000 to 4500). Closer examination of the results reveals that the variation is not random, but is controlled by at least two factors. Saturated hydrocarbons from the heavy oil have the greatest response factor, while those from a light oil have the least. Compositionally, the saturated hydrocarbons from the heavy oil are strongly biodegraded, those from the first medium oil (plotted as solid squares) are very rich in isoprenoids, those of the second medium oil are rich in medium to heavy range n-alkanes and lastly those from the light oil are rich in low molecular weight n-alkanes (cf. Fig. 9). In case of the aromatic hydrocarbons [Fig. 15(b)], the variation in response factor is from about 4000 for the lightest oil to about 6500 for the heaviest oil. With regards to the compositional variation between the four aromatic hydrocarbons, the heavy oil is biodegraded, as noted above, the first medium oil (plotted as solid squares) is richer in alkylnaphthalenes relative to triaromatics, while the second medium oil and the light oil are relatively rich in triaromatics (cf. Fig. 10). In case of the resins and asphaltenes, the trend is opposite to those of the two hydrocarbon groups
Calibration and standardization of Iatroscan
853
Response Factor(Thousands) _..A..- -o 12
Jk--'"
ught o . J
Medium011
I Heavy011 I
X
..........
~ ........... . ~ - a- i - -i' ......... - - - - i -i....... .....
B
SATHC ~AROHC 10
20
24
-g-NSO
30
35
,~ASP 40
I 50
API Gravity 1") Fig. 16. Relationship of oil density (API gravity) and Iatroscan detector response (Response factor) for the 4 principle fractions of oil viz. saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes. The API gravity is inversely proportional to detector response in case of the 2 hydrocarbon fractions (as indicated by the negative slope of the 2 solid linear regression trends) and directly proportional in case of the 2 non-hydrocarbon fractions (as indicated by the positive slope of the 2 dashed linear regression trends).
[Figs 15(c) and (d)]. Unlike the hydrocarbons, the response factors of the heavy oil fractions are lower than those of their lighter counterparts. The significance of this reversal is not exactly clear, but it is suspected that it may be related to lower carbon and hydrogen contents/greater ionization efficiency of the components in the heavier oil's resin and asphaltene fractions. Calibration of Iatrosean standard
and testing of prepared
Clearly, the present results indicate that there cannot be one unique response factor for a given fraction to calibrate Iatroscan. Parrish and Ackman (1985) had also observed variations in the Iatroscan's FID response, on analysing marine lipids. It is controlled to a large extent by the oil's API gravity (which can be expressed mathematically) and the gross composition of the fraction (which is a subjective aspect and cannot be expressed mathematically). In Fig. 16, the API gravity of the 4 oils is plotted against the average response factor of each of the 4 fractions of each of the 4 oils. The variation of the response factors in case of each of the four fractions is more apparent here. While the calculated response factors decrease with increasing API gravity for the two hydrocarbons, it increases for the two nonhydrocarbon fractions. Moreover, there is a remarkable similarity between the two hydrocarbon fractions as regards to their response factor progression with increasing API gravity (the two linear regression trends--solid lines are parallel to each other). Similarly, the linear regression trends of the two nonhydrocarbon fractions (the two dotted lines) progress
nearly parallel to each other, although unlike the hydrocarbons their response factors increase with increasing API gravity. As response factors cannot be calculated for every unit of the API gravity, and there is no unique response factor for a given fraction either, the whole suite of oils has been divided into three principle types for simplicity--heavy (API gravity 10-24°), medium (API gravity 24-35 °) and light (API gravity 35 ° and above). The American Petroleum Institute defines an oil as heavy if it has an API gravity of 20 ° or lower. However, in our case (Fig. 16), the response factor of the polar fraction is lower than that of the saturated hydrocarbon fraction until about 24°, but then crosses over and is subsequently greater than the saturated hydrocarbons. This point was therefore taken as the boundary between the heavy and medium oils. It is not known, however, whether this fact is of any significance or not. Considering the results plotted in Fig. 16, it is assumed that if each of the 4 fractions have 3 different response factors according to the type of oil (one each for heavy, medium and light oils), calibration of Iatroscan will be reasonably good. The mathematical model developed to derive these factors is exemplified below. The slopes of the two hydrocarbon trends are negative while those of the two non-hydrocarbons are positive. Ideally (typically for single synthetic compounds and theoretically for a mixture of similar compounds), R~=A/M
(1)
where Rc = response factor, A = peak area, M = sample amount.
SUNIL BHARATIet al.
854
| $
tl
10
-I 0
cm
I
I~ 10
em
b
A
0
"" 10
cm
0
¢
Fig. 17. TLC-FID chromatograms of prepared standards. (a) A natural standard based on a medium oil (Standard A). (b) A natural standard based on a light oil (Standard B). (c) A synthetic standard comprising several synthetic components (GEO3).
But in the present case,
where C = f ( a ) , a being the API gravity of the oil. This is similar to the linear regression equation
The correction constants can be obtained for each fraction through linear regression, using the slope of the trend and the applicable cr in equations (5), (6), (7) and (8). An example for saturated hydrocarbons is shown below:
y = a + bx
CI(SAT)= --53.3 × 17 = --906
Rc = A / M + C,
(2)
(3)
where 17 is the assumed representath~e of the a Range [tO, 241.
where a is a constant and b is the slope. .'. equation (2) can be written as
Rc=A/M+b
xa
(4)
The derived equations for the 4 components in Fig. 16 are:
Saturated hydrocarbons.
C2CSAT)= -- 53.3 × 29 = -- 1546
where 29 is the assumed representative of the cr Range [24, 35]. C3(SAT)= -- 53.3 × 42 = -- 2238
Re(sAT) = 5292 + (--53.3 x a)
(5)
Aromatic hydrocarbons. Rc(ARO)= 7328 + (--47.3 X a)
(6)
Rc(eoL) = 5393 + (34.6 x a)
(7)
Polars.
Asphaltenes. RclAsp) = 9781 + (66.3 × a)
(8)
Equation (4) above can be considered to be the basic "Correction Equation". This can theoretically have infinite number of forms for every given a, but for simplicity we will consider 3 principle forms suitable for heavy oils, medium oils and light oils. The 3 equations can be written as,
R c H = A / M +CI,
10<~a <~24
(9)
RcM=A/M+Cz,
24<~-N<35
(10)
RcL=A/M+C3,
35
(11)
~<50
where C~, 6"2, C3 are "Correction Constants".
where 42 is the assumed representative of the a Range [35, 50]. Using the correction constants, the response factor (Re) of a given fraction for each oil type can be calculated using equations (5), (6), (7) and (8), as shown below for saturated hydrocarbons as an example [the equation employed in this case is equation (5)]. RcH = 5292 -- 906 = 4386 RcM = 5292 -- 1546 = 3746 RcL = 5292 -- 2238 = 3054 Similarly, the other correction constants and response factors can be calculated for the other fractions and each type of oil. These were calculated and are shown in Table 9. According to the table below, we have a total of 4 fractions x 3 oil types i.e. 12 response factors for use and calibration. It must be noted, however, that these are response factors of the F I D used to analyze the samples in the present study, and they will not be necessarily the same for other users. However, the
855
Calibration and standardization of latroscan Table 9. (a) Calculated correction constants (C) Sat Ct C2 C3
-906 - 1546 -2238
Aro -804 - 1371 - 1987
Pol
Asp
588 1003 1453
1127 1923 2785
(a)
Standard A
80--
60 - -
Fraction o SAT x ARO
/
• POL (b) Calculated response factors (R c)
A ASP
Sat
Aro
Pol
Asp
Heavy oil (Rcs) Medium oil (RcM) Light oil (RcL)
4386 3746 3054
6524 5957 5341
5981 6396 6846
10908 11704 12566
Average (Balk) (R¢s)
3729
5941
6408
11726
, 0
~
20--
o -~"v
relationship of the response factors to each other is more likely to remain the same, if the same standard is used to calibrate the instrument. Therefore, for other users, the 2 tables below (Table 10) could be used to extrapolate the response factors from the given standard, which is based on a medium oil. The relationships in Table 10(a) below are derived directly from the data in Table 9(b). A closer examination of these relationships reveals that in relation to the medium oil values, both the hydrocarbon fractions of heavy and light oils have a rather constant relationship (1.17 to 1.1 times and 0.82 to 0.9 times respectively) and both the non-hydrocarbon fractions of heavy and light oils also have a constant relationship (0.93 times 1.07 times respectively). This further strengthens the interpretation that these variations are not random, but are controlled by distinct factor(s). The user may utilise either Table 10(a) or 10(b) to extrapolate response factors from the given medium oil based standard, Standard A. Two standards have been prepared using topped oils, for calibrating Iatroscan and subsequent use. The strength of the solutions in both cases was aimed in the range 10-15 mg oil/ml solvent, as it has been found through experience that this is the ideal range to ensure adequate individual component amounts, good separation and to avoid overloading. The solvent chosen to dissolve the standards was toluene instead of commonly used dichloromethane, due to toluene's greater stability (higher boiling point) under ambient conditions. Table 10. (a) Relationship of response factors (Re) of a fraction with respect to medium oil
Heavy oil (RcH) Medium oil (R¢M) Light oil (ReL)
Sat
Aro
Pol
Asp
1.17p p 0.82p
1.1q q 0.9q
0.93r r 1.07r
0.93s s 1.07s
p, q , r and s are the user's calculated response factors based on prepared Standard A. (b) Relationship of response factors (Re) of a type of oil w.r.t. saturated hydrocarbons
Saturated HC Aromatic HC Polars Asphaltenes
Heavy oil
Medium oil
Light oil
(RcH)
(RcM)
(RcL)
x 1.49x 1.36x 2.49x
y 1.59y 1.71y 3.12y
z 1.75z 2.24z 4.11 z
x is the user's calculated response factor for saturated hydrocarbon fraction based on prepared Standard A.
0.5
[-
T
I
I
I
I
1.0
1.5
2.0
2.5
3.0
(b) ~
80 --
<
60 --
Standard B
[]
[]
2.0
2.5
40 --
20
0.5
1.0
1.5
3.0
Sample Amount (lad Fig. 18. C a l i b r a t i o n curves f o r the 2 n a t u r a l s t a n d a r d s , A a n d B. (a) P l o t o f s a m p l e a m o u n t versus p e a k a r e a o f the 4 i n d i v i d u a l c o m p o n e n t s o f S t a n d a r d A. (b) P l o t o f s a m p l e a m o u n t versus p e a k a r e a o f the 4 i n d i v i d u a l c o m p o n e n t s o f S t a n d a r d B. N o t e the l i n e a r i t y o f p e a k - a r e a progression w i t h i n c r e a s i n g s a m p l e a m o u n t in case o f e a c h c o m p o n e n t in b o t h the s t a n d a r d s .
Standard A is primarily made from a medium oil and is rich in polars and asphaltenes. In contrast, Standard B is primarily from a light oil and contains minor polars and practically no asphaltenes. The chromatograms of these two standards are shown in Figs 17(a-b). These two standards were analysed by Iatroscan using 5 parallels, with different sample amounts each time. The sample amount varied from 0.5 to 3#1 in each case. Calibration curves were drawn and are shown in Figs 18(a-b), where it can be observed that the peak area progression is rather Table 11. Composition and quantitative data for the two prepared standards
Fraction
Peak area uVs
Corrected Response area factor uVs [cf. Table9(b)]
Saturated HC Aromatic HC Polars Asphaltenes
45792 20847 26794 10645
Standard A 48332 20727 26177 10599
3746 5957 6396 11704
12.90 (60.37) 3.48 (16.29) 4.09(19.14) 0.90 (4.2)
Saturated HC Aromatic HC Polars Asphaltenes
42018 10266 4284 236
Standard B 45537 10022 5100 312
3054 5341 6846 12566
14.91 (84.96) 1.88(10.71) 0.74 (4.22) 0.02 (0. I 1)
Amount ~ul (%)
19.0
18. Stearamide
19. Berberine sulphate
18.2
17. Stearyl alcohol
9.8
15. I-benzothiophene 20.1
12.7
16. Stearic acid
20.2
31.8
NSO1
13. Thenoic acid
100
11.3
5.1
16.4
27.1
8.3
AROI
14. Dibenzothiophene
50.4
12. Phenanthrene
16.5
10. 2-methylnaphthalene
49.6
7.5
9. Biphenylene
I1. 1,3-dimethylanthracene
24.1
3.8
8. Naphthalene
5.5
5. Hexatriacontane
12.1
19.7
4. Tfiacontane
39.8
26.9
3. Hexacosane
7. 1,4-dicyclohexylbenzene
41.4
1. Hexadecane
2. Docosane
MODII POLYI POLY2
6. 2,3-dimethylnaphthalene
SAT1
Compound
23.7
29.5
46.8
NSO2
71.4
28.6
NSO3
55.5
17.8
26.7
NSO4
49
12.8
22.5
15.7
NSO5
20.1
21.7
19.6
38.5
NSO6
Standard Name
33.4
13.4
14.5
13.1
25.6
NSO7
33.3
33.1
33.6
NSO8
100
NSO9
100 100
NSOI0 NSOII
25
75
25
75
25
75
NSOI2 NSOI3 NSOI4
Table 12. List of compounds used to prepare various synthetic standards. The figures indicate the percentage of the compound used in a standard
I00
ASPI
2 g~
-e
z r~
Calibration and standardization of Iatroscan consistent in the case of all the four principle fract i o n s - s a t u r a t e d hydrocarbons, aromatic hydrocarbons, polars and asphaltenes. Corrected peak areas, based on linear regression, were used to calculate the fraction amounts. The response factors used to derive the individual component amounts are derived from Table 9(b). The case for 2 #i sample amounts for both the standards is shown below in Table 11 and taken as correct, as this is the sample amount (sample dissolved in solvent) that is applied on the chromarods.
Synthetic standards Table 12 shows the details of the synthetic standards (total 21 basic standards) prepared in the present study. One basic criterion that was followed while making these standards was that in each standard, the distribution of components should be as close as possible to the distribution commonly encountered in the natural samples. A total of 19 compounds, most of which occur naturally in solvent extracts and/or crude oils, was chosen with objective of obtaining response factors as close as possible to natural mixtures. The first stage in synthetic standard analysis included mainly establishing the correct combination of various compounds which are compatible with each other in a solution. This was therefore a more qualitative objective. The second stage consisted of making a mixture of all the compatible compounds, with at least one compound representative of each of the principle fraction type, and calibrating Iatroscan and deriving the response factors. The qualitative assessment and compound compatibility findings are described below for each fraction type.
857
Saturated hydrocarbons. Squalane is the most commonly used synthetic compound for the purpose of calibration and quantification of saturated hydrocarbons. We, however, chose to use a mixture of 5 different n-alkanes ranging widely in the carbon chain-length (nCl6 to nC36) to prepare the saturated hydrocarbon standard, SAT1 (Table 12). The T L C FID chromatogram is shown in Fig. 19(a). Iatroscan analysis of SATI gave very good results with one dominant peak. This was therefore the only standard made for saturated hydrocarbons. Aromatic hydrocarbons. The standard for monoand diaromatic hydrocarbons (MODII), which was a mixture of 5 compounds (Table 12), resulted in one sharp peak on Iatroscan analysis [Fig. 19(b)]. In addition, a combination of MODI1 components and a poly-aromatic hydrocarbon (phenanthrene) (ARO1) resulted in two well defined peaks on Iatroscan analysis [Fig. 19(c)]. Asphaltene. The criteria for selecting a compound which could serve as a natural asphaltene equivalent were simple: (1) the molecule should be large, (2) the molecule should be polar, (3) the molecule should contain nitrogen, sulphur and oxygen. Berberine sulphate (MW = 822, formula = C40H42N20~5 S) was selected and the standard ASP1 and ASP2 (a much diluted version of ASPI) analysed by Iatroscan. The results were good with one dominant peak at the spotting point [Fig. 19(d)]. Resins. Making the right synthetic standard for resins was the most difficult task in the entire synthetic standard study. Up to 14 different mixtures (Table 12) had to be made to reach the right composition of the resin standard. Initially, the criteria was to include at least one compound containing
E
r-tO
cm
a
0
==,, lO
,% cm
b
OlO
cm C
0
10
, cm
, 0
d
Fig. 19. TLC-FID chromatograms of synthetic standards. (a) Synthetic standard SAT1, comprising 5 n-alkanes with varying carbon chain-length (C16-C36). (b) Synthetic standard MODII, comprising 5 different mono- and diaromatic hydrocarbon components. (c) Synthetic standard AROI, comprising MODI1 and a poly-aromatic hydrocarbon (phenanthrene). (d) Synthetic standard ASP1, comprising a single compound, berberine sulphate, which is thought to be comparable to a natural asphaltene molecule with regards to polarity and elution characteristics during chromatography.
858
SUNIL BHARATIet al.
|
.~~ 10
10
cm a
0
cm
0
e
10
10
i ~
."I
cm b
0
cm
0
f
IO
10
l~',
cm c
0
¢m
0
g
10
, cm
d
10
cm
0
h
Fig. 20. Results of latroscan analysis of the various synthetic resin standards. TLC-FID chromatograms of (a) NSO 1, comprising 6 different components, including 2 acids, an alcohol, an amide and 2 thiophenes. (b) NSO2, comprising 3 components- thenoic acid and 2 thiophenes. (c) NSO5, comprising ASP1 and C18 acid, alcohol and amide. (d) NSO6, comprising thenoic acid and C 18--acid, alcohol and amide. (e) NSO7, comprising NSO6 and ASP1. (f) NSO8, comprising only the 3 C 18 components--acid, alcohol and amide. (g) NSO11, comprising only C18 amide and (h) NSOI4, comprising C18 amide and berberine sulphate (ASP1).
nitrogen, sulphur and oxygen and at the same time, maintaining the individual molecular weight low. NSOI standard was made, but on analysis, the mixture split into three peaks with one peak eluting as an aromatic hydrocarbon [Fig. 20(a)]. NSO2 (made to identify the incompatible component) split into two distinct components [Fig. 20(b)] and therefore abandoned. NSO3, NSO4 and NSO5, which consisted of similar compounds mixed with berberine sulphate, and which were expected to give 2 peaks each, gave variable results. While NSO3 resulted in two peaks, thenoic acid did not elute as much as it should have; NSO4 gave only one peak; and NSO5 seemed to have worked well with the combined CI8acid, alcohol and amide peak separating very well from the berberine sulphate peak [Fig. 20(c)]. This led us to believe that perhaps the two benzothiophenes are the problematic components. To ascertain this,
NSO6 and NSO7 were analysed, but contrary to expectations NSO6 resulted in two peaks [Fig. 20(d)] and NSO7 in three peaks [Fig. 20(e)]. This clearly indicated that the combination of stearic acid, stearyl alcohol and stearamide is also not compatible. This was evident from NSO8 analysis [Fig. 20(f)], where three components gave three peaks. NSO9, NSOI0 and NSO1 i were analysed to identify the incompatible component. All the three gave reasonably good single peaks, but stearamide [Fig. 20(g)] seemed to be most consistent in its retention time. A combination of these three components with berberine sulphate (NSO12, NSO13 and NSOI4) confirmed that stearamide was most compatible with the asphaltene component, berberine sulphate [Fig. 20(h)]. This concluded the qualitative and compatibility assessment of the selected synthetic compounds and the final standard mixture GEO3, which comprised
859
Calibration and standardization of Iatroscan Table 13. Calibration results of the Synthetic Standard GEO3 (based on 10 parallels t~/"2#1 ) Fraction
Mean Area
Area SD
Area (%)
Amount (%)
Amount (/~g)
Rcs
Saturated Hydrocarbons Mono + Diaromatic Hydrocarbons Poly-aromatic Hydrocarbons Resin Asphaltene
14727 4842 3917 8035 6961
962 399 282 688 531
37.51 12.72 9.98 21.13 18.79
41.8 24.5 9.3 14.7 9.6
4.32 2.532 0.96 1.52 0.996
3409 1912 4080 5286 6989
SATI, AROI, NSOI1 and ASP2 was analysed. The resulting T L C - F I D chromatogram is shown in Fig. 17(c). Clearly, the 13 compounds used in GEO3 are compatible and stable in each other's presence, with one peak each of saturated hydrocarbons, mono+diaromatic hydrocarbons, polyaromatic hydrocarbons, resins and asphaltene. The new GEO3, which was subsequently made using fresh chemicals with the purpose of calibrating Iatroscan and deriving response factors, comprised of the following: saturated hydrocarbons 40%, aromatic hydrocarbons 35%, resin component 15% and asphaltene component 10%. Ten parallels of 2 #1 spotting were analysed to establish reproducibility and obtain optimum peak areas. Table 13 above summarizes the findings. The implications of the calculated response factors from synthetic standards (R~s) are discussed in the next section.
Validity of oil type dependent calculated response factors The calculated response factors as shown in Table 9(b) apparently indicate potentially large differences in the resulting quantitative data when these factors are used. It is intended in this section to demonstrate the actual differences that will be obtained if only a single set of bulk response factors are used instead of calculated response factors with oil type dependence. The test is performed for heavy and light oils' response factors, keeping the average bulk response factors constant. In the first case, we assume that Standards A and B are heavy oils and use the corrected areas to calculate the fraction amounts using the heavy oil response factors i.e. RcH, and this will give Amount 1 for both the standards. Next we
use the bulk response factors (Rca) and calculate the fraction amounts (Amount 2). The fraction amounts resulting by employing RoB in Case 1 should be quite close to the actual values of the standards calculated using RcM, and which are shown in Table 11. This is because RoB are closest to Rosa. Next, the response factors for light oil (R~L) are used and the fraction amounts calculated (Amount 3). Finally, the response factors derived from the GEO3 synthetic standard (Rcs from Table 13) are used and the individual fraction amounts calculated (Amount 4). The results are presented below in Table 14. From Table 14 the following conclusion can be made. The first general effect of using a single set of bulk response factors is that the quantity of fractions in heavy oils are overestimated and those in light oils are underestimated. This means, for example, that the concentration of a heavy oil solution is reported as 21.43 and 13.69 mg/ml instead of 19.55 and 11.89 mg/ ml respectively, an overestimate of about 10-12%. Likewise, the light oil solution which should have a concentration of 24.37 and 16.32 mg/ml is reported as 21.43 and 13.69 mg/ml respectively, an underestimate of about 12%. In addition, it must be borne in mind that even the figures quoted in the above table are likely to be slightly erroneous (say about + 5%) and therefore the true error due to use of bulk response factors could be in the range of about 18%, which is undoubtedly a high percentage. However, the relative percentages are less affected and tend to remain rather constant, except in the case of saturated hydrocarbons where the variation is about + 9 % for Standard A, which is relatively depleted in saturated hydrocarbons, and + 3 % for Standard B, which is rich in saturated hydrocarbons.
Table 14. Comparison of quantitative data obtained by using different response factors (R¢., Rca, RcL and R~s) and Standards A & B peak data Fraction
Area mVs
RcH
Amount 1 ul (%)
Sat. HC Aro. HC Polars Asphalt.
48332 20727 26177 10599
4386 6524 5981 10908
11.02 (56) 3.18 (16) 4.38 (22) 0.97 (5)
Total Sat. HC Aro. HC Polars Asphalt. Total
19.55 (99) 45537 10022 5100 312
4386 6524 5981 10908
10.38 (81) 1.54 (12) 0.85 (7) 0.03 (0) 12.80 (100)
R~a
Amount 2 /~1 (%)
Case I: Standard A data 3729 12.96 (60) 5941 3.49 (16) 6408 4.08 (19) 11726 0.90 (4)
R~L
Amount 3 /tl (%)
R~s
Amount 4 #1 (%)
3054 5341 6846 12566
15.83 (65) 3.88(16) 3.82 (16) 0.84 (3)
3409 2996* 5286 6989
14.18 (51) 6.92(25) 4.95 (18) 1.52 (6)
21.43 (99) Case 2: Standard B data 3729 12.21 (83) 5941 1.69(11) 6408 0.80 (5) 11726 0.02 (0)
14.72 (99)
24.37 (100) 3054 5341 6846 12566
14.91 (85) 1.88 ( l l ) 0.74 (4) 0.02 (0) 17.55 (100)
Note: Response factor types: R~H = heavy oil, R~a = bulk (average), RcL = light oil and R~s = synthetic standard. *Average of mono + diaromatic and poly-aromatic hydrocarbons.
27.57 (100) 3409 2996* 5286 6989
13.36 (75) 3.34 (19) 0.96 (5) 0.04 (0) 17.70 (100)
860
SUNIL BHARATI et al.
Oils with high proportions of polars are also likely to be adversely affected with the variation being about _+8%. Assuming that most oils would fall in the category of medium oils, the use of bulk response factors will most commonly not bring about any significant change in the quantitative data. On the other hand, the use of bulk response factors, would result in an underestimation of saturated hydrocarbons in heavyand hydrocarbon-depleted oils which could be crucial. Similarly, an overestimation of saturated hydrocarbons in hydrocarbon depleted light oils is also critical, though this case is less likely to occur. With regard to the use of bulk response factors in the analysis of solvent extracts from reservoir rocks, the degree of inaccuracy should be minimal as the majority of the extracts would in any case fall in the category of medium oils. This is, however, not equally applicable to extracts from source rocks, as source rocks are much more likely to be rich in high molecular weight non-hydrocarbon species, such extracts being comparable to low API gravity oils. In general, therefore, the use of bulk response factors for rock extracts could be substantially erroneous, since for rock extracts it is not only the relative percentage data that is important, but also the net yield data. Yields obtained by using response factors derived from synthetic standards (Rcs) are even more erroneous than those obtained by using bulk response factors. The net saturated hydrocarbon yield is grossly overestimated for heavy and medium oils (28 and 9% respectively), while slightly underestimated for light oil. Aromatic hydrocarbons are most adversely affected by using R~s. The net yield is almost doubled, irrespective of the type of oil, and this is the principle reason for highly erroneous relative percentages. Resins are overestimated for all types of oils, the medium and low API gravity samples being the most affected. Asphaltenes are also highly overestimated (up to 80% for light oils), despite employing a large molecule such as berberine sulphate as an asphaltene standard. Apparently, the response of berberine sulphate is closer to the natural aromatic hydrocarbon fraction from a light oil. All these factors contribute to the highly inaccurate relative percentages for a given extract or oil sample, regardless of its composition and API gravity, if synthetic standards response factors are used for calibration and quantification. SUMMARY AND CONCLUSIONS
Of the 31 oils world wide that were used in the study, only a few were found suitable for the purpose of preparing standards and calibrating the Iatroscan T L C - F I D instrument. For the remaining oils, the primary reason for not qualifying as Iatroscan Standard precursors was their inability to separate into 4 distinct and clean fractions, which were chromatographically pure from the T L C - F I D point of view.
This inability is believed to be primarily due to their inherent composition. However, a few oils covering a wide API gravity range were found to be suitable and were later used to calibrate Iatroscan and prepare standards. As expected, the most problematic fractions were the non-hydrocarbons, due to mainly two reasons. One reason was the limited amounts of these fractions obtained from the oils, and second being the difficulty in obtaining pure, uncontaminated fractions. This problem was eventually minimised through repeated separation and parallel analyses. A major finding of this study is that the response factors of the separated fractions from oil are not unique for all oils, but are instead greatly dependent on the composition of the fraction and the API gravity of the oil. It will therefore be highly erroneous to use a single response factor for, say saturated hydrocarbons, without taking into account the API gravity of the oil. For the sake of simplicity, a set of 3 response factors for each of the 4 fractions, have been derived which are suitable for heavy oils, medium oils and light oils, these being defined as Rcn, RcM and RcL •
Examples of two standards, A (rich in polars and asphaltenes) and B (lean in polars and asphaltenes nearly absent), have been prepared through this study for use in the petroleum industry. A mathematical model has been developed to derive response factors and their relationships to the given standard have been described. These can be readily employed by different Iatroscan users. The use of these standards and the proposed mathematical model will ensure realistic inter-laboratory data comparison and standardization of the Iatroscan technique in the petroleum industry. Based on the results obtained in the present study and bearing in mind the various limitations/problems encountered, we recommend the following: 1. Further work must be performed to better understand the dependability of response factors on API gravity and composition of the oil. More data points are required in this paper's Figs 16 and 17, to make the proposed model better and subsequently derive more reliable and correct response factors. This will require further MPLC separations of oils, GC of hydrocarbon fractions and Iatroscan analyses. 2. The concentration of the solution that will be used to spot samples on the chromarods must be 10-15mg oil/ml solvent, to ensure good chromatography and avoid overloading. In the case of whole rocks, SI data from Rock-Eval should be used to determine the rock weight, but if nothing is known then 3g should be dissolved in 5 ml solvent. 3. An amount of 2 p l solvent containing the sample should be spotted on the rod as a general practice, preferably using an auto-spotter, to avoid band-spreading and to ensure good
Calibration and standardization of Iatroscan
4.
5.
6.
7.
chromatography. Blowing the spotting point continuously using nitrogen has been found to be very effective in quickly removing the solvent, and should be routinely practised. As n-pentane precipitated asphaltene fraction is compositionally not at all comparable to the fraction hitherto referred to as asphaltene in the T L C - F I D technique, we propose the use of the terms Resin 1 for resins (synonymous to the resin or NSO fraction from MPLC) and Resin 2 for the fraction hitherto referred to as asphaltene in the T L C - F I D technique. Resin 2 will therefore essentially comprise of heavier, high molecular weight and large molecules of N S O compounds, relative to Resin 1. Adoption of the terms Resin 1 and Resin 2 will also minimize confusion when the quantitative data from M P L C is compared to quantitative data from Iatroscan. Synthetic compounds should not be used as standards for quantification purposes, as response factors derived from them result in highly erroneous data for real samples. Natural standards (such as the prepared Standards A and B of the present study) should be used for calibration and subsequently should be routinely analysed (say once a fortnight or after every 100 analyses) and the individual response factors updated accordingly, as the response/sensitivity of any F I D alters with time and usage. Bulk response factors must not be used as these are likely to give erroneous results. Different response factors for heavy oils, medium oils and light oils must be used instead, which can be easily calculated and applied by the users.
Acknowledgements--Many thanks to the following oil companies (arranged in alphabetical order) for oil samples: BEB Erdgas und Erd61 GmbH, BHP Petroleum Pty. Ltd, Chevron/Cabinda Gulf Oil Company Limited, Mobil Turkey, Norsk Hydro, OMV Aktiengesellschaft, Occidental Petroleum Corporation of Peru, SANTOS Ltd, Saga Petroleum a/s, Unocal Netherlands B.V., Geolab Nor; and Norsk Hydro, Saga Petroleum a/s, and Statoil for financial support. F. Behar and D. Karlsen are thanked for critical comments and improving the quality of the manuscript.
REFERENCES
Dietz W. A. (1967) Response factors for gas chromatographic analyses. J. Gas Chromatogr. 5, 68 71. Drushel H. V. (1983) Needs of the chromatographer-detectors. J. Chromatogr. Sci. 21, 375-384. Goutx M., Gerin C. and Bertrand J. C. (1990) An application of Iatroscan thin-layer chromatography with flame ionization detection--lipid classes of microorganisms as biomarkers in the marine environment. In Advances in Organic Geochemistry 1989 (Edited by Durand B. and Behar F.), pp. 1231-1237. Pergamon Press, Oxford. Karlsen D. and Larter S. (1989) A rapid correlation method for petroleum population mapping within individual petroleum reservoirs: applications to petroleum reservoir description. In Correlation in Hydrocarbon Exploration, OG 22/3-5---HH
861
pp. 77-85. Norwegian Petroleum Society, Graham & Trotman, London. Karlsen D. and Larter S. (1991) Analysis of petroleum fractions by TLC-FID--applieations to petroleum reservoir description. Org. Geochem. 17, 603-617. Norwegian Industry Guide to Organic Geochemical Analyses (1993) 3rd edition. Jointly published by Statoil, Norsk Hydro, Saga Petroleum, Geolab Nor, IKU and Norwegian Petroleum Directorate. Poirier M. A., Rahimi P. and Ahmed S. M. (1984) Quantitative analysis of coal derived liquids residues by TLC with flame ionisation detection. J. Chromatogr. Sci. 22, 116-119. Radke M., Willsch H. and Welte D. H. (1980) Preparative hydrocarbon group type determination by automated medium pressure liquid chromatography. Anal. Chem. 56, 2538-2546. Ray J. E., Oliver K. M. and Wainwright J. C. (1982) The application of the Iatroscan TLC technique to the analysis of fossil fuels. In Petroanalysis 81, IP Symposium, London, 361-388. Heyden and Son, London. Selucky M. L. (1983) Quantitative analysis of coal derived liquids by thin layer chromatography with flame ionisation detection. Anal. Chem. 55, 141-143. Yamamoto Y. (1988) Analysis of heavy oils by thin-layer chromatography with flame ionisation detection. Sekiyu Gakkaishi 31, 351-362.
APPENDIX
1
Whole rock (cuttings, core-chips, side wall cores) is crushed using either a mill or a pestle and mortar to fine particle size. If the sample is a shale, then the particle size should be 65 ,am or finer. If the sample is a loosely packed sandstone, then it is sufficient that all the individual sand grains are free and finer crushing does not improve the results. About 2 g of whole rock is accurately weighed in a 8 ml glass vial with screw cap (cap with a Teflon lining inside), and 2.0 ml of solvent (dichloromethane: methanol, 93 : 7 v/v) is added and the vial sealed immediately. The sample is allowed to be extracted for 48-72 h (preferably 72 h if the sample is a shale), with vigorous agitation 4-5 times a day. If the rock is very rich in EOM (the colour is dark brown), then an additional 1 ml solvent can be added after one day. After this stage, the procedure is the same as in the case of oils.
APPENDIX
2
List of Ions Specified During GC-MS of Saturated and Aromatic Hydrocarbons 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1I. 12. 13. 14.
m/z 77 83 91 97 99 106 134 142 163 183 177 191 198 217
15.
231
16
253
Compound --benzene ---cyclo-alkanes --Cl-alkylbenzenes --n-alkenes/methylcyclic-hexanes --n -alkanes --alkylbenzenes ~4-alkylbenzenes --methylnaphthalenes --terpanes --isoprenoids ~emethylated triterpanes --triterpanes ~ibenzothiophenes --steranes --triaromatic steranes --mono-aromatic steranes
SUNIL BHARAT1et al.
862
APPENDIX
3
List of Ions Specified During P Y - G C - M S of Resins and Asphaltenes m/z I. 2. 3. 4. 5.
77 91 97 99 106
Compound --benzene --Cl-benzene (toluene) --n-alkenes --n-alkanes --alkylbenzenes
6. 7. 8. 9. 10. I I. 12. 13. 14.
134 142 156 163 170 192 198 206 212
15.
231
16.
253
---C4-alkylbenzenes --methylnaphthalenes --C2-naphthalcnes --terpanes --C3-napht halenes --methylphenanthrenes ---dibenzothiophenes ---C2-phenanthrenes --C2-dibenzothiophenes --triaromatic steranes --mono-aromatic steranes