Chapter 8 The sulfur chemiluminescence detector

E.R.Adlard (Ed.), Chromatography in the Petroleum Industry Journal of ChromatographyLibrary Series, Vol. 56 0 1995 Elsevier Science B.V.All rights reserved

20 1 CHAPTER 8

The sulfur chemiluminescence detector Richard S. Hutte Sievers Instruments Inc., 2500 Central Avenue, Suite HI, Boulder, CO 80301, USA

8.1 INTRODUCTION

All petroleum samples from crude oil to refined products contain varying concentrations of sulfur-containing compounds. These compounds include gases (H2S, COS, SO2 and CS2), aliphatic, and aromatic thiols, sulfides and polysulfides, thiophenic and other sulfur-heterocyclic compounds. The geochemistry of sulfur has been recently reviewed [ 13 and an extensive list of the types and concentrations of sulfur compounds found in petroleum systems is presented. The concentration of sulfur compounds in crude oils varies widely from <0.05% to >14% sulfur by weight [l]. Most of the sulfur is in the form of organic compounds, with only relatively small amount of dissolved H2S and elemental sulfur present. Most of the organo-sulfur compounds are higher molecular weight compounds with boiling points greater than 300°C [l]. Hydrotreating and other processes to remove sulfur compounds from crude oil and other feedstocks result in the conversion of the higher molecular weight materials to H2S and lower molecular weight thiols and sulfides. Thus, a wide range of different types of sulfur compounds is usually present in petroleum products. Sulfur compounds are also intentionally added to petroleum products. Low molecular weight thiols, sulfides and tetrahydrothiophene are used as odorants for natural gas and LPG and certain additives for lubricants and other products contain sulfur. The presence of even parts per billion levels of sulfur can cause numerous problems in the processing of petroleum including poisoning of the expensive catalysts used in the refining process, corrosion of reactors and pipelines and the presence of sulfur-containing compounds imparts undesirable odors to petroleum products. Combustion of petroleum products containing sulfur compounds is a References pp. 227-229

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Chapter 8

major source of SOz air pollution and acid rain. For these reasons, accurate determination of the concentrations and identities of the sulfur-containing species in petroleum products is required. Many analytical techniques have been developed for the measurement of the sulfur content of petroleum and petroleum products. These techniques include methods for the determination of total sulfur content including X-ray fluorescence [2], coulometric [3] and radiometric [4] techniques and chromatographic techniques that permit the quantification of individual sulfur-containing compounds. The most widely used chromatographic technique is gas chromatography, usually coupled with sulfur-selective detectors.

8.2 SULFUR-SELECTIVEDETECTORS FOR GAS CHROMATOGRAPHY The most widely used sulfur-selective detector for gas chromatography has been the flame photometric detector (FPD) IS]. Sulfur-selective detection in the FPD is based on combustion of sulfur-containing compounds in a hydrogenricldair flame to produce diatomic sulfur in an electronically excited state (&*). The emission from Sz* is monitored using a photomultiplier tube positioned near the flame. The problems of the FPD for sulfur detection are well documented [6] and include a non-linear response, compound-dependent response and perhaps most important for petroleum analyses, quenching of the response due to coelution of hydrocarbons. Despite these limitations, the FPD has been successfully used for the determination of sulfur compounds in a wide range of petroleum samples for many years [7-1 l]. The Hall electrolytic conductivity detector (HECD) can also be used in a sulfur-selective mode [ 12,133. Sulfur compounds are combusted in the presence of oxygen in a heated nickel tube to form SOz,which is bubbled into a suitable solvent such as methanol or methanol/water and the electrical conductivity of the solvent is monitored. The HECD is not widely used for sulfur-selective detection, in part because it is viewed as being difficult to operate and maintain and suffers from interferences and quenching of the sulfur response. The atomic emission detector (AED) is a multi-element detector that can be used for sulfur compounds [14-16]. The AED has a linear response for sulfur compounds and does not suffer from quenching, but has poor selectivity for sulfur versus carbon [ 161 that limits detection of low levels of sulfur compounds in hydrocarbon matrices. The AED is also relatively expensive and requires highly skilled operators. The analysis of low levels of sulfur by gas chromatography is complicated by the reactivity of most sulfur compounds. Sulfur compounds are irreversibly ad-

203

The chemiluminescence detector

sorbed or will decompose on many surfaces, especially heated metal surfaces. This results in the loss of sulfur compounds in sample collection equipment and throughout the chromatographic system (syringes, injection ports, columns, and detectors). Thus, precautions must be taken to minimize sorption and loss of low level sulfur compounds in the samples by replacing metal surfaces with glass, fused silica, Teflon, or less reactive metals such as nickel, and by conditioning the chromatographic system by injection of high concentrations of the sulfur compounds of interest. Sorption and loss are particularly noted for H2S, SO2 and mercaptans, while sulfides and disulfides are less reactive. Preparation of standards containing sulfur compounds is also a problem. For gas standards, low level sulfur compounds can be lost on the walls of the gas cylinders, requiring the use of specially treated cylinder and stabilization of gas mixtures. For liquid standards, chemical reactions such as oxidation of mercaptans by air to form disulfides occurs even with refrigeration of the solutions. 8.3 THE SULFUR CHEMILUMINESCENCEDETECTOR The limitations of existing sulfur-selective detectors for gas chromatography led to the development by Benner and Stedman [17] of a new sulfur-selective detector, the sulfur chemiluminescence detector (SCD). Combustion of sulfur compounds in a hydrogen-rich flame produces a number of different sulfur species including HIS, HS, S, S2, SO and SO2, with the ratios of the various species determined by the fuel/air ratio and other factors [6]. Benner and Stedman [I71 noted that sulfur monoxide (SO) is one of the major species formed in the combustion of sulfur compounds in reducing flames and, in fact, is present at 10 times higher concentration than atomic sulfur. Detection of SO produced in a flame should provide a more sensitive and linear detection system for sulfur compounds. Halstead and Thrush [18] reported in 1966, that sulfur monoxide undergoes a chemiluminescent reaction with ozone. Emission in the blue and ultraviolet region of the spectrum (260480nm) was observed from the emission of electronically excited SO2 formed from the reaction of SO with 0,. The SCD combines these reactions to provide sensitive and selective detection of sulfur compounds. Sulfur monoxide is produced by combustion of sulfur compounds in a hydrogen-rich flame, the flame gases are collected by means of a sampling probe and transferred to a chemiluminescent reaction chamber where they are mixed with ozone and the emission monitored using a photomultiplier tube after passing through a wide band-pass filter. Sulfur compounds References pp. 227-229

H,lair flame

> SO + products

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so + 0 3 -3 so2+ o2+ hv The SCD developed by Benner and Stedman is an atmospheric monitor for the measurement of both oxidized and reduced sulfur compounds in ambient air. The detector has a linear response for sulfur compounds, equal molar response and a detection limit of 0.13 ppbv (-1 pg S/s). No quenching of the sulfur response is observed due to C 0 2 and hydrocarbons and this lack of interference provides for improved measurement of low parts per billion level sulfur compounds in ambient air compared with fluorescence and FPD sulfur monitors 1191. At Sievers Instruments, we investigated the use of the SCD as a chromatographic detector by coupling the Benner and Stedman SCD with a gas chromatograph. These preliminary experiments led to the development of a commercial detector for gas chromatography, the Model 3 50 Sulfur Chemiluminescence Detector. A schematic of the SCD is shown in Fig. 8.1. It consists of a hydrogenrich flame, a flame sampling probe, a transfer line, an ozone generator, a chemiluminescent reaction cell, an optical filter, a photomultiplier tube and associated electronics for detection of the chemiluminescent radiation and a vacuum pump. In the GC-SCD, the flame source is a conventional flame ionization detector operated under hydrogen rich conditions. The flow rates of hydrogen and air for the flame in the SCD are 200 ml/min and 400 ml/min, respectively. In contrast, normal FID flow rates are typically 30 mumin for hydrogen and 250-400 ml/min for air. The more reducing flame results in the formation of sulfur monoxide from the combustion of sulfur compounds, yet still provides a flame ionization detector response, although the sensitivity of the FID is reduced by approximately an order of magnitude due to the higher hydrogen flow rates. The flame sampling probe in the SCD is a high purity ceramic tube 8-1 1 cm in length (0.5 mm i.d.) positioned -4 mm above the FID jet. To avoid collection of room air, only -90% of the flame gases are collected by the ceramic probe. The probe is positioned in the flame using a bayonet mount to lock it into a movable mounting base shown in Fig. 8.2. The position of the probe in the flame can be optimized by either adjusting the position of the mounting base or rotating the probe handle to one of three locking positions, thus repositioning the end of the probe relative to the flame jet. The vacuum pump of the SCD serves several purposes; collection of the flame gases, transfer of the flame gases and ozone into the chemiluminescent reaction chamber, and reduction of non-radiative collisional quenching of the emitting species (SO2*) in the chemiluminescent reaction cell. Under normal operating conditions, the chemiluminescent cell pressure is 10-1 5 Torr. A chemical trap is used to remove ozone and oxides of nitrogen from the gas stream before the vacu u m pump and since the flame gases contain high concentrations of water vapor,

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MODEL 350 B SCD REGULATOR FILTER

RESTRICTOR

OZONE GENERATOR

VACUUM UNE

TRANSFER

<

MNT

SAMPUNQ

188EYBLY

FIO

GAS or SUPERCRITICAL FLUID CHROMATOGRAPH

-

-1 I

I I I

I

I

I

B 3 I I

ELECTRONICS

RECORDER

RECORDER

Fig. 8.1. Schematic of the sulfur chemiluminescencedetector.

the pump is operated with the gas ballast open to vent water from the pump. To prevent loss of oil, an oil coalescing filter is used to collect oil in the pump exhaust and the oil is returned to the pump. The transfer line consists of a 1.5-m length of 4.75 mm (3/16 inch) 0.d. black perfluoroalkoxy (PFA) tubing. The use of an inert material minimizes loss of sulfur monoxide in the transfer line and the relatively large inside diameter reReferences pp. 22 7-229

Chapter 8

206

To Chemiluminescence Detector

Ceramic Probe

Fig. 8.2. Schematic of the fixed position interface for mounting the SCD probe on an FID.

duces condensation of water. Residence time in the transfer line and chemiluminescent reaction cell is approximately 1 s and no significant band-broadening or peak tailing is observed. Emission from the SO/03 reaction occurs in the blue and ultraviolet region of the spectrum, therefore a blue-sensitive photomultiplier tube is used to detect the chemiluminescent reaction. The flame gases also contain relatively high concentrations of nitric oxide, which will also undergo a chemiluminescent reaction with ozone. Emission from the N 0 / 0 3 reaction occurs in the red and nearinfrared region of the spectrum and therefore an optical filter that transmits radiation over the wavelength region of 240410 nm is positioned in front of the PMT to eliminate interference from nitric oxide. In the commercial SCDs, two different techniques have been used to process the signal from the photomultiplier tube. The original detector used photon-

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207

counting, while newer versions employ a pico-ammeter. The principal advantage of the analog system is a wider dynamic range and minimal distortion of the chromatographic peak shape from the signal processing electronics.

8.4 PERFORMANCE CHARACTERISTICSOF THE SCD The SCD has a linear response for sulfur compounds over five orders of magnitude, a specified detection limit of <5 pg S/s [20] with a detection limit of <0.5 pg S/s having been reported [21]. This sensitivity corresponds to -10-50 pg of a sulfur compound or parts per billion concentrations, depending upon the sample size, injection technique (split versus direct or splitless), and other chromatographic variables. The selectivity for sulfur compounds versus hydrocarbons of >lo6 [20-221. The SCD response is equimolar for sulfur compounds and the response is not quenched by co-elution of higher levels of non-sulfurcontaining compounds. The excellent selectivity of the SCD is achieved primarily by the combustion of the sample in the flame. Compounds that will undergo chemiluminescent reaction with ozone, such as olefins, are converted to C02 and water, eliminating possible interference. However, under certain circumstances, a positive hydrocarbon response is observed. For example, when a large amount of a compound elutes as a very sharp chromatographic peak, the compound is not completely burned in the flame and the partial combustion products can react with ozone and produce a response. In this situation, it is necessary to either decrease the sample size or to change the chromatographic conditions to broaden the chromatographic peak to permit complete combustion of the components. An SCD response is also observed for arsine, phosphine and their organoderivatives, although the sensitivity of the SCD for these compounds has not been determined. Nitriles also produce a small SCD response, presumably due to CN emission however this response is much lower that the response for sulfur compounds. The equimolar response of the SCD for sulfur compounds has been demonstrated for a wide range of sulfur compounds [21-241. The relative response factors for a range of sulfur compounds (relative to phenyl sulfide) are shown in Table 8.1. As will be described in more detail in the applications of the SCD, the equimolar response permits the determination of the total sulfur content (or total volatile sulfur content) by simply summing the individual components, or when only total sulfur content is desired, then a short chromatographic column, with poor resolution can be employed and the overall detector response integrated for the determination of total sulfur content.

References pp. 227-229

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'TABLE 8.1 RELATIVE RESPONSE FACTORS FOR SULFUR COMPOUNDS USING THE SULFUR CHEMILUMINESCENCE DETECTOR Compound

Relative response factoP

H2S

0.80 0.90

cos so1

CS2 Methanethiol Ethanethiol I -Propanethi01 2-Propanethiol 1-Butanethiol 1 -Methyl- 1-propanethi01 2-Methyl- 1-propanethi01 2-Methyl-2-propanethiol 1-Pentanethiol 1-Hexanethiol I -Heptanethiol 1-0ctanethiol 1-Decanethiol Dimethyl sulfide Methyl ethyl sulfide Diethyl sulfide 2-Methyl-2-propane sulfide Dimethyl disulfide Diethyl disulfide Thiophane Thiophene 2-Methy lthiophene 3-Methyl thiophene 2,j-Dimethyl thiophene 2-Ethyl thiophene 2-n-Propyl thiophene 2-n-Butyl-thiophene Benzothiophene Dibenzothiophene

"Relative to phenyl sulfide, data from [21-241.

1.oo

1.oo I .43 1.18 1.05 1.04 1.11 1.02 1.05 1.11 1.12 1.08 I .oo 1.06 1.04 1.oo

0.88 1.07 1.02 0.95 1 .oo

I .04 1.08 1.10 1.oo 1.05 1.03 I .02 1.oo

1.05 0.87

The chemiluminescence detector

209

8.5 FACTORS INFLUENCING THE SENSITIVITY AND SELECTIVITY OF THE SCD

The key step in the SCD is the combustion of sulfur compounds in the hydrogen-rich flame to form sulfur monoxide and collection of SO by the sampling probe. Therefore flame conditions and the position of the probe in the flame are the most critical factors in determining the sensitivity, selectivity, and stability. The hydrogen/air ratio in the flame determines the products of sulfur combustion. For the best SCD sensitivity and stability, the hydrogen flow rate is set to -200 ml/min and the air flow rate set to -400 mVmin or a fuel/air ratio of -1 :2. Under these conditions, the tip of the sampling probe is heated to sufficient temperatures that a dull red glow can be observed when the probe is rapidly removed from the flame. These conditions also yield sulfur monoxide as the major product of sulfur combustion. With higher air flow rates >400 ml/min (or lower hydrogen flow rates) sulfur dioxide is the principal combustion product and the SCD sensitivity is decreased. The higher air flow rates also produce a hotter flame as indicated by a white glow at the tip of the probe, when removed from the flame. At lower air flow rates (<400 mumin), the yield of SO in the flame is actually increased, but several other factors make operation at lower air flow rates undesirable. Whenever the sampling probe is in the flame and the ozone generator is on, a background signal is observed for the SCD. In part, this background is due to the so-called “ozone-wall reaction”, a background radiation observed when ozone is passed in front of a photomultiplier tube. However, an additional background signal is observed from the reaction of the flame gases (possibly hydrogen atoms) with ozone. Under the normal hydrogen and air flow rates, this background signal is relatively constant and not affected by the elution of non-sulfurcontaining compounds from the GC However, when the flame for the SCD is operated at lower air flow rates, this background signal can be “quenched” by the elution of non-sulfur-containing compounds from the GC. This results in negative peak for these components when the SCD is operated with low air flow rates. In addition to the negative SCD response for non-sulfur species, operating the flame of the SCD at lower air flow rates produces a cooler flame (no glow observed for the tip of the probe) and this lower temperature does not produce stable response over a period of several days. Thus, any increase in sensitivity achieved at the low air flow rates is offset by the reduced stability observed. Thus, the optimum conditions for most GCs are very close to the 200 ml/min hydrogen flow rate and 400 ml/min air flow rate. Examples of the SCD response obtained at different air flow rates are shown in Fig. 8.3. The sample is a test mixture containing five sulfur compounds (and References pp. 227-229

210

Chapter 8

N O %x.Q473

RESPONSE

h n

N

Fig. 8.3. Effect of hydrogen and air flow rates on sensitivity and selectivity of the SCD. (A) Chromatogram of sulfur standard with low air flow rate. (B) Chromatogram of sulfur standard with correct hydrogenhir ratio. (C) Chromatogram of sulfur standard with high air flow rate.

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211

some impurities) in hexane solvent, with 10% benzene. Figure 8.3B shows the SCD chromatogram obtained under the proper gas flow rates (200 mumin H2, 400 ml/min air). No response is observed for -130pg of hexane (2pl split 1O:l). Under the chromatographic conditions employed, benzene co-elutes with thiophene, thus slightly distorting the thiophene peak on the SCD, however no quenching is observed. Figure 8.3A shown the same test mixture using 200 ml/min H2 and 350 mumin of air. Under these more reducing conditions, a negative response is observed for hexane and benzene and a greater response of the detector for the sulfur compounds is obtained. Finally, Fig. 8.3C shows the SCD response for the same test mixture using 200 ml/min H2 and 500 ml/min of air. The response for the sulfur compounds is greatly reduced and in some cases, a positive response for the hydrocarbons can be observed under these more oxidizing conditions. This example illustrates the importance of operating the detector at the proper gas flow rates. When the SCD is operated at the proper gas flow rates, good day-to day reproducibility can be obtained, however, contamination of the probe from column bleed and septa bleed can cause a loss in sensitivity. Silicone compounds from either GC columns or septa when high injection port temperatures are used can deposit on the tip of the probe and decrease the response of the detector for sulfur compounds. In most cases, the contamination can be physically removed by simply inserting a cleaning wire into the probe. To avoid the silicone bleed contamination, the columns (and septa) should be well conditioned and the column operated at as low of temperature as possible for a given analysis. For example, most bonded methyl silicone columns can be operated at temperatures up to 275”C, without any bleed problem, but operation at higher temperatures such as 300°C can cause contamination of the probe and reduced sensitivity. It should be noted that the SCD is more sensitive than the FID to silicone bleed and contamination can occur even though no significant baseline rise is observed on the FID. 8.6 FLAMELESS SULFUR CHXMILUMINESCENCE

A dedicated ceramic burner for the formation of sulfur monoxide has been recently described by Shearer [25] and a schematic of the burner is shown in Fig. 8.4. In this “flameless” SCD, hydrogen and air are mixed with the column effluent in a heated ceramic tube and all of the combustion gases are transferred to the chemiluminescence detector. The typical gas flow rates for the burner are 100 mumin for hydrogen and 20 ml/min of air, which are outside the flammability limits for H2/air. To initiate and sustain the combustion, the burner is heated typically to 800-900°C and direct connection of the chemiluminescence detector (and vacuum pump) to the burner results in a reduced pressure, typically References pp. 227-229

Chapter 8

212 HYDROGEN INLET

1/16' CERAMIC HYDROGEN DELIVERY TUBE

HEATING ELEMENT

REACTION ZONE 1/8" CERAMIC TUBE

THERMOCOUPLE

COLUMN CONNECTION

Fig. 8.4. Schematic of enclosed burner for the conversion of sulfur compounds'[o sulfur monoxide.

-200 Torr in the ceramic tube. This modification of the SCD improves the ease of use of the SCD, by eliminate the need to position a fragile ceramic probe in an FID and also results in improved sensitivity. Detection limits of 2-5 fg S/s were obtained by Shearer [25].The improved sensitivity is due in part to the more reducing conditions of the combustion, increasing the yield of SO and lower pressures in the chemiluminescent reaction cell, reducing non-radiative colli-

The chemiluminescence detector

213

sional quenching of SO2*. As with the flame-based SCD, a nearly equimolar response is observed for sulfur-containing compounds and good sulfur to carbon selectivity (>lo8) is obtained [25]. The burner is available commercially, however the sensitivity is somewhat reduced compared with Shearer, with typical detection limits of >0.5 pg S/s reported [26]. One drawback of the lower gas flow rates of the ceramic burner is that quenching of the sulfur response due to co-elution of hydrocarbons can be observed. In most cases, this quenching can be reduced or eliminated by increasing the hydrogen and air flow rates, while maintaining the 5: 1 hydrogerdair ratio (e.g. 200 ml/min H2, 40 mVmin air), however decreased sensitivity for sulfur compounds is obtained at these higher flow rates. For example, the sulfur response is decreased by approximately a factor of two at 200 ml/min H2, 40 ml/min of air versus the response obtained at 100 ml/min H2, 20 mVmin of air. Another drawback of the ceramic burner is that in order to obtain simultaneous SCD/FID signals it is necessary to use a post-column split. However, for many routine applications and for low level detection of sulfur compounds, the ceramic burner offers many advantages compared to the flame-based SCD. 8.7 COLUMN SELECTION AND SAMPLING TECHNIQUES As previously noted, sulfur compounds can be sorbed and lost in all components of the chromatographic system and in sampling containers. Specially treated gas bombs and cylinders have been developed to minimize the loss of sulfur compounds, but in most cases, passivation of the sampling devices by treatment with high levels of sulfur compounds is required to avoid losses of low levels of sulfur compounds. In many cases, the best sampling containers are older ones that have been in use for long periods of time and thus have been passivated through use. Gas-tight syringes can also be a source of problems. Some syringes are more active than others and cause significant loss of low levels of sulfur compounds. When high levels samples are analyzed, the sulfur compounds can permeate through the Teflon and other syringe components, then slowly outgas, resulting in contamination of future samples analyzed with this syringe. Passivation with high levels of sulfur compounds is also the most common method for minimizing loss in the chromatographic system. Exposing inlet lines, gas sampling valves, sample loops, and injection port liners to high levels of H2S, SO2, mercaptans and other reactive sulfur compounds can reduce the loss of sulfur compounds in the GC system. Decomposition and loss of sulfur compounds can also occur in the chromatographic column. For example, porous References pp. 22 7-229

214

Chapter 8

- 6 4 mV

dibenzothiophene

ETU

3

n-oc tadecanethiol h

0

YlnuU.

5

20

16

10

25

5m x 0.53mm 1.5umu DB-5 FSOTC (J&W SC.) 35OC t o 430% Q 12.O0/min.

r

dibenzothiophene rthianthrene n-octadecanethiol

A 0

Mlnutsr

5

10

16

20

5m x 0.53mm O.lOum,, SPB-1 FSOTC (Supelco) 50°C to 43OoC Q 14.Oo/min.

Fig. 8.5. Illustration of decomposition of sulfur compounds in a fused silica capillary column.

polymers provide good separation of volatile sulfur compounds but low levels of sulfur compounds can be lost in the column and no amount of passivation appears to overcome this problem. The same is true for PLOT columns and the best materials we have found for analysis of low levels of the sulfur gases are the treated silicas such as Chromosil. Decomposition and loss of sulfur compounds can also be observed in fused silica capillary columns. An example of the decomposition of ethylene thiourea (ETU) on a megabore column with a relatively thin film is shown in Fig. 8.5. Analysis of a standard mixture of four-sulfur containing compounds ETU, dibenzothiophene, thianthrene and n-octadecanethiol using a column with a 1.5p m film thickness shows good peak shape for all of the sulfur compounds. Analysis of the same mixture using a column with a 0.1 p m film thickness shown almost complete decomposition of the ETU and severe tailing of the other sulfur compounds. Sorption and loss of reactive sulfur compounds like ETU has been observed even for new fused silica capillary column, indicating that the loss of the reactive compounds is most likely due to interactions with the silica surface and not simply due to an active column. This suggests that whenever possible, the analysis of sulfur compounds should be performed using fused silica column with film thickness of l p m or greater.

The chemiluminescence detector

215

A particularly useful capillary column has been developed for the analysis of sulfur compounds. The column (30 m X 0.32 mm i.d. SPB-I 4,um film thickness, Supelco Inc.) provides separation of H2S from co-eluting COS/S02 at 35°C and can be used for samples up through the diesel range. At -1O"C, COS and SO2 can be separated. Methyl silicone columns with 4 ,urn film thickness are now also available from other column manufacturers. The combination of a thick film methyl silicone and special cross-linking to minimize column bleed permits separation of most sulfur compounds and applications for a wide range of petroleum samples. The combination of high sensitivity and selectivity of the SCD has led to the development of a number of applications for this detector in the measurement of sulfur compounds in petroleum and petroleum products. Some representative examples are given below. 8.8 APPLICATIONS

8.8.1 Refinery gases A major application for the analysis of sulfur compounds is natural gas, LPG, refinery gases and other process gas streams. Since these streams are usually upgraded or sold for heating purposes, accurate measurement of the levels of sulfur compounds is required. Figure 8.6 shows the FID and SCD chromatograms obtained from the analysis of a refinery gas (LP) sample using the flame-based SCD. A 0.1-ml sample was injected onto a capillary column, using a split injection technique (split ratio 1O:l). The FID chromatogram shows that propylene and propane are the major constituents of the sample with low levels of C4 and C5 hydrocarbons. Hydrogen sulfide is the major sulfur contaminant, with lower levels of COS, SO2, mercaptans, sulfides disulfides and thiophenes present in the sample. Not all of the sulfur compounds were identified, however, the equimolar response of the SCD for sulfur compounds can be used to determine the sulfur content of these unidentified components and the total sulfur content of the sample. A response factor (pg S/area) can be determine from the analysis of a standard containing one or more sulfur compounds and this response factor used to quantitate unknown sulfur compounds and total sulfur content can be calculated from the total area. Another important application is the measurement of sulfur compounds in polymer grade ethylene, propylene and other olefins used in the production of plastics. Due to the complications from low levels of sulfur compounds in the polymerization reactions, the desired level of total sulfur in the feedstock is References pp. 227-229

216

Chapter 8

Gas Chomatograph Hervlet-Packard Model 5890 Sulfur Detector Slevers lnst, Inc Model 3506 SCDbn Cokmn 30m x 0 32mm 4p SPBl FSOT (Supelco. Inc ) Column Temperature 3 mn @ -10% to 3ooOC @ 10 0 Ohin Carrier Gas Hekum Q 22 png (3 0 d h n ) Speaal Spkt lryector 250% Split vent Row 32 c d h n FID 300OC kr @ 39 psig H2@ 60 psig Makeup Air Q 30 cm?lMn Manual Injecbon 0 1 mL gas (8 6 O h to column)

SCD Response

FID Response

0

2

4

6

0

10

I2

I6 Yhrtaa

14

Fig. 8.6. SCD and FID chromatograms of a refinery gas

18

20

22

24

26

28

00605018

217

The chemiluminescence detector

Calibration mixture 80 ppb carbonyl sulfide 40 ppb hydrogen sulfide

80 ppb sulfur dioxide

20 ppb carbon disulfide

t

SCD

SCD

FID I 1

I

I

0 min

2

4

I

6

I

I

8

10

Fig. 8.7. Analysis of polymer grade propylene using the SCD.

<5 ppb. To facilitate detection of sulfur compounds at the low ppb levels using the SCD, larger sample sizes and direct injection onto a packed column can be used. Figure 8.7 shows the analysis of a calibration standard and a sample of polymer grade propylene using a Chromosil 310 column. As noted above, the treated silica packed in a Teflon column provide a sufficiently inert system for the detection of low ppb levels of sulfur gases. For this propylene sample, the levels of the individual sulfur compounds were <20 ppb. References pp. 22 7-229

218

Chapter 8

8.8.1.2Gasoline

The concentration of total sulfur in motor gasolines typically ranges from 300 to 500 mg S k g (ppm). The development of new reformulated gasolines is now underway in an effort to improve ambient air quality. Included in the regulations governing fuel reformulation are new limits for the concentration of total sulfur in the gasolines. For example, the California Air Resources Board regulations call for a total sulfur content of less than 40 mgkg in gasoline. These lower sulfur concentrations present many problems, not only in the refining and blending of the fuels, but also to the analytical chemist. The SCD and FID chromatograms obtained from the analysis of a sample of a commercial motor gasoline is shown in Fig. 8.8. This oxygenated gasoline is used in the winter months in the Denver, Colorado, area as part of a program to reduce atmospheric carbon dioxide levels. The SCD chromatogram shows the typical sulfur compounds present in gasoline, thiophene and alkyl thiophene, benzothiophene and the alkyl derivatives. In addition, lower levels of thiols and sulfides are also present in this sample, with a total sulfur content of 337 mg S/kg. The FID chromatogram shows the typical hydrocarbon pattern expected for gasoline, aromatic and aliphatic hydrocarbon, and also shows methyl tert-butyl ether (MTBE), the additive used for the oxygenated fuel program. An example of the SCDKID analysis of a reformulated fuel is shown in Fig. 8.9. The SCD chromatogram again shows the thiophenic sulfur compounds, but lower levels of thiols and sulfide, with a total sulfur content of 36 mg S/kg. The FID chromatogram from the reformulated fuel shows MTBE and higher levels of 2,2,4-trimethylpentaneand other iso-paraffins compared with typical gasolines. 8.8.1.3Diesel fuels

The total sulfur content of diesel fuels is also subject to current and pending limitations, with proposed limit of 500 ppm. Some example of the sulfur and hydrocarbon profiles for diesel fuels are shown. The FID and SCD chromatograms from the analysis of NIST Standard Reference Material No. 1624b (Sulfur in Distillate (Diesel) Fuel Oil) are shown in Fig. 8.10. This material is certified to contain 332 f 3 mgkg sulfur and is useful for calibrating the SCD for total sulfur determinations. Benzothiophene, dibenzothiophene and alkyl derivatives of these compounds are the major sulfur components of this sample and most diesel fuels. For comparison, the FID and SCD chromatograms for a low sulfur diesel (26.5 mg S/1) are shown in Fig. 8.1 1 . 8.8.1.4High temperature gas chromatography

High temperature GC is widely used to determine the boiling range of crude oils and petroleum feedstocks. The flame-based SCD permits simultaneous determination of the hydrocarbon and sulfur compounds boiling range distribu-

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219

Gas Chromatograph: Hewlett-Packard Model 5890 Sulfur Detector: Sievers Inst., Inc. Model 3506 SCD'm Column: 30m x 0.32mm 41.1 SPBl FSOT (Supelco, Inc.) Column Temperature: 1 min. @ 35OC to 3OOOC @ 10.0 O h i Carrier Gas: Helium @ 22 psig (3.0 cm3/min.) Special Split Injector: 250°C Split vent flow: 32 cm3/min. FID: 300°C Air @ 39 psig. H, @ 60 psig. Makeup: Air @ 30 cm3/min. Manual Injection: 2.5pL (8.6% to column)

n .

SCD Response

1

loo

o

u

t

!

5

0

15

10

25

20

minutes

FID Response

0

5

10

IS Minutes

Fig. 8.8. SCD and FID chromatogramsof a commercial gasoline. References pp. 22 7-229

20

25

Chapter 8

220

Gas Chromatograph: Hewlett-Packard Model 5890 Sulfur Detector: Sievers Inst., lnc. Model 3508 SCDtm Column: 30m x 0.32mm 4p SPBl FSOT (Supelco, Inc.) Column Temperature: 1 rnin. @ 3 5 O C to 3OOOC @ 10.0 O/min. Carrier Gas: Helium @ 22 psig (3.0 cm3/min.) Special Split Injector: 250°C Split vent flow: 32 cm3/min. FID: 3OOOC Air @ 39 psig. H, @ 60 psig. Makeup: Air @ 30 cm3/rnin. Manual Injection: 2.5pL (8.6%to columnj

SCD Response

O

I

! 3

1

10

IS

20

23

mutes

FID Response

0

1

13

13

mrts

Fig. 8.9. SCD and FID chromatograms of a reformulated gasoline.

20

23

The chemiluminescence detector

22 I

Gas Chromatograph: Varian Model 3410 Sulfur Detector: Seven Inst., lnc. Model 350A SCDm Column: 1Om x 0.53mm 1 . 5 ~DB5 FSOT (J&W Scienbfic) Column Temperature: 0 min. Q 40°C to 4OOOC Q 10.0 Vmin. Carrier Gas: HeliumQ 2 psig (6.0 CmVtnin.) SPI Injector: 100°C - 400°CQ 20*C/min. FID: 400°C Air Q 40 psig. HZQ 35 psig. Makeup: Helium Q 30 cm3/min. Manual Injection: 0.2pL (Hamiiton #lOOl)

60

SCD Response

-

40

-

20

-

0

f c a

Calibration Analysis

.-f

NIST SRM #1624b Sulfur in Distillate (Diesel) Fuel Oil

8

mV

Sulfur Content: 0.332k 0.003 mass YO Density @ 6G°F: 0.8628 g/cm3

0 0

4

8

12

16

20

24

28

Ylnuirr

32 20828021

FID Response

0

4

8

12

20

16

Yinulrr

24

28

32 20828020

Fig. 8.10. SCD and FID chromatograms of NIST SRM No. 1624 sulfur in distillate (diesel) fuels oil. Referencespp. 227-229

222

Chapter 8

~~~~~~~~

~

Gas Chromatograph: Varian Model 3410 Sulfur Detector Sieves Inst.. Inc. Model 350A SCDbn co(LgMI: 10mx0.53mm 1.5~085 FSOT (J&WScienttfic) Column Temperature: 0 min. Q W< to 400% Q 10.0 O/rnin Carrier Gas: Helium Q 2 psig (6.0 & h n . ) SPI Injector: 100°C 400°C @ 2CFC/min. FID: 400°C Air Q 40 psig. Hi Q 35 psig. Makeup: Helium Q 30 crr?hin. Manual Injection: 0.2pL (Hamilton #7001)

-

SCD Response d l Total Sulfur = 28.5 mas&

0

4

8

li

:F

20

24

28

Y in D i e 8

32 20828011

FID Response 60

40

mv 21)

0

0

4

8

12

20

I6 Yiauiev

Fig. 8.1 1. SCD and FID chromatograms of a low sulfur diesel fuel.

24

28

32 2082801 R

The chemiluminescence detector

1

i

I

223

Gas Chromatograph: Varian Model 3410 Sulfur Detector: Sievers Inst., Inc. Model 350A SCDm Column: 10mx0.53mm 1.w DB5 FSOT (Jaw Scientific) Column Temperature: 0 min. Q 400C to 4OOOC Q 10.0 Ohin. Carrier Gas: Helium Q 2 psig (6.0 cm3hin.) SPI Injector: 100C 400°C Q 20"Chin. FID: 400°C Air Q 40 psig. H, Q 35 psig. Makeup: Helium Q 30 c d h i n . Manual Injection: 0.2pL (Hamilton #7001)

-

30

SCD Response

1

20 MV

10

0

4

a

12

16

20

24

28

32

36 ?Oh27081

28

32

35

Ylnulrr

FID Response '0°

mV

50

1

-

0

4

S

12

16

20

24

Yinutea

Fig. 8.12. SCD and FID chromatograms of West Texas intermediate crude oil. References pp. 227-229

20827088

724

Chapter 8

1 Sulfur Detector: Seven Inst.. Inc. Model 350A SCD" j j Column: 10mx0.53mm 1 . 5 ~DB5 FSOT (Jaw Scientific) 1I

Column Temperature: 0 min. Q 40% to 400°C Q 10.0 */min.

~! Carrier Gas: HeliumQ 2 psig (6.0 cm)hin.) ~!SPI Iqector: 100C - 400C Q MoChnin. i j FID: 400°C Air Q 40 psig. I-$Q 35 pslg. ,

'

Makeup: Helium Q 30 c r n h i n . Manual Injection: 0.2pL (Hamilton W o o l )

75

SCD Response

1

511

-

25

-

i

al u

5

Total Sulfur = 4.471 mg/L

mV

FID Response

0

4

8

I2

16

20

24

Y In u t s s

Fig. 8.13. SCD and FID chromatograms of Ordovician crude oil.

28

22

36 20827098

The chemiluminescence detector

225

Gas Chromatograph: Varian Model 3410 Sulfur Detector: Sievers Inst.. Inc. Model 350A SCDm Column: 10m x 0.53mm 1 . 5 ~DB5 FSOT (JawScientific) Column Temperature: 0 min. Q 4OoC to 4OOOC Q 10.0 Ohin. Carrier Gas: HeliumQ 2 psis (6.0 CrrPMn.) SPI injector: i0OC 400°C Q 20"Chin. FID: 400C Air Q 40 psig. H, Q 35 psig. Makeup: Helium Q 30 crrhtnin. Manual Injection: 0.2pL (Hamilton #7001)

-

SCD Response Feedstock to Hydrotreater Unit

'i

Total Sulfur 5 1.813 maS/k

3 -

.I 2 -

1 -

0

4

6

12

16

20

24

Y in "1.

,'

26

1 .

ih

40

41

20621361

I

Product from Hydrotreater Unit

3 .

. I 2. 1

Total Sulfur = 204.8 maSlL

-

0 - 7

Fig 8.14. SCD chromatograms of gas oil before and after hydrotreating. References pp. 227-229

Chapter 8

226

tions. As previously noted, the SCD is susceptible to contamination and loss in sensitivity due to column and septa bleed and this problem is particularly acute in high temperature GC. The use of well-conditioned columns and low bleed septa are required for high temperature GC with SCD detection. With these precautions, the SCD can be used for the simultaneous hydrocarbon and sulfur compound boiling range distribution. Figure 8.12 shows the FID and SCD chromatograms from the analysis of a West Texas Intermediate crude oil. The major sulfur components are dibenzothiophene and its alkyl derivatives. There is also a relative large amount

FID Response

Feedstock to Hydrotreater Unit

15

mY lb

5

- . 4

0

8

12

16

23

24

28

32

36

Yinrtmm

40

44

20828068

Product from Hydrotreater Unit 25

1

o

l 0

4

E

12

16

20

24 Ylnrtmm

28

Fig. 8.15. FID chromatograms of gas oil before and after hydrotreating.

32

36

40

44

20828088

The chemiluminescence detector

227

of higher molecular weight (bp > C30)sulfur compounds present in this sample as indicated by the unresolved ‘‘hump” observed at the end of the chromatogram. An even higher concentration of these high molecular weight sulfur compounds can be seen in Fig. 8.13, which shows the FID and SCD chromatograms from an Ordovician Crude Oil. The total sulfur content of this sample was found to be approximately three times than the West Texas crude and a large percentage of the sulfur compounds are higher molecular weight species. The SCD can also be useful in monitoring refinery processes by measuring the concentrations of sulfur compounds in feedstocks and resultant products. An example of this is shown in Figs. 8.14 and 8.15 for the feedstock and product of a hydrotreating unit. The SCD chromatograms shown in Fig. 8.14 illustrate that significant reduction in the concentrations of sulfur compounds is achieved by this treatment, while the FID chromatograms, Fig. 8.15, indicates that the major hydrocarbon constituents are relative unchanged.

8.9 CONCLUSIONS The high sensitivity, selectivity and linear response of the sulfur chemiluminescence detector have made this instrument a valuable tool for the measurement of sulfur compounds in petroleum and petrochemicals. The equimolar response for different sulfur compounds and the absence of quenching of the response due to co-elution of hydrocarbons permits determination of low ppb levels of sulfur compounds in hydrocarbon matrices and determination of total sulfur content. The SCD has also found applications in the measurement of sulfur compounds for foods, flavor, and beverages, pesticides and other environmental applications, detection of chemical warfare agents, quality control for pharmaceutics and basic chemicals and a host of other areas [27-331.

8.10 ACKNOWLEDGMENTS The author would like to thank Neil Johansen of Sievers Instruments for providing the chromatograms and his assistance in the preparation of the chapter.

8.11 REFERENCES 1 W.L Orr and C.M. White, Geochemistry of Sulfur in Fossil Fuels, American Chemical Society, Washington DC (1 990).

228

Chapter 8

2 ASTM Standard D2622, Annual Book of ASTM Standards, Vol. 05.02, ASTM Philadelphia, PA (1990). 3 ASTM Standard D3246, Annual Book of ASTM Standards, Vol. 05.03, ASTM Philadelphia, PA (1987). 4 ASTM Standard D4045, Annual Book of ASTM Standards, Vol. 05.03, ASTM Philadelphia, PA (1 990). 5 S. S. Brody and J. E. Chaney, J. Gas Chromatogr. 4 (1966) 42. 6 S. 0. Fanvell and C. J. Barinaga, J. Chromatogr. Sci. 24 (1986) 483. 7 M. Dressler, in: Selective Gas Chromatography Detectors, Elsevier, Amsterdam (1986), p. 133. 8 C. Bradley and D. J. Schiller, Anal. Chem. 58 (1986) 3017. 9 J. L. Buteyn and J. J. Kosman, J. Chromatgr. Sci. 28 (1990) 19. 10 T.R. McManus, Anal. Chem. 63 (1991) 48R. 1 1 R.S. Hutte and J.D. Ray, in: Detectors for Capillary Chromatography, Wiley, New York (1992), p. 193. 12 R.C. Hall, J. Chromatogr. Sci. 12 (1974) 152. 13 R.C. Hall, in: Detectors for Capillary Chromatography, 109. 14 P.C. Uden, Y. Young, T. Wang and Z. Cheng, J. Chromatogr. 486 (1989) 319. 15 P.C. Uden, in: Detectors for Capillary Chromatography, Wiley, New York (1992), p. 219 16 B.D. Quimby and J.J. Sullivan, Anal. Chem. 61 (1990) 1027. 17 R.L. Benner and D.H. Stedman, Anal. Chem. 61 (1989) 1268. 18 C.J. Halstead and B.A. Thrush, Proc. R. SOC.London 295 (1966) 363. 19 R.L. Benner, R.L. and D.H. Stedman, Environ. Sci. Technol. 24 (1990) 1592. 20 Operation and Service Manual Model 350B Sulhr Chemiluminescence Detector, Sievers Instruments, Inc. (1990). 21 R.L. Shearer, D.L. O’Neal, R. Rios and M.D. Baker, J. Chromatogr. Sci. 28 (1990) 24. 22 H.V. Drushel and G.D. Dupre, Sulfur compounds in petroleum by GC/SCD: detector evaluationioptimization and BP/TR correlations, presented at the 1991 Pittsburgh Conference. 23 ASTM Committee D-2 Proposed Standard Test Method for the Determination of Sulfur Compounds in Petroleum Gases and Light Liquids by Gas Chromatography and Chemiluminescence Detection. 24 K.J. Bohler, A.J. McCormack and J.M. McCann, Simultaneous detection of aromatics, sulfur and hydrocarbons in diesel fuels by gas chromatography, presented at American Chemical Society Meeting (1991). 25 R.L. Shearer, Anal. Chem. 18 (1992) 2192. 26 Operation and Service Manual Model 355 Sulfur Chemiluminescence Detector, Sievers Instruments, Inc. (1 993). 27 N.G. Johansen, R.S. Hutte and M.F. Legier, in: Monitoring Water in the 1990s: Meeting New Challenges ASTM STP 1102. 28 H.C. Chang and L.T. Taylor, Anal. Chem. 63 (1991) 490. 29 M.S. Burmeister, C.J. Drummond, E.A. Pfister and D.W. Hysert, J. Am SOCBrew. Chem. 50 ( 1992) 53. 30 R.S. Hutte and R.E. Sievers, Selective detection of HD and VX using the sulfur chemiluminescence detector, presented at 47th Southwest Regional American Chemical Society Meeting ( 1991). 3 1 K. MacNamara, Investigation of medium volatile sulfur compounds in whiskey, presented at the Cognac Symposium (1992).

The chemiluminescence detector

229

32 R. Dominguez, Jr., The determination of total sulfur in fuel, landfill and sewage digester gas using the sulfur chemiluminescence detector, presented at American Chemical Society Meeting (1992). 33 R.L. Shearer, E.B. Poole and J.B. Nowalk, J. Chrornatogr. Sci. 31 (1993) 82.