Recent advances in the low voltage mass spectrometric analysis of fossil fuel distillates

International Journal of Mass Spectrometty and Zon Processes, 92 (1989) l-7 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

RECENT ADVANCES IN THE LOW VOLTAGE MASS SPECTROMETRIC ANALYSIS OF FOSSIL FUEL DISTILLATES

*

THOMAS ACZEL and C.S. HSU Exxon Research and Engineering NJ (U.S.A.)

Company, Corporate Research Laboratories,

Clinton,

(Received 6 September 1988)

INTRODUCTION

Since its invention in the late 1950s by Frank Field, S. Hastings and H.E. Lumpkin in the Baytown Research Laboratories of Humble Oil Co. (now Exxon Research and Engineering) low voltage, molecular ion analysis of complex hydrocarbon mixtures has played an important role in the characterization of fossil fuel distillates. Advantages of the low voltage method include limitation of ionization to olefinic, aromatic and polar components to produce aromatic molecular ions, hence greatly simplifying the complex spectra of these materials; good quantitative reproducibility; feasibility of constructing very extensive sets of calibration data from fundamental properties such as aromaticity; and the ease with which the compound type and carbon number distribution yielded by the low voltage analysis can be used to predict a number of physical properties and processing characteristics. Particularly useful in this respect is the combination of low voltage and high resolution capabilities [l-5]. The use of GC/MS in the low voltage mode adds another dimension to the interpretative capabilities of the method [6]. This paper briefly discusses some of the issues and applications involved in the low voltage GC/MS methodology. DISCUSSION

The overall advantages of using GC/MS in the low voltage mode are essentially self explanatory. GC separation enhances the interpretation capabilities of low voltage mass spectrometry in general, and integration of low * Dedicated to Professor Frank H. Field on the occasion of his retirement from Rockefeller University. 0168-1176/89/$03.50

0 1989 Elsevier Science Publishers B.V.

2

TABLE 1 Low voltage compound type analyses by conventional Z

Compound type

mass spectrometry and CC/MS

wt % Conventional mass spectrometry a

6 8 10 12 14 16 18

Benzenes Indanes/tetralins Ind~es/dinaphthenobe~~es Naphthalenes Biphenyls/acenaphthenes Fluorenes Phenanthrenes

Totals

GC/MS

17.0 35.8 4.3 38.2 4.1 0.3 0.3

16.8 36.0 4.3 38.0 4.1 0.4 0.4

100.0

100.0

a CEC Model 21-103C mass spectrometer (sector instrument) b Fimngan TSQ quadrupole mass spectrometer [8].

b

[7].

and high voltage CC/MS is a particularly powerful tool for this purpose. The very good quantitative agreement found between GC/MS and conventional mass spectromet~ at low voltages allows use of these extended capabilities with a high degree of confidence. Issues discussed in some detail include the following: (1) quantitative agreement between GC/MS and conventional MS at low voltages, (2) improved deconvolution of unresolved GC/MS peaks, (3) determination of empirical boiling points, and (4) differentiation between olefins and naphthenes. A major advantage of low voltage GC/MS over conventional low voltage mass spectrometry is the ability to relate composition to boiling range. This can be achieved by boiling point vs. elution time calibration as in GC distillation, by dividing the c~omatogram into desired boiling range segments and then by integrating the low voltage spectra across each of the segments to obtain the information on their composition. The validity of the approach hinges, however, on demonstration of good agreement between conventional MS and GC/MS in the low voltage mode. We found that such an agreement indeed exists, as illustrated by the compound type data reported in Table 1 and the carbon number distribution data shown in Table 2. Improved deconvohtion

of unresolued GC/ MS peaks

CC/MS analysis of middle distillates, 300-8~ o F and higher, from fossil fuels is hindered by the presence of a very large number of components,

3 TABLE

2

Low voltage carbon Compound

number

type

distribution

by conventional Average carbon Conventional mass spectrometry

Benzenes Indanes/tetralins Indenes/dinaphthenobenzenes Naphthalenes Biphenyls/acenaphthenes Fluorenes Phenanthrenes

mass spectrometry number

GC/MS

b

a

12.1= 12.3’ 13.3 12.3 13.8 14.0 14.8

a CEC model 21-103C sector mass spectrometer. b Finnigan TSQ quadrupole mass spectrometer. c Lower value owing to rearrangement peaks believed to be more accurate.

and GC/MS

12.8 12.5 13.5 12.3 13.8 14.1 15.0

(pseudo-molecular

ions).

GC/MS

value is

homologs and isomers, with similar spectra. Most of the GC/MS peaks are unresolved, and might contain as many as ten or more components. This is also true, albeit to a lesser extent, for fractions separated by HPLC. GC/MS spectra at both high and low voltage conditions can lead to unequivocal determination of at least the aromatic molecular ions and carbon number homologs. The typical procedure involves: (1) obtaining high and low voltage spectra at identical operating conditions; (2) matching the retention times of the components (these are generally within 5-10 s in runs with up to 4000 s total elution time (Table 3)); (3) detailed examination of the high voltage spectrum of each peak, or each narrow elution segment (this step elucidates the structures of the individual components within the peak or segment); of the aromatic types using the low voltage spectrum (this (4) deconvolution leads to essentially unequivocal identifications). The procedure is extremely time consuming, as steps (3) and (4) have to be repeated for each peak or segment (300-400+ times in a typical spectrum). Use of an expert program [7] reduces the time and manpower requirements to a minimum. The deconvoluted molecular ions can be easily related to the corresponding fragmentation spectra and, although specific isomeric identifications are often difficult, physical properties can be correlated with structural characteristics. Examples in recent work include differentiation between isomeric compound types, such as indanes and tetralins (C,,H,,_,); indenes and octahydro-phenanthrenes (C,H,,_ rO) biphenyls and acenaphthenes (C,H2,_,,); fluoranthenes and pyrenes (C,H,,_,,) [6].

4

TABLE 3 Retention times of selected components in replicate GC/MS Component

A B C D E F G H

experiments

Retention time (s) High voltage GC/MS

Low voltage GC/MS

1020 1439 1786 2121 2769 3127 3456 4318

1025 1447 1794 2125 2778 3131 3462 4327

This approach combines the complementary capabilities of GC/MS, low voltage molecular ion analysis and high voltage spectra/structure correlations.

Determination of empirical boiling points Mass spectral information can be used to determine the distillation characteristics of complex hydrocarbon mixtures, as well as to predict the composition of hypothetical distillation cuts from the analysis of the wide boiling feedstock. The approach requires the determination of concentrations of individual compounds and the assignment of boiling points or boiling point ranges to each. Individual concentrations of components within narrow boiling point ranges (1-5 o F) are then summed to construct a simulated distillation curve, which essentially plots the summed concentrations against the corresponding boiling ranges. Low voltage GC/MS provides us with an ideal means to determine the empirical boiling points to be used in this context. This is a significant contribution as there are no literature values available for the boiling points of most components likely to be present in fossil fuel streams. A “boiling point” type column is easily calibrated to determine the relationship between retention time and boiling point using a blend of n-paraffins, C,-C,. Mixture components are identified in terms of aromatic carbon number homologs and are assigned boiling points using a simple computerized interpolation routine. In cases of multiple isomers, we calculate both the average boiling point, weighted according to the individual isomeric concentrations and boiling points, and the overall boiling range of a given homolog. The procedure is illustrated in Figs. 1 and 2. Agreement with

Benzene,

Z e s-

176 (vs 176)

mlz 76

Toluene, 229 (vs 231)

ml2

I

I

/ 201

I

I

I

92

I

1

-E E t

C,

276\

EL E s-

I

I

1

mlz 106

I

I

I

350 c

Ih

I

263 (vs 264)

I

303

/

Benzene&

I

C, Benzenes,

I

Retention Time/Boiling

322

ml2

I

120

I

I

Point (‘F) +

Fig. 1. GC/MS results for alkyl benzenes. Mass chromatograms of C,-C,

homologs.

literature boiling points, where available, is very good for olefins and single ring aromatics; condensed ring aromatics elute faster than one would expect from their atmospheric boiling points. This phenomenon is due to the

400

Naphthalene,

I

I

400 (vs 426)

I

m/z 126

I

I

I

I

443

C,, Naphthalenee,

I

445 (vs 469)

I

m/z 142

I

I

1

475 C,, Naphthalenes,

I

C,, Naphthalenes,

I

I

462 m/z 156

I

I Retention Time/Boiling

520

I

I

536

I

I

mlz 170 I,

I

Point (OFI --)

Fig. 2. GC/MS results for naphthalenes. Mass chromatograms of C,,-C,,

homologs.

6

crossing of the vapor pressure/ temperature curves between n-paraffins and condensed ring aromatics at sub-atmospheric pressures (phenanthrene “boils” higher than C,, n-paraffin at 760 torr, but lower at 10 torr) and, in fact, is analogous to what occurs in a vacuum distillation. Differentiation between olefins and naphthenes Olefins and naphthenes (~ycloparaff~s) cannot be reliably differentiated by electron impact mass spectromet~, as they possess identical molecular weights and extremely similar spectra: monolefins = one ring naphthenes diolefins and cyclic olefins = two ring naphthenes triolefins, cyclic diolefins, dicyclic olefins = three ring naphthenes. This uncertainty is resolved by combining data from high and low voltage CC/MS spectra. The approach is based on the fact that olefins have higher low voltage intensities than cycloparaffins. (The voltages used in practice are slightly above the theoretical low voltage values, 10 eV, to increase sensitivity.) As shown by the examples listed in Table 4, the ratio of the low voltage molecular ions vs. the total high voltage io~zation is about ten times as high in olefins and aromatics as in paraffins and cycloparaffins. This fact can be used to distinguish between the two possibilities for individual components well resolved by GC/MS. The use of the separation capabilities of the

TABLE 4 Combination of low voltage, high voltage CC/MS oiefins and naphthenes

allows differentiation between isomeric

Component

Ratio

Benzene Toluene Ethyl-benzene o-Xylene Me-pentene-2 3-Hexene Me-pentadiene 3-Heptene Me-hexadiene 3-Me-pentane Me-cyclopentane Me-cyclohexane Tome-cyclopen~e DiMe-cyclopen~e

11.9 12.1 7.9 12.4 10.1 9.8 9.9 8.9 8.6 0.1 1.7 0.9 0.6 1.0

7

GC/MS is essential in this case, as the approach would not be valid for unresolved components or a total stream. In the latter cases, cycloparaffinic concentrations ten times as high as the olefinic ones would give the same molecular ion to total ionization ratios. The method has been successfully used to complement and corroborate spectral and retention time information, where available, and also on its own. CONCLUSIONS

The examples of applications given in this paper illustrate the great value of the low voltage technique in the detailed analysis of complex fossil fuel streams. It is indeed remarkable that a technique introduced more than 30 years ago is still capable of giving unsurpassed detailed information, as in the case of the high resolution, low voltage approach, and is amenable to new developments, as in the case of the GC/MS applications. ACKNOWLEDGMENTS

Experimental work discussed in this paper was carried out by Ms. Linda Kwiatek and Mr. Gary Deckert. We are also indebted to Dr. Terry Ashe, Imperial Oil Research, Sarnia, Ont., for particularly useful advice. REFERENCES 1 H.E. Lumpkin and T. Aczel, Anal. Chem., 36 (1964) 181. 2 T. Aczel and H.E. Lumpkin, 19th Annual Conf. Mass Spectrometry, Atlanta, GA, May 1971, p. 328. 3 B.H. Johnson and T. Aczel, Anal. Chem., 39 (1967) 682. 4 T. Aczel, Reviews in Analytical Chemistry, Freund Publishing House, Tel-Aviv, Vol. 1, No. 3, (1971) 226. 5 T. Aczel, D.E. Allan, J.H. Harding and E.A. Knipp, Anal. Chem., 42 (1970) 341. 6 C.S. Hsu and T. Aczel, Proc. 36th ASMS Conf., June 1988, San Francisco, p. 326. 7 T. Aczel, S.G. Colgrove and Lan Le, Proc. 35th ASMS Conf., May 1987, p. 377.