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Sampling System for Mass Spectrometry of High Boiling Mixtures Using Gas Chromatography-Mass Spectrometry
Joseph E. Schiller* and Curtis L. Knudson Grand Forks Energy Research Center, Grand Forks, N.D. 5820 1
Hydrocarbon type analysis of petroleum derived samples by mass spectrometry has been widely applied ( I , 2). Less volatile materials, such as coal tar and coal liquefaction products, have been studied in an analogous way by both low resolution and high resolution methods (3, 4 ) . Type analysis of coal liquefaction distillates was attempted in our laboratory using a Du Pont 21-491B mass spectrometer. The sample was introduced via the batch inlet a t maximum temperature, 150 "C, but we noted that high molecular weight fragments were absent or of low intensity in the mass spectra obtained under these conditions. These highboiling mixtures require correspondingly high inlet temperatures (typically 300-350 "C) in order that complete vaporization of the sample is assured. The standard batch inlet systems of most mass spectrometers are limited in this respect. Using the mass spectrometer coupled with a Varian Model 2740 gas chromatograph, however, we have achieved successful volatilization of both high and low molecular weight species in such mixtures using the GC inlet. T o do this, the gas chromatographic column was replaced with a 75-ml stainless steel cylinder (Minnesota Valve and Fitting Co., Minneapolis, Minn. 55423) connected to a 1.5-ft X inch stainless steel column packed with uncoated 100-140 mesh Gas Chrom P (Applied Science Laboratories, Inc., State College, Pa.). All other parts of the system were left unchanged, so this modification is as rapid and economical as changing GC columns. Figure 1 shows a diagram of the modified GC inlet. The packed column was used only to cause a pressure drop between the inlet and splitter. A sample injected through the septum and carried into the large dead volume would bleed into the mass spectrometer for several minutes. Figure 2 shows the sample introduction profile as indicated by the FID. A t a mass spectrometer scan rate of 10 sec/decade, spectra could be obtained near the maximum with a negligible change in concentration during a mass scan. If needed, 10 or more mass spectra could have been obtained while the sample was passing into the ion source. At a helium flow of 30 ml/min through the chromatograph, another sample could be run in 20 min. T o confirm that the composition of material entering the ion source of the mass spectrometer was constant during the profile and representative of the whole sample, mass spectra were taken at various times after the sample was injected into the GC. Relative peak heights were identical, within experimental error, for spectra taken 1.2, 2.5, 4.0, and 10 min after injection. These times correspond to before, at, and after maximum sample concentration was obtained. All parts of the GC, MS, and transfer line were maintained a t 250 "C, and the ionizing voltage was 70 volts. Operation a t temperatures of 300 "C or above is not recommended for the Teflon ferrules used in the GC and transfer
Column O v e n
Figure 1. Diagram of modified GC inlet system for the gas chromatograph-mass spectrometer system
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Figure 2. Concentration as a function of time of samples entering the mass spectrometer through the modified GC inlet
line connections. An all-stainless steel GC and transfer system could be operated a t 350-400 "C, however. Samples analyzed were 0.1-111 injections of distillation fractions of hydrogenated solvent refined lignite ( 5 ) (bp 110-190 "C a t 2.5 Torr and 140-200 "C a t 1.0 Torr). These materials vaporized essentially instantaneously, since the FID response (Figure 2) for these samples was the same as when acetone was injected. A sample of bp 200-240 "C a t 1.0 Torr volatilized gradually with the injector port of the GC. For this sample, the FID response reached a maximum after about 10 min, and the sample bled into the MS for a t least 1 hr. I t appears that the least volatile component of a mixture must have a vapor pressure of 5-10 Torr for successful analysis using the system described. No decomposition of these samples was noted, but a metal inlet system may cause difficulty with heteroatom compounds. The standard batch inlet was unsatisfactory for these ANALYTiCAL CHEMISTRY, VOL. 48, NO. 2 , FEBRUARY 1976
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Figure 3. Mass spectra of sample 65-3 with sample introduced via the standard batch inlet and the modified GC inlet T
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Figure 4. Mass spectra of sample 31-3 with sample introduced via the standard batch inlet and the modified GC inlet
Table I. Comparison of Results for High-Boiling Components Sample 3 1 - 2 , 5% Gulf Compound type
Pyrenes Chrysenes Benzopyrenes
Table 11. Results for Standard Addition to Coal Liquids Sample 3 1 - 3 ,
Research
GFERC
Gulf Research
1.6
1.9 1.6 0.2
9.1 3.9 1.9
1.4
0.2
GFERC
Percent added
Percent increase f o u n d
Naphthalene Phenanthrene
6.7
6.0
10.1
11.1
9.5
4.6 2.7
mixtures as shown in Figures 3 and 4. The deficiencies in high molecular weight fragments for the samples introduced through the conventional batch inlet a t 150 "C are obvious. The peak at mle = 202 (pyrene group) ( 4 ) was the base peak in both samples when introduced through the GC, but appeared as only a minor peak in the spectra using the batch inlet. Further, the high mass peaks were relatively much more intense when using the modified GC inlet. Mixtures analyzed using the modified GC inlet gave results for high boiling compound types that agreed with those obtained for the same samples by Gulf Research and Development Company, Pittsburgh, Pa., Table I. The results for samples run using the standard batch inlet were found to be excessive in low molecular weight types, and low in higher boiling compound types. Table I1 shows the results for standard additions of compounds to samples. 454
C o m p o u n d added
The modified GC inlet is being used in our laboratory as a means of sample introduction for type analysis of coal derived liquids.
LITERATURE CITED (1) R. J. Clerc, A Hood, and M. J. O'Neil, Anal. Chem., 27, 868 (1955). (2) S. H. Hastings, Anal. Chem., 28, 1243 (1956). (3) A. G.Sharkey, Jr., J L. Shultz, C. E. Schmidt, and R. A. Friedel, "Mass Spectrometric Analysis of Coal Derived Fuels," Pittsburgh Conference on Analytical Chemistry and ADDlied SDectroscoDy. . . 26th Annual Meetina, Cleveland, Ohio, 1975 (4) J. T. Swansiger, F. E. Dickson, and H. T. Best, Anal. Chem., 46, 730 11974). (5) J. Y. F. Low, et al. "Batch Autoclave Hydrogenation of Solvent Refined Lignite," 169th National Meeting, American Chemical Society, Philadelphia, Pa., April 1975.
RECEIVEDfor review July 21, 1975. Accepted October 14, 1975. References to specific brands, makes, or models of equipment are made for identification only and do not imply endorsement by the Energy Research and Development Administration.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
