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Anal. Chem. 1986, 58, 2788-2791
to permit their use as standards in isotope dilution analysis. There are several advantages to the approach used. The required starting material benzene-U-13C is readily available and the synthetic procedure (three steps) is brief, minimizing material losses. Since the method of synthesis is similar to the commercial route to PCBs, the congeners present in the isotopically enriched preparation closely mirror those found in commercial preparations. The 13C-enriched PCB mixture itself could be used as an isotopic dilution standard for the analysis of commercial PCB mixtures. Registry No. Benzene-U-13C,32488-44-1; iodobenzene-U-13C, 104130-35-0;biphenyl-U-13C, 104130-36-1;2,2’,3,5,6-pentachlorobiphenyl-U-13C, 104130-37-2;2,2’,3,4,5’-pentachlorobiphenyl-U-13C,104130-38-3; 2,2’,4,5,5’-pentachlorobiphenyl-U-13C, 104130-39-4; 2,3’,4,4’,5-penta~hlorobiphenyl-U-~~C, 104130-40-7; tetrachlorobiphenyl-U-13C,104092-27-5;pentachlorobiphenyl-UI3C, 104092-28-6; hexa~hlorobiphenyl-U--’~C, 104092-29-7; heptachlorobiphenyl- U-13C, 104092-30-0.
LITERATURE CITED (1) Gebhart, J. E.; Hayes, T. L.; Alford-Stevens, A,: Budde, W. L. Anal. Chem. 1985, 5 7 , 2458. (2) Cooper, S. D.: Moseby, M. A,: Pellizzari, E. D. Anal. Chem. 1985, 57, 2469. (3) Slivon, L. E.: Gebhart, J. E.; Hayes, T. L.; Alford-Stevens, A. L.; Budde, W. L. Anal. Chem. 1985, 5 7 , 2464. (4) Swartz, T. R.; Campbell, R. D.; Stalling, D. L.: Little, R. L.; Petty, J. D.; Hogan, J. W.; Kaiser, E. M. Anal. Chem. 1984, 5 6 , 1303.
(5) Voyksner, R. D.; Bursey, J. T.; Pack, T. W.; Porch, R. L. Anal. Chem.
1986, 5 6 , 621. (6) Daves, G. D., Jr.; Smith, R. G.; Valkenburg, C. A. Methods Enzymol. 1983, 9 4 , 48. (7) Bartos, F.; Bartos, D.; Grettie, D. P.; Campbell, R. A,; Marton, L. J.: Smith, R. G.: Daves. G. D., Jr. Biochem. Biophys. Res. Commun. 1977, 75, 915. (8) Mullin, M. D.; Pochini, C. M.; McCrindle, S.; Romkes, M.: Safe, S. H. Environ. Sci. Techno/. 1984, 16, 468. (9) Moron, M.; Sundstrom, G.; Wachtmeister, C. A. Acta Chem. Scand. 1972, 2 6 , 830. (10) Sundstrom, G.;Wachtmeister, C. A. Acta Chem. Scand. 1972, 2 6 , 3816. (11) Sundstrom, G. Bull. Environ. Toxlcol. 1974, 1 1 , 39. (12) Sundstrom, G. Acte Chem. Scand. 1973, 2 7 , 1109. (13) Sundstrom, G. Chem. Commun., Unlv. Stockholm 1974, 10, 1. (14) Sundstrom, G.;Wachtmeister, C. A. PCB Conf., [Collect. Lect.], 2nd, 1972 1973, 73. (15) Hoizumi, K.; Moriya, T. J. Labelled Cornpd. 1974, IO, 499. (16) Brinkman, U. A. T.; Seettz, J. W. F. L.: Reymer, H. G. M. J. Chromatogr. 1976, 116, 353. (17) Krupcik, J.; Kriz, J.; Prusova, D.; Suchanek, P.; Cervenka, Z. J. Chromatogr. 1977, 142, 797. (18) Kaminsky, L. S.;Fasco, M. J. J. Chrornatogr. 1978, 155, 363. (19) Issaq, H. J.; Klose, J.: Muschik, G. M. J. Chromatogr. 1984, 302, 159. (20) Webb, R. G.; McCall, A. J. J. Assoc. Off. Anal. Chem. 1972, 5 5 , 746. (21) Sisson, D.; Welti. D. J. Chromatogr. 1971, 6 0 , 15.
RECEIVED for review April 17,1986. Accepted July 9,1986. The authors thank AT&T Technologies, Inc., for support of this work as a Master of Science Thesis topic and for permission to publish.
Trimethylsilyl Ions for Selective Detection of Oxygenated Compounds in Gasoline by Gas Chromatography/Chemical Ionization Mass Spectrometry Ron O r l a n d o a n d B u r n a b y Munson* Department of Chemistry, University of Delaware, Newark, Delaware 19716
The trhnethylsilyl ions from mixtures of tetramethylsllane with helkm react raplcUy with rknple alcohols and ethers and react slowly, If at ail, with hydrocarbons. Consequently, gas chromatography/chemical lonlzatlon mass spectrometry with tetramethylsllane as the reagent gas constltutes a simple method for the analysis of these compounds In gasoline wlthout prior separation of the oxygenated compounds from the hydrocarbons. The alcohols and ethers can be Identified by their chemical Ionization mass spectra and chromatographic retention times and quantitated by selected ion monitoring at characteristic masses. Quantltatlve analyses were made on gasoline mlxtures containing 0.50-10.0 vol % of C,-C, alcohols and methyl teri-butyl ether with relative standard devlailons of a few percent. Good precfskn requires control of the amount of (CH,)3SIOH,+ In the reagent gas spectrum. The method should be applicable for the analysis of most oxygenated compounds in gasoline.
As the permissible amounts of lead allowed in gasoline are lowered, refiners must find new ways to maintain both production and octane ratings. One possible solution to this problem is the use of short chain alcohols and ethers as additives. The most common of these are methyl tert-butyl ether and tert-butyl alcohol (1). Ethanol can also be used for this purpose (2). Methanol is an attractive blending agent because 0003-2700/86/0358-2788$0 1.50/0
it provides the greatest octane improvement per unit volume ( I ) ; however, problems from phase separation lead to the use of cosolvents such as propanols and butanols ( 3 ) . Several techniques for the analysis of these oxygenated compounds in gasoline blends have been previously reported. The amount of ethanol has been quantitated by near-infrared (4) or infrared (5) spectroscopy. Recently, NMR spectroscopy has been applied to the determination of alcohols in gasoline blends (3). Gas chromatography has often been employed in this analysis (6-8). Methanol, ethanol, and tert-butyl alcohol were extracted into water and then analyzed by gas chromatography while methyl tert-butyl ether was separated from the matrix by adsorption chromatography and then quantitated by GC or LC (6). Multiple column (7) or capillary column (8)techniques have been used to eliminate the initial extraction procedure. In this paper, we report a technique using gas chromatography/chemical ionization mass spectrometry (GC/CIMS) that does not require any sample preparation or prior separation of the blending agents from the hydrocarbons present in gasoline. This method takes advantage of a selective chemical ionization reagent gas that has a high reactivity with compounds containing nucleophilic centers, in this case, alcohols and ethers.
EXPERIMENTAL SECTION The samples (0.01pL) were injected into a Varian Moduline 2740 gas chromatograph, with a direct interface to a Du Pont 492 @ 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13. NOVEMBER 1986
2789
B mass spectrometer that has been modified for CI operation (9). The column was 6 ft X l / * in. Carbopack 1500 to separate the alcohols and ether, Helium was used as the carrier gas at a flow rate of 30 mL/min. The chromatographic operating conditions were as follows: injection port temperature, 190 O C ; detector temperature, 230 OC; oven temperature, 50 "C for the first 3 min and then increased at 8 OC/min to 130 O C . Spectra were obtained approximately every 3 s over the mass range of 50-200 with a Du Pont 492 mass spectrometer having a Hewlett-Packard 21MX computer and a Du Pont data system. The ion accelerating voltage was approximately 1750 V. The source repeller voltage was kept at 0 V to maximize ionic residence times. The reagent gas, tetramethylsilane (Me4Si, Aldrich 99.9+%) was introduced through the CI inlet system while the helium from the gas chromatograph served as the diluent gas. The reagent gas mixture of 25% Me4Si was mixed within the ion source. The total pressure was 0.4 torr, as measured by a capacitance manometer (MKS Instruments, Burlington, MA) through a hollow glass probe connected directly to the ion source. The source temperature was kept at 230 "C. The standard solutions were made by diluting the additives to the desired concentration in gasoline. Standard solutions of the alcohols and ether were also prepared using mixed hexanes as the solvent, and no differences in response or sensitivitieswere found.
RESULTS AND DISCUSSION Tetramethylsilane was introduced as a CI reagent gas because of the high reactivity of the major reagent ion, (CH3)&+, m / z 73, with classes of compounds containing a nucleophilic center. The major products formed for many compounds with this reagent gas are adduct ions, (M + 73)+ (IO). The Me4Si CI spectra of alcohols contain adduct ions, (M 73)+,and (CH3)3SiOH2+ions at m / z 91 for those alcohols with at least two carbon atoms and alkyl ions, as well, for alcohols containing a t least four carbon atoms ( I I , I 2 ) . The ratios of ionic abundances, (M + 73)+/91+ and (M 73)+/R+, in the Me4%CI spectra increase with increasing concentration of the alcohols since the (M 73)' adduct ions are formed by a two-step process (11, 13)
+
+
+
--
+ C,H2,+10H (CH3)&3iOH2+ + ROH
(CH3)3Si+
(CH3I3SiOH2++ C,H2,
(1)
+ 73)+ + H20 (2) The Me4Si CI spectrum of CH30H contains only (M + 73)+ (ROH
adduct ions a t m / z 105. The Me4% CI spectra of simple dialkyl ethers with at least two carbon atoms in one alkyl group contain (M + 73)+ adduct ions, ions resulting from the loss of olefin from these ions, (M + 73 - CxHzx)+,and alkyl ions for C4and higher ethers. The dominant ions in the Me4Si CI spectrum of methyl tert-butyl ether are (M + 73 - C4Hs)+at m / z 105 and C4H9+. The trimethylsilyl ions at m / z 73 are nonreactive with the hydrocarbons in gasoline. No decreases (within &2%) were noted in the ion current a t m / z 73 as the hydrocarbons were separated by the gas chromatograph and passed through the ion source of the mass spectrometer. In addition, the characteristic ions, 91+ for the alcohols and 105' for methanol and methyl tert-butyl ether, are not formed by reactions of 73+ with any of the gasoline components: there were no detectable peaks in a selected ion current trace for m/z 91 and 105 during the separation and analysis of a gasoline sample without the alcohols or methyl tert-butyl ether. Protonated trimethylsilanol or the hydrated trimethylsilyl ion, a t m / z 91, was present to a small extent in all of the spectra because of the reactions of the trimethylsilyl ion with trace amounts of water present in the system. Because of this nonreactivity of trimethylsilyl ions with hydrocarbons, the oxygenated compounds need not be separated from the hydrocarbons for the analysis. The only requirement of the gas chromatographic separation in this analysis is the resolution of the oxygenated compounds. Figure
U
Time
Figure 1. GC and GCICIMS traces for gasoline samples: (a) flame ionization detector (FID) trace of gasoline without additives; (b) FID trace of gasoline with 1.5 vol % of five oxygenated compounds; (c) GCICIMS trace for analysis of oxygenated additives in gasoline. Ion trace for sum of ion currents of 91+, (M + 73)+,and 105'. Key: a. methanol; b, ethanol; c, isopropyl alcohol; d, n-propyl alcohol; e, methyl tert-butyl ether.
1 shows a typical analysis of a gasoline sample. Figure l a shows the FID (flame ionization detector) trace for a sample of gasoline containing none of the additives. Figure l b shows the FID trace of a sample of gasoline containing 1.5 vol % of each of five oxygenated compounds. It is apparent from a comparison of parts a and b of Figure 1 that the alcohols are present but that their quantitation would be difficult because of poor chromatographic resolution. Figure IC demonstrates the ease of detection and quantitation of each additive from the selected ion trace for the sum of the ion currents a t the characteristic masses, 91+ and (M 73)+ for the alcohols and 105' for methanol and methyl tert-butyl ether. The individual alcohols can be identified from the characteristic ion at rnlz 91, the (M + 73)+ adduct ions which give the molecular weights, and the retention times for the alcohols under the chromatographic conditions. The ether can also be identified from the tetramethylsilane CI mass spectrum and the retention time. Linear calibration curves were obtained from several mixtures of these alcohols in gasoline with 5.0 vol % of 2-propanol as an internal standard, using the sum of the ion currents of both 91+ and (M + 73)+ for the C2-C4 alcohols and 105+ for both methanol and methyl tert-butyl ether. One could easily detect small amounts (0.2%) of the alcohols or methyl tertbutyl ether from the selected ion traces with these 0.01-pL sample sizes. Since 91+ is one of the characteristic ions used for quantitation, the larger the amount of 91+ present as a background ion in the reagent gas spectrum (the base line of Figure l ) , the greater the uncertainty in detecting small amounts of alcohols or the higher the detection limits. One
+
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Table I. Replication of GC/CIMS Analysis of Oxygenated Compounds in Gasoline
compund
trial I*
CHSOH CzH50H n-C,H,OH t-C,H,OH MTBEe
5.2 f 0.3 5.1 f 0.2 5.1 f 0.2 5.0 f 0.3 5.1 f 0.2
volume percent mixture 1" trial 2' trial 3d 5.1 f 0.2 5.0 f 0.2 5.0 f 0.2 4.9 f 0.1 5.1 f 0.2
4.9 f 0.2 4.9 f 0.1 5.1 f 0.2 4.8 f 0.3 5.1 f 0.2
mixture 2f
0.48f 0.03 0.56 f 0.05 0.54 f 0.06 0.47 f 0.07
Average value f standard deviation from three analyses, 5.0 vol % of each component; 0.01 g single size. *Three replicate GC/
CIMS analyses in rapid succession. Three replicate GC/CIMS analyses over one day. dThree replicate GC/CIMS analyses in rapid succession, 2 weeks after trial 1. 'MTBE, methyl tert-butyl ether. f Average value f standard deviation from three analyses, 0.50 vol % of each Component; 0.1 ILLsamDle size.
can quantitate these compounds a t the levels of a few tenths of a percent by injecting larger (0.1 rL) samples onto the gas chromatograph. No efforts were made to extend the analyses to very low concentrations of the alcohols or ether. The data of trial 1 of Table I show the short-term reproducibility of three replicate analyses of a mixture of compounds in gasoline obtained in rapid succession. The second trial in this table shows the precision of three analyses taken at widely separated times during the course of one day. The third trial shows the results of triplicate analyses of the same mixture obtained 2 weeks after the first analysis, using the original calibration curve. Both the short-term and the long-term relative precisions of the analyses are approximately the same, within a few percent. Replicate analyses were done with several mixtures of these compounds. The average standard deviation from 29 triplicate determinations of six mixtures containing 1-10 vol % of two to five of these compounds was 10.2. For the same set of experiments, the average value of the ratio of the experimental concentrations to the theoretical concentrations was 1.00 f 0.04. The precision of the analysis for lower concentrations of alcohols or ether can be improved by increasing the size of the sample injected into the gas chromatograph. Table I also shows the results from the analysis of a mixture of 0.50 vol % of four additives using 0.1 KL sample size under the same experimental conditions. For analyses with this sample size, the average standard deviation of 12 triplicate determinations on three mixtures from 0.5 to 2.0 vol '70of each component was f0.05. The average value of the ratio of experimental concentrations to theoretical concentrations was 1.01 f 0.05. One of the experimental parameters that affects the reproducibility of the analysis is the amount of (CH3I3SiOH2+ in the reagent gas spectrum (the background). It is difficult to control the amount of 91+ in the background spectrum because it depends on trace concentrations of water in the source of the mass spectrometer. Figure 2 shows the effect of water (amount of 91+ in the background) on the relative responses for some of the alcohols and methyl tert-butyl ether. The relative response is the integrated area of the selected ion peaks for each component divided by the area of the selected ion peaks for 5.0% i-C3H70H. The same mixture was used for each experiment. The smallest amount of 91' that we were able to achieve routinely was 2-3% of the reagent gas ionization. Small (but undetectable on the pressure gauge) amounts of water were added from the heated oven of the mass spectrometer. No effect of increasing 91+ concentration was observed on the relative responses for ethyl alcohol, isobutyl alcohol, or tert-butyl alcohol because the mechanisms for the formation
0
5
10
20
15
Protonated trimethylsilanol. 91',
25
35
30
X o f reagent spectra
Figure 2. Effect of water on relative responses of oxygenated compounds (2-propanol = 1).
of the 91' and (M + 73)+ ions from these alcohols and from isopropyl alcohol are the same, reactions 1 and 2. The detection limits for these alcohols, of course, increase with increasing 91' in the reagent ion spectrum because of the increasing background correction. One notes, however, that the relative response for methyl alcohol increases significantly with the increasing amount of 91'. A likely explanation for this observation is that the displacement reaction (eq 3) is faster for methanol than the collisionally stabilized adduct formation (eq 4). Consequently, (CH3)3SiOH2++ C H 3 0 H (CH3)3Si+ + C H 3 0 H
+
[ (CH3)3Si+CH30H]* M
-
--
+
(CH3)3Si+CH30H H 2 0
(3) [(CH3)3Si+.CH30H]* (4a) (CH3)3Si'CH30H
(4b)
an increase in the abundance of the protonated trimethylsilanol a t mlz 91 would increase the rate of formation of the methanol adduct of the trimethylsilyl ion. The trimethylsilyl ion reacts with methyl tert-butyl ether to form two major product ions ( C H ~ ) ~ S It+ t-C4HsOCH3
I-CdHs'
(Sa)
(CH3)3SiOHCH3+
(5b)
i:
of which the 105' ion, but not the C4HS+ion, was used for quantitation. It has been shown that trimethylsilyl adduct ions, (CH3)3SiB+,react with many compounds to give different distributions of products than those observed from reactions of (CH3I3Si+(14). Consequently, it is possible that the 91+ ion, (CH3)3SiOH2+, reacts with methyl tert-butyl ether to give more 105' then does (CH3)3Si+,and therefore the relative response increases with increasing abundance of the solvated trimethylsilanol, 91+. This GC/CIMS procedure provides a rapid qualitative and quantitative analysis for simple aliphatic alcohols and ethers in gasoline with no sample preparation or extraction procedures because of the selective reaction of the trimethylsilyl ion with the oxygenated compounds. Other oxygenated compounds, such as ketones, aldehydes, esters, higher alcohols, and other ethers, can also be analyzed because all of these compounds give simple Me$i CI spectra with high sensitivities. The precision and accuracy of the method are satisfactory for routine analysis of complex samples if care is taken to maintain a low level of (CH3)+3iOH2+in the background (reagent gas) spectrum. Registry No. TMS, 75-76-3; MTBE, 1634-04-4;C2H50H, 64-17-5; n-C3H70H, 71-23-8; t-C,H,OH, 75-65-0; CH,OH, 67-56-1.
Anal. Chem. 1986, 58, 2791-2796
LITERATURE CITED (1) Chem. Eng. News 1985, 6 3 , 30. (2) Anderson, E. Chem. Eng. News 1966, 64, 18-19. (3) Renzoni, G. E.; Shankland, E. G.; Gaines, J. A,; Callis, J. B. Anal. Chem. 1985, 57, 2864-2867. (4) Wong, J. L.; Jaseiskis, B. Anawst (London) 1982, 107, 1282-1285. (5) Battiste, D. R.; Fry, S. E.; Whlte, F. T.; Scoggins, M. W.; McWiliiams, T. B. Anal. Chem. 1981, 5 3 , 1096-1099. (6) Pauls, R. E.; McCoy, R. W. J. J . Chromatogr. Sci. 1881, 19,
558-561. (7) Luke, L. A.; Ray, J. E. Analyst (London) 1984, 109, 989-992. (8) Lockwood, A. F.; Caddock, B. D. Chromatographia 1983, 17, 65-68. (9) Spreen, R. C. Ph.D. Thesis, University of Delaware, 1983.
2791
(IO) Odiorne, T. J.; Harvey, D. J.; Vouros, P. J . Phys. Chem. 1972, 7 6 , 3217-3220. (11) Blair, T. A.; Phiiiipou, G.; Bowie, J. H. Aust. J . Chem. 1979, 32, 59-64. (12) Clemens, D.; Munson, 8. Org. Mass Spectrom. 1985, 20, 368-373. (13) Orlando, R.; Munson, 8. Presented at 34th Annual Conference on Mass Spectrometry and Allied Topics, Cincinnati, OH, June 1986. (14) Ciemens. D.; Munson, B. Anal. Chem. 1985, 57, 2022-2027.
RECFJVED for review May 9,1986. Accepted July 3,1986. This work was supported by the National Science Foundation (CHE-8312954).
Gas Chromatography/Fourier Transform Infrared/Mass Spectrometry Using a Mass Selective Detector John R. Cooper, Ian C. Bowater,' and Charles L. Wilkins* Department of Chemistry, University of California-Riverside, Riverside, California 92521
Automated correlatlon of FT-IR and mass selective detector (MSD) reconstructed chromatograms Is demonstrated for a linked GC/FT-IR/MSD system. Interactlve software Is described which compares and tabulates the results from data generated for the analysis of two sample mixtures. Ail compounds of a slmpie 14-component mixture were identified utilizing colncident library search results and molecular ion asslgnments. The analysls of a second mlxture contalnlng 23 components resulted in 17 correct ldentlficatlons, 4 not Identifled, and 2 not detected uslng the automated software.
As a result of continuing advances in analytical instrumentation and computer technology, linked gas chromatography/Fourier transform infrared/mass spectrometry (GC/ FT-IR/MS) systems have become practical alternatives for analysis of organic mixtures. Numerous authors have discussed the value of the complementary information thus available (1-5). Following our initial report of a packed column GC/FT-IR/MS system (61, it was soon demonstrated that use of support-coated open tubular (SCOT) columns was feasible (7).Subsequent work has relied almost entirely upon use of SCOT or wall-coated open tubular (WCOT) columns and has demonstrated the potential generality and analytical utility of the technique (8-11). The use of a linked (vs. discrete) system has been described as the next step in effective nontarget analyses of complex environmental samples to confirm identifications, chemical class, and/or functionality (12-1 3). A major advantage of such a linked multiple detector analytical system is the use of a common chromatographic separation, which facilitates correlation of multiple detector responses for individual mixture components. A reliable, automated method to correlate these multiple detector responses is of substantial importance for computer-aided interpretation of GC/FTIR/MS data. However, the algorithm used in the correlation should not be affected by the potential differences in detector selectivity and sensitivity. For correlation of GC/FT-IR and GC/MS information, a combination of manual peak location and use of a first-order polynomial Present address: Department of Chemistry, Swinburne Institute of Technology, Melbourne, Australia, 3122.
fitting procedure has been used by previous workers to correlate reconstructions for a serially linked GC/IR/MS system (14). In the present work, a somewhat simpler procedure is described. Applications to analyses of a 14- and a 23-component mixture are presented. A sample analysis format that minimizes use of manual data interpretation is described.
EXPERIMENTAL SECTION Instrumentation. A Hewlett-Packard 5970 mass selective detector (MSD) was interfaced via a parallel gas chromatography effluent split using a 27 cm X 0.05 mm i.d. fused silica line heated to 250 "C. The restrictor resulted in a ca. 955 (1R:MS) effluent split ratio. The majority of the GC sample went to the lightpipe of a Nicolet 7199 GC/FT-IR system, which employed an 18 cm X 1.33 mm i.d. gold-coated Pyrex lightpipe maintained at 240 "C. Helium makeup gas was used to bring the total flow rate (column + makeup gas) through the lightpipe to 4 mL/min. Compressed air passed through a heatless regenerative air drier and carbon dioxide scrubber was used to purge the Nicolet FT-IR spectrometer. The 2048-point interferograms were collected with six scans (interferograms) coadded per data file in real time. Each interferogram data file was Fourier transformed (postrun) to yield an 8-cm-' resolution spectrum over the range 745-4000 cm-'. The total acquisition time of each data file was 1.0 s. A narrow band HgCdTe (MCT) detector (D*= 2.5 X 10'O cm Hz'/'/W) with a low frequency cutoff of about 745 cm-' was used. The detector element size was 1 mmz. A Varian 3700 gas chromatograph equipped with a 27 m X 0.33 mm i.d. fused silica column with a 1bm thick methylpolysiloxane liquid phase was used for the separations. The column end was interfaced to the split tee described above. A column head pressure of helium carrier gas was maintained at 0.8 bar to give a flow rate of 2 mL/min at 80 "C. An injector split ratio of 1 0 1 was used for both samples. Temperature programming was used for both separations. The temperature program for the 14-component mixture was 10 "C/min from 80 to 170 "C. For the 23-component mixture a program of 6 "C/min from 90 to 200 "C was used. The MSD was tuned prior to data collection using perfluorotri-n-butylamine (PFTBA) as calibrant to obtain average spectral peak widths of 0.6 amu at half-height. The MSD source temperature was ca. 250 "C. The background pressure was maintained at 2.0 X lo4 torr. Scan ranges of either 29-245 amu or 29-280 amu were used. For the 14-component mixture, MSD scans were collected every 0.71 s. For the 23-component mixture, MSD scans were collected every 0.55 s. Samples. Two mixtures were prepared for evaluation of system performance. Chemicals were purchased from either Aldrich Chemical Co. or Matheson Coleman and Bell and used without
0003-2700/86/0358-2791$01.50/0 0 1986 American Chemical Society
