Determination of Alkenes in Hydrocarbon Matrices by Acetone

Anal. Chem. 1997, 69, 1550-1556

Determination of Alkenes in Hydrocarbon Matrices by Acetone Chemical Ionization Mass Spectrometry Stilianos G. Roussis* and Jim W. Fedora

Research Department, Products and Chemicals Division, Imperial Oil, Sarnia, Ontario, N7T 8C8 Canada

The use of acetone and acetone-d6 as reagent gases for the selective determination of alkenes in hydrocarbon samples by chemical ionization mass spectrometry (CI MS) is reported. An intense (M + 43)+ or (M + 46)+ adduct ion peak is produced in the acetone or acetoned6 CI mass spectra of alkenes. The same adduct ion is not observed in the acetone or acetone-d6 CI mass spectra of cycloalkanes. This selective formation of the acetyl adduct can be used to differentiate alkenes from cycloalkanes. The suitability of the method to be used for the analysis of alkenes in hydrocarbon mixtures is demonstrated by the analysis of a whole gasoline sample and its olefin extract. The capabilities of the method for quantitative analysis are illustrated by the analysis of a light gas oil sample. Quantitative analysis of alkenes in hydrocarbon samples is very important to the petroleum industry due to the possible adverse effects that olefins might have on the quality of petroleum products. Conventional 70 eV electron ionization (EI) mass spectrometry (MS) cannot differentiate olefins from cycloparaffins because both chemical types have the same molecular weight and chemical formula (isomers), and produce very similar mass spectra. Olefins and cycloparaffins can be simultaneously present in petrochemical samples. An American Society for Testing and Materials (ASTM) standard MS test method1 (ASTM D 2424-67) exists for the characterization of olefins in petroleum samples, but the method requires that the sample contain no cycloparaffins. The most widely used method for the determination of the concentration of olefins in petroleum products is by fluorescent indicator adsorption (FIA) (ASTM D 1319-89).2 However, this method is long and manually intensive and provides only the total volume percent of the olefinic content in the sample. Method ASTM D 1319-89 does not provide information about the distribution (e.g., molecular weight, boiling point, etc.) of the olefins in the sample. Most importantly, the use of method ASTM D131989 is limited to the analysis of low-boiling petroleum fractions (e.g., boiling point lower than 315 °C). Chemical derivatization procedures that involve the conversion of structures containing double bonds to other chemical structures that retain their structural information upon analysis by mass spectrometry or other analytical techniques have been developed in the past.3-10 However, these procedures are often time(1) Method ASTM D 2424-67 is not currently supported by the ASTM committee. (2) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 273-278. (3) Wolff, R. E.; Wolff, G.; McCloskey, J. A. Tetrahedron 1966, 22, 3093-3101.

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consuming and cannot be easily implemented for the routine analysis of olefins. A faster and more sensitive approach is to selectively ionize the olefinic structures in the chemical ionization (CI) source of a mass spectrometer by ion-molecule reactions. Such an approach for the characterization of alkenes has been the reaction of alkenes with Fe+ as reagent ion.11-14 Unfortunately, this technique is not highly suitable for routine analysis due to the long-term adverse effects of the metal reagent ion on the ionization source and the difficulties of the approach associated with gas chromatography/mass spectrometry (GC/MS) analysis.13,14 The use of organic reagent compounds that are less aggressive to the ionization source and the vacuum system would be more suitable for the routine analysis of alkenes in petroleum fractions. Several studies have previously investigated the suitability of different organic reagent ions for the analysis of alkenes by gas phase ion-molecule reactions.15-18 Emphasis has been placed primarily on the capabilities of the different reagent ions to obtain structural information by locating the double bond position on the structure, rather than the differentiation of the alkenes from cycloalkane isomers. The differentiation of the alkenes from cycloalkane isomers is a common problem encountered in the analysis of complex petrochemical samples by mass spectrometry. A selective mass spectrometric method that could differentiate the alkenes from cycloalkanes would be highly desirable. Previous work in our laboratory19 investigated the use of organic reagents for the analysis of alkenes. We found that an acetylating reagent compound reacts in the chemical ionization source of a mass spectrometer with an alkene to generate an intense (M + 43)+ adduct ion. The reaction of an acetylating (4) Niehaus,W. G.; Ryhage, R. Anal. Chem. 1968, 40, 1840-1847. (5) Murata, T.; Ariga, T.; Araki, E. J. Lipid. Res. 1978, 19, 172-176. (6) Kidwell, D. A.; Biemann, K. Anal. Chem. 1982, 54, 2462-2465. (7) Poirier, M. A.; George, A. E. Fuel 1982, 61, 182-184. (8) Cervilla, M.; Puzo, G. Anal. Chem. 1983, 55, 2100-2103. (9) Buser, H.-R.; Arn, H.; Guerin, P.; Rauscher, S. Anal. Chem. 1983, 55, 818822. (10) Schuerch, S.; Howald, M.; Gfeller, H.; Schlunegger, U. P. Rapid Commun. Mass Spectrom. 1994, 8, 248-251. (11) Peake, D. A.; Gross, M. L.; Ridge, D. P. J. Am. Chem. Soc. 1984, 106, 43074316. (12) Peake, D. A.; Gross, M. L. Anal. Chem. 1985, 57, 115-120. (13) Peake, D. A.; Huang, S.-K.; Gross, M. L. Anal. Chem. 1987, 59, 15571563. (14) Gross, M. L. Adv. Mass Spectrom. 1989, 11A, 792-811. (15) Field, F. H. J. Am. Chem. Soc. 1968, 90, 5649-5656. (16) Budzikiewicz, H.; Busker, E. Tetrahedron 1980, 36, 255-266. (17) Chai, R.; Harrison, A. G. Anal. Chem. 1981, 53, 34-37. (18) Einhorn, J.; Kenttamaa, H. I.; Cooks, R. G. J. Am. Soc. Mass Spectrom. 1991, 2, 305-313. (19) Roussis, S. G.; Fedora, J. W. Proceedings of the 42nd ASMS Annual Conference on Mass Spectrometry and Allied Topics; American Society for Mass Spectrometry: Chicago, IL, 1994; p 1164. S0003-2700(96)01210-3 CCC: $14.00

© 1997 American Chemical Society

reagent compound with a cycloalkane structure does not produce a similar (M + 43)+ adduct ion. The preferential formation of the (M + 43)+ adduct by the acetylation reaction can be used to differentiate alkenes from cycloalkanes. This report describes the use of the acetylation reaction to selectively detect acyclic alkenes in petrochemical samples. The experimental procedures used and the results obtained for the analysis of alkenes in typical gasoline and gas oil samples are described below. EXPERIMENTAL SECTION Mass Spectrometer. A JEOL (Peabody, MA) JMS-AX505WA double focusing sector mass spectrometer was used for the experiments. The instrument was tuned for ∼1500 resolution and calibrated in the electron ionization mode using perfluorokerosene (PCR, Inc., Gainesville, FL). The ionization mode was then changed to chemical ionization using acetone or acetone-d6 as reagent compounds. The JEOL mass spectrometer uses a combined EI/CI source. Changing to the CI mode involves the introduction of a pneumatically driven CI chamber into the ionization chamber. CI pressure conditions were achieved by using the direct insertion probe to seal the CI chamber. Upon introduction of the CI reagent compounds, the instrument was retuned for maximum sensitivity. Retuning of the instrument in the CI mode did not affect the mass calibration achieved in the EI mode. The temperature in the chemical ionization source was 230 °C. Reagent Compounds. The reagent compounds (acetone, acetone-d6) were introduced into the ionization source via a heated (90 °C) reservoir inlet. Approximately 20 µL of reagent compound was injected into the reservoir. A needle valve was used to regulate the flow of the reagent compound into the ionization source. The ionization source pressure was monitored by using the pressure readout in the source housing (∼0.6 × 10-5 Torr). Stable chemical ionization conditions, suitable for GC/MS experiments, were obtained for the duration of the experiments (2-3 h). Additional reagent compound was injected in the heated reservoir as required. Sample Introduction. Samples were introduced into a Hewlett-Packard 5890 Series II gas chromatograph coupled with the JEOL JMS-AX505 mass spectrometer. A 30 m × 0.25 mm i.d. J&W Scientific (Folsom, CA) DB-1 fused-silica capillary column with a 0.5 µm film thickness was used for the analysis of the model compounds and the gas oil sample. For the determination of the relative response factors, a mixture of model alkene compounds was prepared and analyzed by programming the GC oven temperature from 50 to 300 °C at a rate of 10 °C/min. The same GC conditions were used for the analysis of the gas oil sample. The gasoline samples were analyzed using a 100 m × 0.25 mm i.d. × 0.5 µm film thickness Supelco Inc. (Bellefonte, PA) Petrocol DH column. The GC oven temperature was programmed in the following three steps: (1) 0 (15 min) to 50 °C (0 min) at a rate of 1 °C/min, (2) 50 (0 min) to 70 °C (0 min) at a rate of 2 °C/min, and (3) 70 (0 min) to 270 °C (0 min) at a rate of 4 °C/min. A sample size of 0.2 µL was injected in the split mode (50:1) into the GC. The injector temperature was 275°C. The GC/MS interface temperature was 280°C. Chemicals. Acetone was purchased from Burdick and Jackson (Muskegon, MI). Acetone-d6 was obtained from Isotec Inc. (Miamisburg, OH). The alkene and cycloalkane standards, obtained from Aldrich Chemical Co. (Milwaukee, WI), were used without further purification. The gasoline and light gas oil samples

Figure 1. Chemical ionization mass spectrum of acetone (acetone used as reagent compound).

Figure 2. Acetone chemical ionization mass spectrum of 2-heptene.

Figure 3. Acetone chemical ionization mass spectrum of 1-octene.

were obtained from Imperial Oil Ltd. Products & Chemicals Division (Sarnia, ON). RESULTS AND DISCUSSION Acetone CI Mass Spectra of Model Compounds. The chemical ionization mass spectrum produced when acetone was used as the reagent compound is shown in Figure 1. Most of the ion current consists of protonated H(CH3COCH3)n+ clusters (n ) 1, 2). The acetyl (m/z 43) and protonated acetone (m/z 101) ions are other major ions in the mass spectrum. These results are in good agreement with the results obtained in previous work.20-22 The acetone CI mass spectra of 2-heptene and 1-octene are shown in Figures 2 and 3. The acetone CI mass spectra of cycloheptane and cyclooctane are shown in Figures 4 and 5. These spectra are used as examples to summarize the observations made in the study of the acetone chemical ionization of (20) MacNeil, K. A. G.; Futrell, J. H. J. Phys. Chem. 1972, 76, 409-415. (21) Hass, J. R.; Nixon,W. B.; Bursey, M. M. Anal. Chem. 1977, 49, 10711073. (22) Vairamani, M.; Siva Kumar, K. V.; Viswanadha Rao, G. K. Org. Mass Spectrom. 1990, 25, 363-367.

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Figure 4. Acetone chemical ionization mass spectrum of cycloheptane.

Figure 5. Acetone chemical ionization mass spectrum of cyclooctane.

model compounds. Intense (M + 43)+ adduct ions are observed in the spectra of the alkenes (Figures 2 and 3), indicating facile acetylation. Intense (M + 43)+ adduct ions are not observed in the acetone CI mass spectra of the cycloalkanes (Figures 4 and 5). Work with other model compounds produced similar results: facile acetylation of the alkenes and no acetylation of the cycloalkane isomers. The tendency of alkenes to undergo facile acetylation by acetone CI has been observed previously.22 The inability of acetylating reagent ions to produce stable acetyl adduct ions of typical alkanes under chemical ionization conditions has been observed in other studies21,23 and has been rationalized on the basis of solution Lewis acid/base chemistry.23 In this work, the selectivity of the acetylation reaction has been used to differentiate alkenes from cycloalkanes in hydrocarbon mixtures. The use of acetone-d6 as reagent compound has the advantage, relative to acetone, of reducing the possibilities of interference from (M - 1)+ ions produced by acetone chemical ionization of alkanes. Upon acetone-d6 chemical ionization, alkene adduct ions are formed that are shifted by 3 mass units. This is illustrated in the acetone-d6 CI mass spectrum of 1-octene shown in Figure 6. The (M + 43)+ adduct ion observed in Figure 3 at m/z 155 is observed at m/z 158 in the acetone-d6 CI mass spectrum (Figure 6). Although possible interference in the acetone CI mass spectra from the formation of (M - 1)+ alkane ions can be generally neglected due to large differences in the relative sensitivities, it can become nonnegligible when the relative concentrations of the alkanes are very high compared to those of the alkenes. Use of acetone-d6 eliminates this interference since the acetylated alkene ions are shifted to an even mass number and the (M - 1)+ alkane ions are still produced at odd mass numbers. Other possible interferences may be due to compounds containing heteroatoms (23) Bursey, M. M.; Elwood, T. A.; Hoffman, M. K.; Lehman, T. A.; Terasek, J. M. Anal. Chem. 1970, 42, 1370-1374.


Figure 6. 1-octene.

Acetone-d6 chemical ionization mass spectrum of

with unshared electron pairs23 and some aromatic compounds22 that tend to undergo acetylation. An approach used to reduce the possibility of interference in the analysis of alkenes is to reduce the complexity of the sample by extracting the aromatic and polar hydrocarbons from the mixture. Alternatively, it is possible to use high-resolution methods to directly separate the acetylated alkene ions from other hydrocarbons without prior extraction procedures. For practical reasons, however, low-resolution mass spectrometry is preferred for the analysis. The main advantages of low-resolution MS over high-resolution MS using a magnetic sector instrument are higher sensitivity and simplicity of operation. The use of acetone-d6 CI, low-resolution MS, coupled with gas chromatography will be demonstrated below by the analysis of alkenes in typical petroleum samples. Relative Sensitivities of Acetylated Alkenes Under Acetoned6 CI MS. Although it is possible to use the ion current measured for a set of selected ions corresponding to different alkenes (carbon number), and compare their relative concentrations in samples of interest, it is useful to convert the measured ion current into weight percent of alkenes in the samples. This can be achieved by predetermination of the relative response factors (sensitivities) of the different analytes in the mixture and by subsequent application of the factors to the measured ion current. In the case of a complex mixture such as petroleum, with the number of possible isomers increasing extraordinarily with molecular weight (or carbon number), it is not possible to obtain response factors for all compounds in the sample. Several simplification assumptions are usually necessary. These are based on the observation of general trends for compounds with similar chemical structures. Several model compounds have been analyzed to address the issue of relative sensitivities of alkenes by acetone chemical ionization. An equimolar mixture of alkene compounds was prepared and analyzed by acetone-d6 CI GC/MS. The relative response factors for the (M + 46)+ adduct ions are given in Table 1. It would be highly desirable to determine the relative response factors of heavier alkene compounds (e.g., carbon number higher than C20). Unfortunately, these heavier alkene compounds are not commercially available. The results obtained for the smaller alkenes provide a useful insight about the general trends of the response factors. It is expected that the relative sensitivity trends continue for the heavier alkene compounds. Table 1 contains the (M + 46)+ ion relative response factors obtained for the analysis of model alkene compounds by acetoned6 CI GC/MS. The results have been normalized to the ion abundance obtained for 1-heptene. The average relative response

Table 1. Relative Response Factors of (M + 46)+ Ion Obtained from the Analysis of Model Alkene Compounds by Acetone-d6 Chemical Ionization Mass Spectrometry compound

rel response factora

std devb

1-heptene 2,4,4-trimethyl-1-pentene 1-nonene 3-nonene 2-methyl-1-octene 2-methyl-2-octene 2,2-dimethyl-1-heptene 1-decene 5-decene 1-dodecene

1.00 1.22 1.04 1.20 0.72 0.65 1.05 0.76 1.20 1.13

0.16 0.09 0.24 0.11 0.04 0.17 0.22 0.13 0.14

a The (M + 46)+ ion abundance of each compound is normalized to the corresponding ion abundance of 1-heptene. b Average of three replicate measurements made on different days over a 2-week period with different instrumental tuning conditions.

Figure 8. Total ion chromatogram of a reference gasoline obtained by acetone-d6 chemical ionization mass spectrometry. The entire mass range is acquired (m/z 33-800) to illustrate the contribution of the chemical ionization plasma ions to the total ion signal during the analysis.

Figure 7. Total ion chromatogram of a reference gasoline obtained by electron ionization mass spectrometry.

factor obtained from all measured compounds is 1.0 with a standard deviation of 0.21. These results indicate that, in the absence of measured relative response factors, unity can be used for the analysis of alkenes by acetone-d6 CI with an error of ∼20%. In the present work, conversion of the measured ion current to weight percent is achieved by assuming unity relative response factors. A larger set of model compounds should provide a more accurate conversion of the measured ion currents to weight percent values. Acetone-d6 CI Analysis of Alkenes in Gasoline. Figure 7 displays the total ion chromatogram of a reference gasoline obtained by electron ionization mass spectrometry. High-resolution chromatographic conditions were employed to ensure maximum peak separation. This analysis is routinely performed using a GC equipped with a flame ionization detector (FID). The objective of this analysis is to obtain a quantitative report on all chemical compounds present in the gasoline. In order to achieve this, the identity of each compound in the gasoline has to be known. This is typically done by using model compounds and retention time indexes to identify the peaks in the chromatogram. Whenever model compounds are not available, mass spectrometry is used to characterize the unknown peaks. However, standard 70 eV electron ionization mass spectrometry cannot differentiate the alkenes from the cycloalkanes. It would be highly desirable if another more selective MS ionization method could be used to differentiate the two compound types. Acetone-d6 chemical ionization mass spectrometry has been used for the detection of alkenes in gasoline. Figure 8 shows

Figure 9. Analysis of C7 alkenes in gasoline: (A) part of total ion chromatogram obtained from the analysis of whole gasoline by EI GC/MS; (B) part of selected ion chromatogram obtained from the analysis of the corresponding gasoline olefin extract by acetone-d6 CI MS; (C) part of selected ion chromatogram obtained from the analysis of whole gasoline by acetone-d6 CI MS.

the acetone-d6 CI total ion chromatogram of the reference gasoline previously analyzed by electron ionization (Figure 7). The total ion chromatogram shown in Figure 8 is significantly simplified due to the selectivity of acetone. A wide mass range (m/z 33800) was acquired to illustrate the contribution of the background chemical ionization plasma ion current to the total ion signal and its stability over the period of the GC/MS analysis. The background ion current due to the CI plasma is considerably stable over the duration of the GC run (∼130 min). Since the background signal is in considerable excess compared to the signal of the analytes, very small quantities of analytes detected Analytical Chemistry, Vol. 69, No. 8, April 15, 1997

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in the experiment are not shown in Figure 8. Selected analyte ion extraction or exclusion of the background ions will show the peaks measured for the weaker analyte concentrations in the sample. To verify the capabilities of acetone-d6 CI MS to selectively detect alkenes in gasoline, a gasoline olefin extract was obtained by modifying the FIA method (ASTM D 1319-89) to permit collection of the olefin fraction. To achieve that, additional time was allowed for complete elution of the olefin fraction from the column. Figure 9A shows a part of the total ion current corresponding to the C7 alkene isomer distribution obtained for the analysis of the whole gasoline by electron ionization MS. No positive identification can be made for the alkenes in the gasoline from the EI GC/MS results. Figure 9B shows the corresponding C7 alkene distribution obtained from the analysis of the olefin extract by acetone-d6 CI MS. Ion m/z 144 (M + 46)+ has been selectively monitored in the acetone-d6 CI GC/MS chromatogram. Due to the selected extraction of the olefins from the gasoline by the modified FIA method, a much better chromatographic separation is obtained in the acetone-d6 CI chromatogram (Figure 9B) compared to the EI chromatogram (Figure 9A). However, extraction of olefins is not always possible for other higher boiling materials (e.g., gas oils, lube oils, etc.) by this method. Direct analysis of the alkenes, without prior extraction, is desired for these higher boiling materials. To examine the capability of acetone-d6 CI to perform this task, the whole gasoline was analyzed and the corresponding C7 ion current was selectively monitored

in the chromatogram (C). An excellent agreement is obtained between the acetone-d6 olefin extract and the acetone-d6 whole gasoline analysis (Figure 9B vs C) demonstrating the suitability of acetone-d6 CI MS for use in the selective detection of alkenes. The S/N ratio is lower in Figure 9C for the direct analysis of the C7 alkene in the whole gasoline compared to the S/N ratio obtained in the analysis of the olefin extract (Figure 9B) for the same total amount introduced into the GC/MS (0.2 µL). This is expected since only a portion of the whole gasoline sample consists of olefins whereas the total olefin extract amount injected into the GC/MS consists only of olefins. Additional examples of the C8 and C9 alkene distributions in gasoline are shown in Figures 10 and 11, respectively. It is successfully demonstrated that acetone-d6 CI MS can selectively detect alkenes in gasoline. The method has been used to validate the assignment of alkenes in the EI GC/MS and FID GC gasoline chromatograms. Acetone-d6 CI Analysis of Alkenes in Gas Oil Samples. The complexity of hydrocarbon samples increases considerably with boiling point. This is illustrated in the EI GC/MS total ion chromatogram obtained for the analysis of a light gas oil sample (Figure 12). The modified FIA method was examined for the extraction of the olefins, but the effort was unsuccessful due to the overlap of the olefin band in the column with other hydrocarbons. In order to decrease the complexity of the sample, a saturated hydrocarbon fraction was obtained by open column chromatography (extraction from silica gel using cyclohexane solvent), which contained the alkenes but not the aromatic and


Figure 12. Total ion chromatogram of a light gas oil sample analyzed by EI GC/MS.

Figure 14. Analysis of C10 alkenes in the saturated hydrocarbon fraction of a light gas oil sample: (A) selected ion chromatogram obtained by EI GC/MS (m/z 140); (B) selected ion chromatogram obtained by acetone-d6 CI GC/MS (m/z 186).

Figure 13. Total ion chromatogram of the saturated hydrocarbon fraction of a light gas oil sample analyzed by acetone-d6 CI GC/MS.

polar compounds. The corresponding total ion chromatogram obtained by acetone-d6 CI GC/MS is shown in Figure 13. Although the chemical complexity of the sample was reduced in the analysis of the saturated hydrocarbon fraction, the chromatographic separation was not improved (Figure 13). The analysis of alkenes in such a complex sample is not possible by conventional mass spectrometric methods. This is demonstrated in Figure 14A, which displays the selected ion chromatogram of the C10 alkene (m/z 140) monitored in the EI GC/MS chromatogram of the light gas oil saturate fraction. The ion signal produced by the alkenes is interfered by the ion signal produced by other hydrocarbons. Additionally, inadequate chromatographic separation does not permit characterization of the alkenes by the use of retention time information. However, detection of the alkenes in the sample is possible by using acetone-d6 CI GC/MS. The selected ion chromatogram of the C10 alkenes is shown in Figure 14B (m/z 186). The corresponding results obtained for the C11 alkenes are shown in Figure 15. Although for a given carbon number, chromatographic separation does not permit the quantitation of the individual alkene peaks, the ion current can be monitored to obtain information about the distribution of the different alkene isomers in the sample. The selected ion chromatograms representing the distributions of the alkenes in the light gas oil saturate fraction are shown in Figure 16. As expected, the average boiling point of the alkenes increases with carbon number. Quantitative analysis of the alkenes in the sample is possible by using the information obtained in relative response factors study for acetone-d6 CI MS. Knowledge of the relative response factors permits the conversion of the measured ion currents to weight percent values. Normalization to the total ion current measured for the alkenes allows the determination of relative

Figure 15. Analysis of C11 alkenes in the saturated hydrocarbon fraction of a light gas oil sample: (A) selected ion chromatogram obtained by EI GC/MS (m/z 154); (B) selected ion chromatogram obtained by acetone-d6 CI GC/MS (m/z 200).

differences between samples in the distributions of the alkenes. The use of external or internal standard calibration procedures permits absolute quantitative analysis. The results obtained using an external standard calibration approach for the analysis of the alkenes in the gas oil are shown in Figure 17. For this type of analysis, long GC/MS experiments are not required. The experiment can be simplified by the introduction of the sample using a batch-type heated inlet. Although no information on the individual alkene carbon number isomer distributions can be obtained using a batch inlet, quantitative information on the alkene distributions Analytical Chemistry, Vol. 69, No. 8, April 15, 1997

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Table 2. Relative Sensitivity of the Analysis of Model Alkene Compounds by Electron Ionization vs Acetone-d6 Chemical Ionization Mass Spectrometry

compound

total peak area ratioa EI:acetone CI

compound

total peak area ratioa EI:acetone CI

2-heptene 1-octene 1-nonene

24:1 21:1 19:1

1-decene 1-undecene 1-dodecene

17:1 20:1 15:1

a Average of triplicate measurements conducted with identical chromatographic conditions and mass spectrometer ion detector gain. The mass ranges m/z 33-350 and 135-350 were scanned for the EI and CI experiments, respectively. The total area was obtained by integration of the TIC signal above the baseline.

Figure 16. Selected ion chromatograms of C10-C22 alkenes in the saturated hydrocarbon fraction of a light gas oil sample obtained by acetone-d6 CI GC/MS.

Figure 17. Weight percent distribution of alkenes in the saturated hydrocarbon fraction of a light gas oil sample obtained by acetoned6 CI GC/MS.

as a function of carbon number can be obtained at a fraction of the GC/MS analysis time. Relative Sensitivity of the Analysis by EI vs Acetone-d6 CI. To evaluate the relative sensitivity of the acetone-d6 CI method vs 70 eV electron ionization, a set of model compounds was analyzed under identical gas chromatographic conditions. To enable direct comparison of the two methods, the mass spectrometer ion detector gain was kept at the same value for both experiments. Table 2 contains a summary of the results obtained from the analysis using the two methods. For the CI experiment, the mass range m/z 135-350 was acquired in order to avoid the contribution of the background signal due to the acetone-d6 CI plasma ions. The total peak area was obtained by integration of the TIC signal above the baseline. Due to the different nature of the two ionization techniques, the comparison provides only an estimate of the relative sensitivities for the analysis of the same compounds. The results obtained by the analysis of the model alkenes (Table 2) indicate that EI is ∼19 times more sensitive 1556 Analytical Chemistry, Vol. 69, No. 8, April 15, 1997

than acetone-d6 CI based on the comparison of the signal obtained from many ions in the mass spectrum. Typical quantitative analysis, however, is done by selective ion monitoring (SIM) of the most intense ion(s) in the spectrum. Therefore, the EI: acetone-d6 CI peak area ratio obtained in Table 2 (∼19:1) represents an upper limit of relative sensitivity ratio when SIM quantitative analysis is performed. CONCLUSION The analysis of olefins has traditionally been a difficult analytical problem. The use of 70 eV EI mass spectrometry, although highly successful for hydrocarbon compound-type analysis, has been very limited in the analysis of olefins due to its inability to distinguish olefins from isomeric cycloalkanes. Olefins are generally more reactive than cycloalkanes and are related to quality issues in a number of petroleum products. Alternatively, when olefins are the major product, undesired cycloalkane byproducts can affect the quality of the olefin product. In this work, the selective acetylation reaction of acetone and acetone-d6 with alkenes has been used to differentiate acyclic alkenes from isomeric cycloalkanes. An intense acetyl adduct (M + 46)+ ion peak is produced in the acetone-d6 CI mass spectra of alkenes but not in the mass spectra of cycloalkanes. The selectivity of the method for the analysis of alkenes has been tested by comparison of results obtained in the analysis of a gasoline olefin extract and the direct analysis of a whole gasoline sample. Excellent agreement was obtained between the two samples by using acetone-d6 CI MS, demonstrating the suitability of the method to be used for the selective analysis of alkenes in hydrocarbon samples. The capability of the method to determine the amount and distributions of alkenes was further demonstrated by the analysis of a light gas oil. Acetone and acetone-d6 CI MS have been demonstrated in this work to enhance the capabilities of 70 eV EI MS to differentiate alkenes from cycloalkanes. The method is simple, reliable, and suitable for routine GC/MS applications. Preliminary investigation for the analysis of linear and cyclic olefins with more than one degree of unsaturation by acetone CI MS has shown that these compounds undergo acetylation; however, the complexity of the mass spectra increases with the degrees of unsaturation. Received for review December 2, 1996. February 5, 1997.X

Accepted

AC961210Q X

Abstract published in Advance ACS Abstracts, March 15, 1997.

Determination of Alkenes in Hydrocarbon Matrices by Acetone

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