In the Laboratory
A New GC-MS Experiment for the Undergraduate Instrumental Analysis Laboratory in Environmental Chemistry: Methyl-t-butyl Ether and Benzene in Gasoline Dinh T. Quach, Nancy A. Ciszkowski, and Barbara J. Finlayson-Pitts* Department of Chemistry, University of California, Irvine, Irvine, CA 92697-2025
Recent additions to the American Chemical Society (ACS) undergraduate curriculum focus on environmental chemistry. This new option is especially relevant to the analytical/ instrumental laboratory, where the analysis of environmental samples can be used to illustrate the theory and application of important techniques such as combined gas chromatography– mass spectrometry (GC-MS). This paper describes a new experiment in an undergraduate instrumental analysis laboratory, in which GC-MS is employed to measure methyl tert-butyl ether (MTBE) and benzene in gasoline. Driven in part by the 1990 Clean Air Act Amendments, reformulated gasolines containing significant levels of oxygenated compounds such as MTBE and ethanol are now in widespread use in the U.S. (1, 2). There have been numerous reports of the analysis of gasoline using GC (e.g., 3, 4), GC-FTIR (5–7 ), and GC-MS (e.g., 8–12). However, relatively few have been directed to the oxygenated additives (3, 5, 10–12). Of these, none are designed specifically for use as a teaching tool for combined GC-MS in the undergraduate curriculum. The experiment reported here was developed to focus on the use of GC-MS in analysis of the highly volatile MTBE as well as benzene, a known carcinogen designated as a hazardous air pollutant (HAP) (13). Organics associated with gasoline use react in the presence of nitrogen oxides to generate ozone and other substances manifested as photochemical smog (14–16 ). Gasoline contributes to photochemical smog formation in two ways: first, by direct evaporation of gasoline vapors into air, and second, through the emission of compounds generated during its combustion. In addition to the contribution of gasoline-associated organics to photochemical smog formation, their toxicity is also of concern (13). New control approaches in the U.S. call for the use of reformulated gasoline in the most polluted areas, in which aromatic and olefinic compounds are reduced and oxygenates such as MTBE, methanol or ethanol are introduced (1, 2). The addition of oxygenated compounds compensates for the reduction in aromatic and olefinic compounds by increasing *Corresponding author.
the octane number. Oxygenates are also believed to decrease CO emissions (1, 2). However, MTBE and methanol, which are also designated HAPs (13), can increase formaldehyde and perhaps NOx emissions as well (1). This experiment focuses on the identification and measurement of some components of gasoline which are HAPs or which lead to ozone formation in air, specifically MTBE and benzene. It illustrates not only the principles of gas chromatography and mass spectrometry, but also the critical role of selected ion monitoring in some analyses. Finally, the application of the method of standard additions for quantitative analysis and the use of internal standards to improve precision are demonstrated. Experimental Procedure A Hewlett Packard benchtop combined GC-MS (G1800A GCD) spectrometer equipped with a capillary column (HP-5, 5% diphenyl–95% dimethylsiloxane, 0.25 mm i.d., 30 m, 0.25 µm film thickness) was used. The column temperature was held at 35 °C for the first minute, then increased from 35 to 60 °C at 2°/min, and finally increased to 200 °C at the rate of 70 °C/min, where it was held for 6 min. Two solvent delays, one from 0 to 1.0 minute and another from 17.5 to 21.9 minutes, were used. The first turned off the detector while air present in the sample eluted. The second at 17.5 minutes turned off the detector while the solvent eluted, starting at 18.4 minutes. Because MTBE and benzene elute so quickly (~1.8 and 2.5 minutes, respectively), a low-boiling solvent could not be used because it would overload the detector by eluting at the same time as the compounds of interest. As a result, a solvent which elutes after MTBE, benzene, and the gasoline components used as internal standards (toluene and o-xylene, in this case) must be used. We chose 1-chlorohexadecane. For qualitative analysis, the gasoline (California Certification Fuel, Lot W-615, California Air Resources Board) was spiked with small amounts of the pure compounds MTBE, benzene, toluene, and o-xylene. A combination of retention
Table 1. Mixtures Used for Analysis of MTBE and Benzene in Gasoline Ratio of Peaks b ± 2σ
Volumea/µL
Soln No.
Gas
73/91
73/106
78/91
78/106
1
750
0
0
0.97 ± 0.14
5.40 ± 0.54
0.124 ± 0.010
0.687 ± 0.050
MTBE Benzene
2
750
30
3
1.27 ± 0.40
7.39 ± 3.66
0.170 ± 0.046
0.988 ± 0.446
3c
750
50
7
1.59 ± 0.22
9.17 ± 1.84
0.261 ± 0.024
1.51 ± 0.22
4d
750
100
10
2.16 ± 0.20
12.9 ± 1.6
0.316 ± 0.022
1.89 ± 0.18
5
750
150
15
2.57 ± 0.28
14.8 ± 1.8
0.400 ± 0.028
2.31 ± 0.20
a1-Chlorohexadecane was added to a total volume of 1.00 mL. bAll ratios are the average of three injections unless otherwise noted. cAverage of six injections. dAverage of five injections.
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(toluene), and 106 (o-xylene) were chosen to monitor these components of gasoline. Figure 3 shows the extracted ion chromatograms for m/z = 73, 78, 91, and 106. In contrast to the total ion chromatogram in Figure 1, the peaks due to the four components of interest can be clearly seen. When multiple peaks occur in the single-ion chromatogram (e.g., for m/z = 91 and 106 in Fig. 3c and 3d, respectively) the component of interest can be identified by the mass spectra. In the case of o-, m-, and p-xylene, spiking the gasoline with the component is necessary for identification of the individual isomers.
Relative Intensity
times and mass spectra was used to identify which peak corresponded to each component. CAUTION: Because benzene is a carcinogen, it must be handled with appropriate precautions, as must the gasoline which contains benzene and other toxic compounds. Of course, this experiment can be modified to focus on less toxic components of gasoline other than benzene. For quantitative analysis, the method of standard additions was used. Five solutions of total volume 1.0 mL (Table 1) were made with 750 µL of gasoline, varying amounts of MTBE (99.9%, Fisher Scientific) and benzene (99.9%, Fisher Scientific), and sufficient 1-chlorohexadecane (95%, Aldrich) to give a total volume of 1.0 mL. The toluene and o-xylene present in the gasoline itself were used as internal standards, since they are present in sufficient concentrations that they can be assumed to be constant (17). The use of sealed vials with septa in the preparation of the standard solutions is crucial to prevent evaporation, especially of the highly volatile MTBE. It is also advisable to make each standard solution immediately before the sample is injected into the GC-MS. Three injections of 0.05 µL were typically carried out for all solutions in Table 1, and mass spectra from m/z = 10–110 were recorded.
Relative Intensity
Relative Intensity
Qualitative Identification of Gasoline Components Figure 1 shows a typical total ion chromatogram for gasoline in 1-chlorohexadecane. At short retention times where MTBE and benzene elute, the various components do not appear as well-resolved peaks using MS detection. However, MTBE, benzene, toluene, and o-xylene can be clearly discerned using the technique of single-ion extraction in the data analysis. This technique is based on using the software to extract intensity–time profiles for selected ions that are characteristic of each compound. Figure 2 shows the mass spectra of authentic samples of each of these species. From these spectra, peaks at m/z = 73 (MTBE), 78 (benzene), 91
Relative Intensity
Results
Figure 1. Total ion chromatogram (TIC) for the gasoline sample in 1-chlorohexadecane (1:10 v/v).
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Figure 2. Reference mass spectra of gasoline components obtained using authentic samples.
Journal of Chemical Education • Vol. 75 No. 12 December 1998 • JChemEd.chem.wisc.edu
In the Laboratory
This portion of the experiment demonstrates the use of a combination of mass spectra and retention times in the identification of various components in the complex gasoline mixture. In addition, the students learn about the fragmentation patterns of various organics. For example, the peak at m/z = 73, due to loss of a –CH3 group during electron impact ionization, is unique to MTBE and hence only one peak occurs in the single ion chromatogram (Fig. 3a). On the other hand, numerous peaks occur at m/z = 91 (Fig. 3c), due to fragmentation of larger aromatics in gasoline. The mass spectra as well as the retention time can be used to identify toluene.
Figure 3. Extracted ion chromatograms for (a) m/z = 73 (MTBE), (b) 78 (benzene), (c) 91 (toluene), (d) 106 (o-xylene) for a typical gasoline sample. Retention times (in min) are shown above the peaks.
Quantitative Measurement of MTBE and Benzene Using the Method of Standard Addition MTBE and benzene in gasoline were quantified by the method of standard additions. Various quantities of MTBE and benzene were added to prepare solutions of gasoline in 1-chlorohexadecane (Table 1). To minimize variations in peak areas due to irreproducibility in sample size, toluene and o-xylene, which are normal components of gasoline present at sufficient levels that they can be assumed to be constant, were used as internal standards (17). As an alternate approach, a standard which is not present in gasoline may be used, such as perchloroethene. For each sample injection, single-ion chromatograms were first developed for m/z = 73, 78, 91, and 106 (Fig. 3). The peak areas at m/z = 73 due to MTBE, 78 due to benzene, 91 due to toluene, and 106 due to o-xylene were measured. The ratios of the peak areas for MTBE and benzene to those for the internal standards, toluene and o-xylene, respectively, were calculated for each injection. Table 1 shows the ratios of these peak areas for each of the solutions: 73/91 and 73/106 for MTBE/toluene and MTBE/o-xylene, respectively, and 78/91 and 78/106 characteristic of benzene/toluene and benzene/o-xylene, respectively. Taking MTBE as an example, the ratios of the MTBE peak to those of the internal standards (RMTBE/STD) (e.g., 73/91 using toluene as the internal standard and 73/106 with o-xylene as the internal standard) were then plotted as a function of the volume of MTBE, VMTBE, added to the mixture. As seen in Figure 4a, these plots were linear, with slope m and intercept b: RMTBE/STD = m VMTBE + b
(1)
It can be shown (a good student exercise) that the ratio of the intercept to the slope of these lines is given by b/m = f MTBE Vgas
Figure 4. (a) Ratio of peak areas due to MTBE and toluene (73/ 91) and MTBE and o-xylene (73/106) as a function of volume of MTBE added to gasoline sample. (b) Ratio of peak areas due to benzene and toluene (78/91) and benzene and o-xylene (78/106) as a function of volume of benzene added to gasoline sample.
(2)
where Vgas is the (constant) volume of gasoline used in each mixture (750 µL in this case) and f MTBE is the volume fraction of MTBE in the gasoline. Hence the volume fraction of MTBE can be obtained from the slope and intercept of plots such as those in Figure 4. Of course, the same analysis applies to benzene (Fig. 4b). Table 2 summarizes the slopes (m) and the intercepts (b) of the data in Table 1 plotted according to eq 1. The volume percent MTBE is 12.1 ± 1.8% (2σ) using toluene as the internal standard and 11.7 ± 2.6% (2σ) using o-xylene. For benzene, the volume percent is 0.87 ± 0.10% (2σ) using toluene and 0.83 ± 0.14% (2σ) using o-xylene as the internal standard. Hence consistent results are obtained using either internal standard. These results are also in excellent agreement
JChemEd.chem.wisc.edu • Vol. 75 No. 12 December 1998 • Journal of Chemical Education
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Table 2. Analysis of MTBE and Benzene Using Two Internal Standards Parameter
MTBE (± 2σ), using
o-Xylene
Toluene
o-Xylene
Slope (m)
0.0109 ± 0.0012
0.0646 ± 0.0102
0.0188 ± 0.0013
0.112 ± 0.011
Intercept (b)
0.992 ± 0.101
5.67 ± 0.86
0.122 ± 0.0115
0.696 ± 0.097
Volume %
12.1 ± 1.8
11.7 ± 2.6
0.87 ± 0.10
0.83 ± 0.14
with the analysis provided with this sample of certification gasoline, 10.9% MTBE and 0.95% benzene (no error estimates provided). The use of internal standards greatly improves the precision of the analysis. For example, for solution #3 in Table 1, the 2σ error of the 73/91 and 78/91 peaks is 14% and 9%, respectively. On the other hand, the corresponding variability in the absolute areas for MTBE and benzene for these injections is ~80%. The poor precision for the absolute peak areas is due to the difficulty in reproducibly injecting very small volumes to avoid overloading the mass spectrometer. Hence this experiment also illustrates the utility of internal standards in improving precision. Discussion This experiment illustrates not only the theory and application of GC-MS to a current environmentally relevant sample, but also the critical need for the use of certain approaches to data analysis for some specific compounds. As seen in Figure 1, the total-ion chromatogram is not adequate for MTBE and benzene analyses, because these substances co-elute with a number of compounds at short retention times. The crucial need for single-ion monitoring or extraction is evident (Figs. 1 and 3). This experiment can also be used to illustrate two different approaches to single-ion analysis. In the case described here, all masses over the chosen mass range were continuously scanned, and the time dependencies of selected ions characteristic of MTBE, benzene, toluene, and o-xylene were extracted from these spectra using software approaches after the experiment. An alternate approach is to monitor only individual selected ions during the run, rather than all ions within a given mass range. Such single-ion monitoring (SIM) results in more time spent monitoring each ion and hence a higher signal-to-noise ratio. Using the SIM mode is advantageous when detecting low levels of components in a sample, which is not the case for gasoline analysis. In this experiment, we have focused on MTBE and benzene, with toluene and o-xylene as internal standards. However, the experiment is easily adapted to focus on other components and internal standards, depending on the particular composition of the available gasoline. For example, ethanol rather than MTBE is used as the oxygenated additive in some areas. Less toxic components than benzene can also be employed. It should be noted that the composition of gasoline varies with the source, season, regional location, and age of the gasoline, so that the results presented here will not be representative of all gasoline samples. Conclusions This experiment demonstrates the application of gas chromatography–mass spectrometry in analytical and en1598
Benzene (± 2σ), using
Toluene
vironmental chemistry to the analysis of oxygenates in gasoline. During the course of the experiment, students learn to interpret chromatograms and mass spectra. Furthermore, they learn the method of standard additions for quantification of chosen components and the use of internal standards to improve precision. They are also introduced to the critical need to use single-ion approaches either through ion extraction methods in the data analysis or through selected ion monitoring during the experiment. Acknowledgments We are grateful for the support of the Camille and Henry Dreyfus Foundation and the University of California, Irvine. We also thank B. Croes, R. Pasek, and R. Susnowitz of the California Air Resources Board (CARB) for helpful discussions and for providing the certified gasoline sample, M. Shu for experimental assistance, J. Osaki of Hewlett Packard for technical support, and A. R. Chamberlin, R. J. Cicerone, and J. C. Hemminger at the University of California, Irvine, for helpful discussions and support of this curriculum development. Literature Cited 1. Calvert, J. G.; Heywood, J. B.; Sawyer, R. F; Seinfeld, J. H. Science 1993, 261, 37. 2. Freed, C. N. In Environmental Fate and Effects of Gasoline Oxygenates, Symposium in the Division of Environmental Chemistry, National Meeting of the American Chemical Society, San Francisco, CA, April 13–17, 1997; Extended Abstracts, Vol. 37, 1997, pp 366–368. 3. Brazdil, L. C. J. Chem. Educ. 1996, 73, 1056. 4. Welch, W. C.; Greco, T. G. J. Chem. Educ. 1993, 70, 333. 5. Diehl, J. W.; Finkbeiner, J. W.; DiSanzo, F. P. Anal. Chem. 1992, 64, 3202. 6. Diehl, J. W.; Finkbeiner, J. W.; DiSanzo, F. P. Anal. Chem. 1995, 67, 2015. 7. Deihl, J. W.; Finkbeiner, J. W.; DiSanzo, F. P. Anal. Chem. 1993, 65, 2493. 8. Ragunathan, N.; Krock, K. A.; Wilkins, C. L. Anal. Chem. 1993, 65, 1012. 9. Kostecka, K. S.; Rabah, A.; Palmer, C. F.; J. Chem. Educ. 1995, 72, 853. 10. Kanai, H.; Inouye, V.; Goo, R.; Chow, R.; Yazawa, L.; Maka, J. Anal. Chem. 1994, 66, 924. 11. Hardman, J. S.; Hill, M. A. W.; Mills, G. A. Fuel 1993, 72, 1563. 12. Teng, S. T.; Williams, A. D.; Urdal, K. J. High Resol. Chromatogr. 1994, 17, 469. 13. Kelly, T. J.; Mukund, R.; Spicer, C. W.; Pollack, A. J. Environ. Sci. Technol. 1994, 28, 378A. 14. Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiley: New York, 1986. 15. Carter, W. P. L. J. Air Waste Manage. Assoc. 1994, 44, 881. 16. Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Science 1997, 276, 1045. 17. Skoog, D. A.; Holler, J. F.; Nieman, T. A. Principles of Instrumental Analysis, 5th ed.; Harcourt Brace: London, 1998.
Journal of Chemical Education • Vol. 75 No. 12 December 1998 • JChemEd.chem.wisc.edu
