In the Laboratory edited by
Topics in Chemical Instrumentation
David Treichel Nebraska Wesleyan University Lincoln, NE 68504
Analysis of Semivolatile Organic Compounds W in Fuels Using Gas Chromatography–Mass Spectrometry Tal M. Nahir Department of Chemistry, California State University, Chico, CA 95929-0210;
[email protected] Petroleum fuels are complex samples made of hundreds of components with a wide range of sizes and properties. Their analysis is important in industrial applications and because of environmental concerns (1). Recent articles in this Journal have presented detailed procedures for examining mostly volatile compounds such as methanol, ethanol, MTBE, benzene, toluene, and xylenes in gasoline by gas chromatography (2) and mass spectrometry as part of an analytical or environmental laboratory (3, 4). This work focuses on larger organic compounds such as long-chain hydrocarbons and polycyclic aromatic hydrocarbons (PAHs), which have also been isolated and identified in fuels and combustion products (5, 6 ). The laboratory procedure described here focuses on the application of the GC–MS technique to analyze semivolatile molecules in fuels. It is also an attractive complement to a headspace analysis of volatiles in gasoline.1 The results provide an opportunity to investigate aspects of gas chromatography as well as mass spectrometry in an advanced analytical chemistry course. Experimental Procedure Approximately 0.1-µL liquid samples of gasoline or diesel fuels (from a local service station) were injected at a 50:1 split ratio into a Hewlett-Packard HP 6890 GC System connected to a HP 5973 Mass Selective Detector. The column was a HP-5MS (cross-linked 5%-diphenyl–95%-dimethylsiloxane) 30 m × 0.25 mm × 0.25 µm. The temperature program was a single 10 °C/min linear ramp from 50 to 300 °C. Kovats indices were determined from isothermal runs at the specified temperatures. An initial solvent delay of four minutes was included to protect the detector when gasoline samples with volumes larger than 0.1 µL were injected; no solvent delay 10
16
Abundance / 106
8
12 6
20
4
air 2
8
24 5
10
15
20
Results and Discussion Our best results were obtained from diesel fuel, which is composed of compounds heavier than those found in gasoline (7). However, the relatively large molecules are also detectable in gasoline, even though they are present in trace amounts.
Gas Chromatography A typical total-ion chromatogram of a diesel fuel sample is presented in Figure 1. The most notable peaks are those of straight-chain alkanes, which are distinctly separated from each other and other compounds. A wide range of alkanes, from n-hexane (at 1.5 min) to nonacosane (at 24.6 min), could be identified. As expected, the gradually increasing retention times of members in this homologous series are closely related to larger molecular sizes and higher boiling points. In addition to alkanes, many peaks on the chromatogram correspond to polycyclic aromatic hydrocarbons and related derivatives. Generally, PAHs elute faster than alkanes with similar boiling points. This is usually attributed to their higher polarity and weaker interaction with the relatively nonpolar stationary phase. To express the retention of compounds with relation to straight-chain alkanes, we used a retention index that was proposed for programed-temperature chromatography (8): I = 100 n + i
0
0
was necessary for the diesel samples. The volumetric flow rate was set to 1.2 mL/min, which corresponded to an average linear velocity of approximately 40 cm/s; pressures ranged from about 10 (at 50 °C) to 25 (at 300 °C) psi. Identification of compounds in fuel samples was by comparison with measured retention times of standards or by matching the mass spectra to those of compounds in a library database (NIST Mass Spectral Search Program, version 1.6). Naphthalene, biphenyl, phenanthrene, fluoranthene, pyrene, and isooctane were purchased from Aldrich (Milwaukee, WI). Several solutions, each containing a single PAH compound, were prepared in isooctane at approximately 0.1% by weight. One-microliter samples of these standards were analyzed under the same conditions as shown above for the fuel samples, with a two-minute solvent delay. CAUTION: Diesel fuel and gasoline are volatile, flammable, and toxic materials.
25
Time / min
Figure 1. Total-ion chromatogram for a sample of diesel fuel. The numbers indicate the number of carbons in n-alkanes.
t r unknown – t r n tr n + i – tr n
(1)
where n is the number of carbons in the smaller alkane, n + i is the number of carbon atoms in the larger alkane, and tr is the retention time of the specified compound. Unlike the re-
JChemEd.chem.wisc.edu • Vol. 76 No. 12 December 1999 • Journal of Chemical Education
1695
In the Laboratory 3.0
Table 1. Data for Polycyclic Aromatic Hydrocarbons and Related Compounds in Diesel Fuel Primary Molecular Ion(s)
Calcd
Kovats Index
Naphthalene
1191
1190 at 100 °C
128
Methylnaphthalenes
1310
—
115, 142
b
Dimethylnaphthalenes
1425
—
141, 156
Trimethylnaphthalenes
1550
—
155, 170
Biphenyl
1387
1403 at 150 °C
154
Fluorene
1595
1597 at 150 °C
166
Phenanthrene
1797
1838 at 200 °C
178
Methylphenanthrenes
1930
—
192
Dimethylphenanthrenes
2055
—
191, 206
Fluoranthene
2085
2132 at 225 °C
202
2140
2181 at 225 °C
Pyrene
index calculated from the temperature-programed chromatogram according to eq 1. bRetention index from isothermal runs. I values for methylated PAHs are the representative average of several isomers.
tention index suggested by Kovats (9), the value of I in eq 1 will be different depending on the pair of alkanes that are chosen for the calculation. This can be explained by observing the unequal spacing between the alkane peaks in Figure 1. The retention indices in Table 1 were computed using two alkanes: the one immediately before and the one after the peak of interest (i = 1).
Mass Spectrometry Since many PAHs do not break apart to a large extent in the ionization chamber, their major molecular ions have the same mass as the molecule itself. While the mass spectra are very different from those of alkanes, it was nonetheless difficult sometimes to locate these compounds on the chromatogram of a complex mixture such as diesel fuel. This difficulty usually occurred when the abundance of a compound was low or when the peaks of neighboring compounds could not be resolved. To alleviate this problem, the identification of PAHs was done in the single-ion-mode analysis. For instance, phenanthrene was resolved easily on an ion chromatogram at m/z = 178. The quality of spectra in fuel samples was significantly improved by applying a background subtraction. Usually, the spectrum at the baseline of the chromatographic peak was subtracted from the spectrum at the apex of the same peak. Figure 2 shows the marked improvement for a peak that appeared at 17.818 min, after what appears to be alkanerelated background is subtracted. The NIST library identified the background-subtracted spectrum as that of 2,3-dimethylphenanthrene (match quality of 93). Acknowledgments Preliminary work on this project was conducted by Alicia Yashowitz at East Stroudsburg University, PA. The diesel fuel used in this work was a kind donation from Bill Wallace.
2.0 1.5 1.0 0.5 0.0 0
50
100
150
200
250
m/z 3.0
B 2.5
202
aRetention
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I a
Abundance / 104
Compound
A 2.5
2.0
(CH3)2
1.5 1.0 0.5 0.0 0
50
100
150
200
250
m/z Figure 2. Mass spectra of dimethyl phenanthrene. A: At 17.818 min. B: Background-subtracted spectrum of A, with a background spectrum at 17.854 min.
Notes W Supplementary materials for this article are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/Dec/ abs1695.html. 1. 1 µL gasoline vapor, temperature program from 40 to 100 °C at 5 °C/min.
Literature Cited 1. Stout, S. A.; Uhler, A. D.; Naymik, T. G.; McCarthy, K. J. Environ. Sci. Technol. 1998, 32, 260A–264A. 2. Brazdil, L. C. J. Chem. Educ. 1996, 73, 1056–1058. 3. Quach, D. T.; Ciszkowski, N. A.; Finlayson-Pitts, B. J. J. Chem. Educ. 1998, 75, 1595–1598. 4. Kostecka, K. S.; Rabah, A.; Palmer, C. F. Jr. J. Chem. Educ. 1995, 72, 853–854. 5. Kelly, G. W.; Bartle, K. D. J. High Resol. Chromatogr. 1994, 17, 390–397. 6. Lewis, A. C.; Robinson, R. E.; Bartle, K. D.; Pilling, M. J. Environ. Sci. Technol. 1995, 29, 1977–1981. 7. Spiro, T. G.; Stigliani, W. M. Chemistry of the Environment; Prentice Hall: Upper Saddle River, NJ, 1996; p 21. 8. Van den Dool, H.; Kratz, P. D. J. Chromatogr. 1963, 11, 463–471. 9. Harris, D. C. Quantitative Chemical Analysis, 5th ed.; Freeman: New York, 1998; p 681.
Journal of Chemical Education • Vol. 76 No. 12 December 1999 • JChemEd.chem.wisc.edu
