In the Laboratory
Identification of Glue Vapors Using Electron Impact and Chemical Ionization Modes in GC–MS
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Jeremy Richer, John Spencer, and Michael Baird* Department of Chemistry, Wheeling Jesuit University, Wheeling, WV 26003; *
[email protected] In the following experiment, designed for an instrumental analysis course, students use both electron impact (EI) and chemical ionization (CI) modes of operation in gas chromatography–mass spectrometer (GC–MS) analysis for identifying volatile compounds emitted from various commercial glues. The analysis of glue vapors presented an interesting and challenging lab assignment for a class of 12 students. It represented a common environmental and health issue as vapors from glues can be toxic and harmful to human health. Being inexpensive and readily available, glues are occasionally purchased by people with a chemical dependency for sniffing. The chemical vapors from glues can produce effects similar to anesthetics, which slow down the body’s functions. Toluene and chlorinated hydrocarbons, common vapors from glues, have been linked to liver, kidney, and brain damage (1). In recent GC–MS experiments reported in this Journal (2–16) only EI was used for ionizing the separated compounds. This high-energy process of EI often results in fragmented species, which makes compound identification difficult owing to the absence of the molecular ion. Also, from recent articles in this Journal (2, 4, 7–10, 12, 15, 16), identification was made by computer match of the mass spectrum of the unknown to stored mass spectra in the NIST library. This experiment introduced students to both EI and CI for compound identification. In the “softer” CI ionization mode, the molecular ion, M+, is usually observed in the mass spectrum as (M+1)+ or (M−1)+ ions, depending on the type of reaction (17). In this experiment, students used headspace analysis to sample the vapors emitted from various commercial glues and obtained the mass spectra of major vapor components using both EI and CI modes of GC–MS analysis. Preliminary identification of the mass spectrum from EI operation was made using the NIST Library of mass spectra data. Additional confirmation was provided by the mass spectrum from CI ionization. Final confirmation was made from spectra obtained
from standards that were run in both EI and CI modes of operation. An operational feature of our Varian Saturn 2000 GC–MS enabled both EI and CI operation to be performed within 30 minutes. This capability allowed six student-pairs to complete the analysis in two 3-hour lab sessions. Experimental Procedure
Samples Six commercial glues were purchased from local grocery or drug stores. The names and volatile components of each, as described on the label, are listed in Table 1. Standards The vapors from the following reagent-grade chemicals were used as standards: (a) toluene, n-heptane, isopropanol, and ethyl alcohol purchased from Fisher Scientific, (b) tetrachloroethylene from J. T. Baker Chemical Co., and (c) acetone purchased from Pharmoco. Analysis A Varian Saturn 2000 ion trap GC–MS was used for compound analysis. A small blob of wet glue was placed on the end of a solid-glass rod and inserted into the bottom of a 2-mL glass vial that was immediately capped. A 10-µL syringe was used to sample 5 µL of glue vapors from the headspace in the vial and was introduced into the GC injector that was at 150 ⬚C and with a split ratio set at 10:1. The syringe was cleaned after each injection by using a vacuum to pull a small quantity of methanol through the barrel and needle. The temperature of the air-conditioned room was 20 ⬚C. A 30-m × 0.25-mm fused silica DB-5 capillary column was used. The column oven was heated from 40 ⬚C to 100 ⬚C at a rate of 5 ⬚C兾min with a constant He flow rate of 1.0 mL兾min. The column was held at 100 ⬚C for 5 minutes, however all glue components eluted from the column before 100 ⬚C was reached. The mass selective detector was scanned
Table 1. Toxic Vapors From Glue Samples Sample
Compounds Identified on the Label
Compounds Identified by GC–MS
1
Industrial Strength Graft Adhesive E6000
tetrachloroethylene
toluene (3.35)a tetrachloroethylene (4.0)
2
Aleene’s Platinum Bond 7800 All Purpose Adhesive
tetrachloroethylene
tetrachloroethylene (4.0)
3
Plastic Wood
acetone, n-butyl acetate, isopropanol
acetone (1.50), n-butyl acetate (4.2)
4
Goop Household Contact Adhesive and Sealant
toluene, petroleum distillates
toluene (3.37), petroleum distillates as long chain hydrocarbons
5
Bond-It Foam Glue
acetone, ethanol
acetone (1.55)
6
Elmers Craft Bond Rubber Cement
mixed hexanes
isomers of methyl-subsituted hexane
Glue
a
The number is the retention time in minutes.
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In the Laboratory
Figure 1. TIC chromatograms of vapors from glue 1: (A) EI mode and (B) CI mode.
from 34 to 350 amu. The lower mass setting avoids detection of N2 and O2, but allows CO2 to be detected, which assures that the syringe was not plugged. Since a solvent was not used, the filament delay was set at 0.20 seconds. The mass spectral data of the separated compounds were first acquired while operating the mass spectrometer in the EI mode, then the spectrometer was switched to the CI mode and data were collected. Methane was used as the ionization source, and it took less than 1 minute to change from the EI mode to the CI mode, a specific feature of a Varian GC– MS. The methods used for both operations were predetermined by the instructor. The first lab period was used to train the students on the operation of the GC–MS and to analyze the standards that were chosen from the list of active ingredients given in Table 1. In the second lab period each pair of students ran their assigned glue sample by both the EI and CI modes of operation. The total analysis time for each glue sample (or pair of students) was approximately 30 minutes. Compound identification was made via the NIST Library and from the retention times and mass spectra of the standards. Hazards The vapors from the glues are toxic and the transfer of glue samples to the vials should be done under a hood. The chemical standards are also toxic and gloves must be worn when handling these solutions. The wearing of safety goggles is required. Results and Discussion Example data from the analyses of glue 1 are shown in Figures 1–4. Figure 1 shows the total ion count (TIC) chromatograms from EI and CI analysis of the headspace vapors from glue 1. The two peaks at retention times (RT) of 3.35 and 4.0 minutes in both chromatograms are two volatile compounds from glue 1. As shown in Figure 2, the retention times of the two peaks from glue 1 match the retention times from www.JCE.DivCHED.org
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Figure 2. Components in vapor from glue 1 compared to standards: (A) glue vapor, (B) toluene standard, and (C) tetrachloroethylene standard.
the toluene and tetrachloroethylene standards, respectively. The mass spectra of the first peak (RT = 3.35 min) from the vapor of glue 1 are compared to the mass spectra of the toluene standard in both EI and CI modes of operation in Figure 3. A similar comparison for the second peak (RT = 4.0 min) from the vapor of glue 1 is made in Figure 4. The most abundant molecular ion, M+, for both toluene and tetrachloroethylene are observed in Figures 3 and 4 at m兾z values of 92 and 166, respectively. The utility of CI is illustrated by the (M+1)+ ions of 93 for toluene and 167 for tetrachloroethylene. With the retention time and mass spectrum match from the standards, the compounds in the vapor of glue 1 were identified as toluene and tetrachloroethylene. The small peak at a RT of 1.3 minutes from the CI mode of operation in Figure 1B is CO2 from the atmosphere that was in the headspace volume. The small peak at a RT of 3.8 minutes was identified as an alkane and its mass spectrum was similar to the mass spectrum of the background taken after 3.9 minutes. Each pair of students was required to explain their mass spectra using chemical reactions. For glue 1, the mass spectrum of the first peak (RT = 3.35 min) in Figure 3A was identical to the EI mass spectrum for the toluene standard in Figure 3B and to the EI mass spectrum for toluene from the NIST library. The reactions explaining the mass fragments from Figure 3 are C6H5CH3 + e− *
C6H5CH3+ + 2e−
(1)
which shows the EI ionization of toluene to produce the molecular ion at m兾z = 92, and the fragmentation of the molecular ion to C6H5CH2+ (m兾z = 91): C6H5CH3+
C6H5CH2+ + H•
(2)
The mass fragments in Figure 3 are in agreement with the data reported in the mass spectra reference book of organic
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compounds (16) that shows the base peak (100% intensity) for toluene at m兾z = 91, the parent ion (M+) with m兾z = 92 having an intensity of 68% of the base peak and the (M+1)+ peak being 5.3% intensity. The mass spectrum from CI operation (Figure 3C and D) shows an intense peak at m兾z = 93, which represented the molecular ion (M+1)+ for hydride transfer reactions (17) as a result of chemical ionization using methane gas. In CH4 chemical ionization, high-energy electrons react with CH4 to produce three primary ions (CH4+, CH3+, CH2+). These ions react with methane to produce the following reactive ions: CH4+ + CH4
CH5+ + CH3
(3)
CH3+ + CH4
C2H5+ + H2
(4)
2C3H5+ + 5H2
(5)
2CH2+ + 4CH4
Field and Munson reported (18) the reactive ions and their abundances from CH4 in the CI mode at a total pressure of 1 torr as CH5+ (48%), C2H5+ (41%), and C3H5+ (6%). To produce the molecular ion for toluene, the students postulated the following proton transfer reaction:
Figure 3. Mass spectra of first peak, glue 1: (A) glue, EI mode, (B) toluene, EI mode, (C) glue, CI mode, and (D) toluene, CI mode.
CH5+ + C6H5CH3 m/z = 92
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(6)
The peak at m兾z = 91 represents the (M−1)+ ion and was produced by the following hydride transfer reaction: C2H5+ + C6H5CH3
Figure 4. Mass spectra of second peak in glue 1: (A) glue vapor, EI mode, (B) tetrachloroethylene standard, EI mode, (C) glue vapor, CI mode, and (D) tetrachloroethylene standard, CI mode.
CH4 + C6H5CH4+ m/z = 93
C2H6 + C6H5CH2+ (7)
The second volatile component in the vapors of glue 1 (RT = 4.0 min) was identified as tetrachloroethylene (m兾z = 164), and its mass spectra from EI and CI analyses are shown in Figure 4. The abundances of the mass fragments at m兾z = 164, 166, 168, 170, and 172 are similar to the isotopic abundances for hydrocarbon compounds with 4 chlorine atoms and represent the relative intensities of the molecular ion, C2Cl4+ where the chlorine atoms can be atoms of either Cl35 or Cl-37. The mass fragments at m兾z = 117, 119, 121, and 123 indicate chlorinated compounds of CCl3+ where the abundances of 100, 96, 30, and 3%, respectively, have been reported (19). The mass fragments at m兾z = 63 and 65 appear to be a monochlorinated species. The identified components of vapors from the six glue samples from this experiment are listed in the last column in Table 1. For glue 1, toluene was detected by GC–MS analysis, but was not listed on the label. Isopropanol and ethanol were listed on the labels of glues 3 and 5, respectively, but were not detected by our analyses. The students rationalized that isopropanol and ethanol probably evaporated from the glue during sampling. As for toluene, it is possible the manufacture did not list it on the label. The extensive syringe cleaning between injections ruled out contamination during the sampling process.
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Conclusions This experiment introduced students to a simple method for sampling vapors and to chemical ionization that complemented the common electron impact ionization mode of operation for chemical analysis. The interpretation of both the EI and CI mass spectra gave students an understanding of the processes that take place in mass spectroscopy. This experiment supported the lecture material on GC–MS. W
Supplemental Material
Instructions for the students, notes for the instructor, and data from the experiment are available in this issue of JCE Online. Acknowledgments We are grateful to the National Science Foundation CCLI program (#0088357) and to West Virginia EPSCoR for funding the GC–MS. We also acknowledge the West Virginia NASA Space Grant Consortium for financial support. Literature Cited 1. Glaser, H. H.; Massengale, O. N. J. Am. Med. Assoc. 1962, 181, 301. 2. Mowery, K. A.; Blanchard, D. E.; Smith, S.; Betts, T. A. J. Chem. Educ. 2004, 81, 87–89. 3. Fong, L. D. J. Chem. Educ. 2004, 81, 103–105. 4. Knupp, G.; Kusch, P.; Neugebauer, M. J. Chem. Educ. 2002,
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79, 98–100. 5. Hardee, J. R.; Long, J.; Otts, J. J. Chem. Educ. 2002, 79, 633– 634. 6. Witter, A. E.; Klinger, D. M.; Fan, X.; Lam, M.; Mathers, D. T.; Mabury, S. A. J. Chem. Educ. 2002, 79, 1257–1260. 7. Schultz, E.; Pugh, M. E. J. Chem. Educ. 2001, 78, 944–946. 8. Sodeman, D. A.; Lillard, S. J. J. Chem. Educ. 2001, 78, 1228– 1230. 9. Slawson, C.; Stewart, J.; Potter, R. J. Chem. Educ. 2001, 78, 1533–1534. 10. Bender, J. D.; Catino, A. J., III; Hess, K. R.; Lassman, M. E.; Leber, P. A.; Reinard, M. D.; Strotman, N. A.; Pike, C. S. J. Chem. Educ. 2000, 77, 1466–1468. 11. Wilson, R. I.; Mathers, D. T.; Mabury, S. A.; Jorgensen, G. M. J. Chem. Educ. 2000, 77, 1619–1620. 12. Hodgson, S. C.; Casey, R. J.; Orbell, J. D.; Bigger, S. W. J. Chem. Educ. 2000, 77, 1631–1633. 13. O’Malley, R. M.; Lin, H. C. J. Chem. Educ. 1999, 76, 1547– 1555. 14. Nahir, T. M. J. Chem. Educ. 1999, 76, 1695–1696. 15. Kjonaas, R. A.; Soller, J. L.; McCoy, L. A. J. Chem. Educ. 1997, 74, 1104–1105. 16. Guisto-Norkus, R.; Gounili, G.; Wisniecki, P.; Hubball, J. A.; Smith, S. R.; Stuart, J. D. J. Chem. Educ. 1996, 73, 1176– 1178. 17. Harrison, A. G. Chemical Ionization Mass Spectroscopy, 2nd ed.; CRC Press: Toronto, Canada, 1992; p 15. 18. Field, F. H.; Munson, M. S. B. J. Am. Chem. Soc. 1965, 87, 3289. 19. McLafferty, F. W.; Tureck, F. Interpretation of Mass Spectra, 4th ed.; University Science Books: Sausalito, CA, 1993; p 340.
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