In the Laboratory
Gaseous-Ion Fragmentation Mechanisms in Chlorobenzenes by GC/MS and GC/MS/MS: A Physical-Chemical Approach W for Undergraduates† Steven M. Schildcrout Department of Chemistry, Youngstown State University, Youngstown, OH 44555-3663;
[email protected] The recent availability of inexpensive tabletop computercontrolled gas chromatography–mass spectrometry (GC/MS) instruments has led to reports in this Journal of their incorporation into undergraduate laboratories. This has involved qualitative or quantitative analysis in general, organic, or analytical courses (1). Rarely, however, have these exercises involved student interpretation of the mass spectra, and then the purpose has been to identify unknown compounds (2). In contrast, MS has hardly been used in the undergraduate physical chemistry laboratory (3) with the concomitant approach characteristic of this course. Instead of an analytical approach, which seeks to identify, characterize, or quantitate unknown samples, in physical chemistry we use data for known systems to discover and elucidate underlying fundamental principles and molecular-level properties involving energetics, structure, and mechanism. Here we apply this approach to MS by considering the ion chemistry that occurs in a mass spectrometer, typically unimolecular fragmentation reactions of gaseous, matrix-free ions. We also consider some physical aspects of GC. Using the capabilities of a commercial quadrupole-iontrap GC/MS, we have designed and used for three years an experiment for physical chemistry students in which they use a solution of chlorobenzene (CB), 1,2-dichlorobenzene (o-DCB), and 1,4-dichlorobenzene (p-DCB) to obtain electron-ionization and MS/MS positive-ion mass spectra for each of the three separated components. Although it is not possible in simple magnetic-sector or axial-quadrupole instruments, the ion-trap mass spectrometer (4) permits the MS/MS experiment, whereby a precursor ion (here the fragment resulting from loss of a Cl atom from the molecular ion) is selected, according to its m/z value, to undergo resonance radiofrequency excitation and collision with helium atoms. This produces ion fragments, which are detected, revealing steps in the overall fragmentation mechanism and accounting for the electron-ionization spectrum. There is no previous report of an undergraduate lab experiment involving MS/MS. The experiment, including data manipulation on the computer, is accomplished well within a three-hour lab period. Students become familiar with the operation and capabilities of a major computer-controlled instrument. They interpret the results for some simple chemical systems in terms of the physical-chemical concepts of chromatographic retention times, isotope distributions and average atomic mass, and fragmentation patterns and gaseous ion reaction mechanisms. They are also led to discover two rules of MS, a special case of the nitrogen rule and the even-electron rule. †
Presented in part at the 215th National Meeting of the American Chemical Society, Dallas, TX, March 29–April 2, 1998.
Instrumentation The Finnigan GCQ instrument uses a capillary GC, whose eluents flow directly into the mass spectrometer’s ion source, where electron ionization and ion fragmentation occur. The resulting ions are extracted into the ion-trap region, where appropriate rf fields permit storage of the ions. In the full-scan mode, the ions are resonance-ejected from the trap sequentially according to their m/z values and are detected to give the mass spectrum. In the MS/MS mode, trapped ions with only the chosen m/z value are saved. These are then subjected to resonance rf excitation, imparting kinetic energy that converts to internal energy when they collide with He bath gas. This causes secondary fragmentation, giving an MS/MS spectrum. The instrument control and data processing software are Windows-based. We set the recommended instrumental parameters and store them in method files to be retrieved and confirmed or modified by the students. Interpretation of Results
Gas Chromatography To appreciate the sensitivity of the method, students calculate the mass of each of the three chlorobenzenes injected. Knowing the measured split flow rate, the column dimensions, and the average linear velocity of He, which the instrument controls, they calculate the split ratio and the amount of each component actually passing through the column. By comparing the electron-ionization and the MS/MS chromatograms, they recognize that the latter exclude most impurities, since only selected ions (and their products) are used. From the column length and average linear velocity they can find the carrier-gas hold-up time t M and calculate the adjusted retention time and the retention factor k for each compound. The latter is the ratio of the time the compound spends in the stationary phase to the time in the mobile phase t M. Of more fundamental interest is the distribution constant K c , the ratio of the concentration of a compound in the stationary phase to that in the mobile phase. This is similar to k, but is corrected for the relative volumes of the two phases. From K c, a ∆G ° of adsorption is calculated. Students can then see how tR , boiling point, or K c depends on polarizability (DCBs > CB) and polarity (o-DCB > CB > p-DCB). Mass Spectrometry Neglecting the minor isotopes of C and H, students calculate relative abundances expected for the isotopologs for an ion containing various numbers of Cl atoms using known values for the natural abundances of 35Cl and 37Cl. They use
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In the Laboratory
the electron-ionization mass spectra to decide how many Cl atoms are in the ion giving each major peak. Consistent with the known chemical formulas of the original compounds, they can then assign an unambiguous formula for each major peak in the electron-ionization spectra. They then reverse the calculation and use the observed relative intensities of isotope peaks of any ions containing chlorine to calculate the atomic abundances of the isotopes and the average atomic mass of Cl. The students assign the major peaks observed in the MS/MS spectra. Based on both electron-ionization and MS/MS evidence, they deduce mechanisms for positive-ion fragmentation of CB and for the two DCBs, whose spectra are essentially the same. An abundant fragment ion in an MS/MS spectrum is taken to be an immediate product of the selected precursor ion (4a). Thus, as shown in the scheme below, we confirm that the molecular ion of CB loses mainly neutral Cl followed by C2H2 (5), but the MS/MS spectrum of C6H5+ does not show C4H2+ so the latter must come directly from the molecular ion in a process less facile than Cl loss. Similarly, for DCB, the MS/MS spectrum of m/z 111 shows m/z 75 but not m/z 50. Chlorobenzenes – Cl
C6H5Cl+ M
+
m /z = 112
–
C6 H5 +
– C 2H 2
m/z = 77
C
C4 H3 +
l
C 4 H2 + m /z = 50
Dichlorobenzenes C6H4Cl2+ M
m/z = 146
– Cl
–
C6H4Cl+ m /z = 111
C
– HCl
C6 H 3 + m /z = 75
2H 2C
l2
C4 H2 + m /z = 50
For each ion in the mechanisms, the parity (even or odd) of the m/z value and the parity of the number of electrons in the ion are opposite. Thus students discover a special case (the no-nitrogen case) of the nitrogen rule in mass spectrometry. Considering the mechanisms further, they discover general rules regarding the parity of the number of electrons in a precursor ion and the parities of the respective numbers of electrons in its ionic and neutral products. According to
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Acknowledgments I thank John A. Jackson, whose collaboration was instrumental in our acquiring the funding to purchase the GC/MS, the National Science Foundation (DUE-9551683) for funding towards that purchase, and Howard D. Mettee and our students with whom this exercise was tested. W
Supplemental Material
The following supplemental material for this article is available in this issue of JCE Online: student handout, instructor’s notes, instrumental parameters, and examples and summaries of chromatographic and mass spectrometric results. Literature Cited
m /z = 51
2H 3C
+
the even-electron rule, when an even-electron ion fragments, it produces two even-electron products rather than two radicals, reflecting the greater stability of paired-electron species. This is evident especially for the DCBs, which do not lose two Cl radicals sequentially, as one might naively expect, but instead lose one Cl, which is followed by a rearrangement to eliminate HCl as the main fragmentation pathway.
1. The following are some examples. Amenta, D. S.; DeVore, T. C.; Gallaher, T. N.; Zook, C. M.; Mosbo, J. A. J. Chem. Educ. 1996, 73, 572–575. Rubinson, J. F.; Neyer-Hilvert, J. J. Chem. Educ. 1997, 74, 1106–1108. Rhoads, C. M.; Farquar, G. R.; Wood, W. F. J. Chem. Educ. 1997, 74, 1220–1221. Quach, D. T.; Ciszkowski, N. A.; Finlayson-Pitts, B. J. J. Chem. Educ. 1998, 75, 1595–1598. 2. Eichstadt, K. E. J. Chem. Educ. 1992, 69, 48–51. McGoren, E. C.; Melton, C.; Taitch, D. J. Chem. Educ. 1996, 73, 88–92. O’Malley, R. M.; Hsiao, C. L. J. Chem. Educ. 1999, 76, 1547– 1551. 3. McKay, C. F. In Physical Chemistry—Developing a Dynamic Curriculum; Schwenz, R. W.; Moore, R. J., Eds.; American Chemical Society: Washington, DC, 1993; Chapter 26. Henchman, M.; Steel, C. J. Chem. Educ. 1998, 75, 1042– 1049. 4. (a) de Hoffman, E. J. Mass Spectrom. 1996, 31, 129–137. (b) March, R. E. J. Mass Spectrom. 1997, 32, 351–369. (c) Jonscher, K. R.; Yates, J. R. III Anal. Biochem. 1997, 244, 1– 15. 5. Cody, R. B.; Burnier, R. C.; Freiser, B. S. Anal. Chem. 1982, 54, 96–101. Douglas, D. J. J. Phys. Chem. 1982, 86, 185– 191.
Journal of Chemical Education • Vol. 77 No. 4 April 2000 • JChemEd.chem.wisc.edu
