Anal. Chem. 1983, 55, 295-297
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Kinetic Ener!gy Release in the Structural Determination of Brominated ICompounds by Gas Chromatography/Mass Spectrometry J. Ronald Hass," Y. londeur, and R. D. Voyksner Laboratory of Environmental Chemistry, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709
The kinetic energy release for a number of aliphatic and aromatlc compounds Is reported. The [MI' -+ [M Br]' and [M Br]' [M 2Br]+ transitions are monitored to determine compound classlficetion and ald in assigning substltution patterns. The ability to dlfferentiate klnetic energy releases of nanogram quantltles olf the gas chromatograph makes this technique applicable for many types of analysis.
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The introduction of combined gas chromatography/mass spectrometry to organic analysis has led to significant progress in numerous areas of research (1-4). By incorporation of high-resolution gas chroimatography (HRGC) and sophisticated computer-controlled data acquisition the analyst has available a reliable elemental composition in addition to the electron impact induced fragmentation pattern of an unknown molecule (5,6) with nanogram quantities of the compound introduced onto the GC column. The use of a specialized high-resolution selected ion monitoring technique makes possible elemental composition measurements of analytes for which picogram quantities can be injected on a gas chromatographic column (7,8). An unambiguous structural assignment can be made by combining the information obtained from the mass spectrometer with that available through a judicious choice of chromatographic columns, assuming that a complete set of reference compounds is available. One can see that even for moderahe molecular weight the complete set of standards is usually unavailable. Consider, as an example, the identification of 2,,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) in enviroinmental samples. Only a few laboratories even claim to have a complete set of the 22 possible TCDD's and the synthetic and characterization procedures reported are certainly lese, than unambiguous (9). Even if all 22 isomers were available in quantity and had been charac terized by unambiguous means, they still fall far short of all reasonable structural forms of CI2H4Cl4O2. Recently, the use of taindem mass analysis (MS/MS) has gained in popularity as a trace analytical tool. From earlier applications involving the irapid screening of complex mixtures for targeted compounds (10, 11) this technique has been combined with GC to provide structural information (12) or as a means of confirmatory analysis (13). In addition to the fragmentation pattern of a mass selected ion, a mass analyzed ion kinetic energy (MIKE) spectrum contains information in the peak shapes (14). In particular, the secondary ions are in fact subjected to a kinetic energy analysis with the m/z of the mass analyzed ion beam being deduced from relative kinetic energies through the application of the law of conservation of energy (15). As the fragmenting ion dissociates, any excess energy in the reaction coordinate and/or the reverse activation energy (if any) will result in a broadening of the peak for that daughter ion. This internal energy, which is converted into translational energy of the products has been termed the kinetic energy release (T) (16) and is a very sen-
sitive probe of the reaction mechanism since it depends upon the structures of the precursor and products, as well as that of the neutral product. For those cases in which the structure of the fragmenting ion is related to that of the neutral molecule, the T value should be a sensitive probe of chemical structure (17-19). This technique has not been applied to trace organic analysis for two principal reasons: sensitivity and sample purity requirements. The T measurements are typically performed on several milligrams of a pure sample which is hardly relevant for trace analysis in complex mixtures. However, the development of combined HRGC/MIKES (20) with the appropriate controlling and data reduction software (21) makes it practical to perform T measurements on samples in complex mixtures. In this paper we report the measurement of T in conjunction with HRGC and evaluate i t as a means of distinguishing isomeric brominated compounds.
EXPERIMENTAL SECTION
AU mass spectrometric measurements were performed on a VG Micromass ZAB-2F instrument. For the T measurements a Finnigan-Incos 2300 data system using previously described custom software (21)was used for data acquisition and processing. Briefly, the magnet is set to transmit the parent ion of interest with a reference sample introduced into the ion source via the direct probe. The unimolecular transistions were observed in the second field-free region which was at 7 X lo4 torr. The computer is then used to scan the electric sector voltage over a small region, sufficient to measure the peak profile, centered in the middle of the fragment ion peak of interest at the rate of 0.4-0.8 s/scan. After direct on-column injection of the sample on a cold 25 M, DB-5 fused silica capillary column (J & W Scientific),the GC oven was programmed from 70 to 150 "C. Data acquisition was commenced after the GC temperature program was under way, and with each scan acquired, the complete peak profile was stored. Postrun the collected ion current profile was examined, and the data over the GC peak were integrated. Thus maximum sensitivity could be achieved without sacrificing GC resolution. This integrated signal was used to determine the T value of the respective reaction. The T values were estimated using standard methods (22, 23) and were corrected for main beam width (24). The reported T values are averages of two to four measurements off the GC, with standard deviations usually less than 10% of the given value. The chemicals were obtained from commercial sources and used without any purification other than the HRGC associated with the measurements.
RESULTS AND DISCUSSION A. Monobromo Compounds. The applicability of T measurements to trace organic analysis has been limited by a lack of sensitivity and the requirement that the sample be pure. The latter consideration can be addressed by simply employing HRGC with a data collection method whose characteristics are similar to those employed in conventional GC/MS. This is illustrated by the reaction C7H7Br+. C7H7+ + Br. for benzyl bromide. The T was measured for this reaction with varying amounts of sample injected on the GC column (Table I). The resulting peak profiles (Figure 1)show acceptable signal/noise ( S I N )ratio when as little as 10 ng is
This article not subject to US. Copyright. Published 1983 by the American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 2, FEBRUARY 1983
Table I. Kinetic Energy Release (Measured at Half Maximum) for the Loss of Br from the Molecular Ion of Benzyl Bromide as a Function of the Quantity Injected on a Gas Chromatographic Column amt,ng
T,meV
amt,ng
T,meV
144 72 36
1.5 1.5 1.4
18 9 1
1.3 1.8 1.3
144 ng
72 ng
Table 11. Kinetic Energy Release and Relative Retention Time for the Isomeric Forms of C,H,Bra compound
T , meV
std dev
RRT
benzyl bromide o-bromotoluene m-bromotoluene p-bromotoluene
1.7 73.2 271 162
0.3 3.7 22 14
1.38 1.00
CH,Br
36 ng
18
1.04
6
ng
a The kinetic energy releases were measured at 50% height for the loss of Br from the molecular ion.
injected. In fact, the T measurement from the 1-ng injection was within experimental error of the values obtained from larger quantities, in spite of the low S I N ratio. The SIN for identical quantities of the other bromotoluenes were better than for the benzyl bromide signal. It can be seen that sensitivity is adequate for many purposes. The T values for the C7H7Br+- C7H7++ Br- transition for the isomeric bromotoluenes and benzyl bromide were determined (Table 11)to provide a measure of the sensitivity of this parameter to molecular structure. Each bromotoluene gives a T value which is significantly different than the others (0rtho:para:meta = 1.0:2.2:3.7). The benzyl bromide, on the other hand, has a T value that is approximately a factor of 40 removed from the closest value. As a comparison to the more commonly used relative retention time (RRT) a~ a means of compound identification, the retention time of benzyl bromide and p-bromotoluene relative to o-bromotoluene was measured to be 1.00:1.04:1.38for 0rtho:para:a. The comparable T ratios are 43:96:1.0. In this example, the T measurement is a more sensitive probe of isomeric structure than RRT, and the same GC information is available in both experiments. In the course of making these measurements, a rather substantial difference in response was noted for these three compounds. They were 2.46.41 for 0rtho:para:a. Thus, if “quantitative ion intensity measurements are compared between conventional GCIMS and GC/T analyses, more structurally significant information is available. B. Dibromo Compounds. The inclusion of more than one bromine atom complicates the identification problem in that it is now possible to have bromotoluenes substituted in either aliphatic or aromatic positions or both. In order to investigate the potential utility of T measurements for structural characterization with compounds containing more than one bromine, we measured the T values for the loss of bromine from the molecular ion for the isomeric bromobenzyl bromides, dibromomethylbenzene, and 2,5-dibromotoluene (Table 111).
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Flgure 1. Peak profile for the loss of bromine from benzyl bromide
at various concentrations off the gas chromatograph. The profile shown is the integral of all scans recorded during the elution of the sample off the GC. As would be predicted from the earlier results, the compound containing only aliphatic bromine is readily distinguished from the compound containing only aromatic bromine. Not surprisingly, the compounds with bromine in both types of position give intermediate results. As before, all compounds have significant differences in their T values indicating once again the potential utility of this parameter for structural assignment. The three dibromobenzene isomers show a much smaller variance in the kinetic energy release with structure. The ortholparalmeta dibromobenzene ratio was 1/1.19/1.28. It would be reassuring to have differences of an order of magnitude or greater so that T measurements could be confidently used for structural assignment in the absence of a complete set of reference compounds. Although that might be possible, insufficient data are available at this time to pass judgement. Using this example, one can see that the measurement of T can give some insight into the chemical structure and thus at least permits a reduction in the number of reference compounds that must be synthesized for positive identification. The monitoring of [ M - Br]+ [ M - 2Br]+ shows little difference in the T values with compound structure. The metastable peak seems to have a structure not dependent on the precursor. The T values are in the 20-35 meV range for all the dibrominated benzenes and toluenes, resulting in little structural information. The loss of two bromines from the
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Table 111. Kinetic Energy Release Measurements for Selected Dibrominated Compounds T, meV T , meV compound ([MI + [M - Br]+ ([M - Br]+ + [M- 2Br]+ dibromomethylbenzene 0-bromobenzyl bromide
m-bromobenzyl bromide p-bromobenzyl bromide 2,B-dibromotoluene 0-dibrornobenzene m-dibromobenzene p-dibromobenzene
2.2 4.9 10.1 7.O 51.3 11.7 25.2 23.4
20.3 21.5 36.6 25.1 31.7 19.8 25.7 21 .o
std dev 0.1
0.5 1.o
0.6 1.7 0.4 1.7 1.5
ANALYTICAL CHEMISTRY, VOL. 55, NO. 2, FEBRUARY 1983
Table IV. Kinetic Energy Release for the Loss of Br from the Molecular Ion of Selected Brominated Compounds compound bromoform 1,2-dibromopropalne 1,2-dibromoethane 2-bromobutane dibromomethme 1-bromohexane 3-bromometh ylheptane
bromobenzene
T,meV
std dev
3.2 7.8 9.8 13.3 15.2 18.0 18.5 24.0
0.4 0.9 1.3
1.3 1.9 0.9 1.1 0.6
molecular ion did not give a measurable signal. The ability to obtain kinetic energy release measurements from a mixture of two or more isomers could prove quite useful in identifying overlapped components in GC peaks or resolve isomers from a direct probe run. The spectrum of a mixture should be the sum of t h e individual components. The deconvolution of this mixture signal with the standard peak should show if the standard is present and its contribution to the overall intensity of the signal recorded. Ratios of 1/5, 213, 312, and 5/1 of 2,5-dibromotoluene to o-bromobenzyl bromide were introduced by direct probe into the mass spectrometer. The spectra of the mixtures were digitized and input into a Radio Shack color microcomputer to perform the deconvolution. The equations found to describe the standards of 2,5-dibromotoluene and o-bromobenzyl bromide are y = 190e - ( 0 . 0 4 ~and ) ~ y = 190e - ( 0 . 1 1 ~ )respectively, ~, were the value 190 is a scaling factor and x is the energy loss in volts. Each mixture was decorivoluted and the ratio of the two Gaussian peaks was obtained. The kinetic energy release of each component in the mixture could be extracted and was in agreement with the standard value. The peak intensity for 2,5-dibromotoluene extracted from the composite signal did vary linearly with the concentration of the components in the mixtures. The slight nonlinearity is attributed to the different rates of distillation of the components off the probe cup. C. O t h e r Bromo Compounds. This investigation was completed by the study of compounds selected to illustrate typical T values for aliphlatic and aromatic systems (Table IV). From the range of values found, we see that the T for the loss of bromine is not of any particular use in the identification of a compound unless it is accompanied by other information. This point illustrates the possible role of such measuremenb in trace organic analysis. That is, after a sample has been analyzed by traditional GC/MS measurements, and as much information extracted as possible from the data, the
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T measurements for carefully chosen fragmentations can quite likely prove useful to answer remaining questions or test structural assignments. Registry No. Benzyl bromide, 100-39-0; o-bromotoluene, 95-46-5; m-bromotoluene, 591-17-3;p-bromotoluene, 106-38-7; dibromomethylbenzene, 618-31-5; o-bromobenzyl bromide, 3433-80-5; m-bromobenzyl bromide, 823-78-9; p-bromobenzyl bromide, 589-15-1; 2,5-dibromotoluene, 615-59-8; o-dibromobenzene, 583-53-9; n-dibromobenzene, 108-36-1; p-dibromobenzene, 106-37-6; bromoform, 75-25-2; 1,2-dibromopropane, 78-75-1; 1,2-dibromoethane, 106-93-4;2-bromobutane, 78-76-2; dibromomethane, 74-95-3; 1-bromohexane, 111-25-1;3-bromomethylheptane, 18908-66-2;bromobenzene, 108-86-1. LITERATURE CITED (1) Tlndall, G. W.; Wlnlnger, P. F. J. Chromatogr. 1980, 196, 109-119. (2) Domino, E. F.; Domino, S. E. J . Chromatogr. 1980, 197, 258-262. (3) Horning, E. C.; Carroll, D. I.; Dzidlc, I.; Stillwell, R. N.; Thenot, J. P. Assoc. Off. Anal. Chem. 1978, 8 1 , 1232. (4) Gates, S. C.; Smisko, M. J.; Ashendel, C. L.; Young, N. D.; Holland, J. R.; Sweeley, C. C. Anal. Chem. 1978, 50,433-441. (5) Melli, J.; Walls, F. C.; McPherron, R.; Burlingame, A. L. J . Chromatogr. Scl. 1979, 17, 29. (6) Hadden, W. F. "High Performance Mass Spectrometry: Chemical Applications", Gross, M. L., Ed.; American Chemical Society: WashIngton, DC, 1978; ACS Symp Ser. No 70, Chapter 6. (7) Harvan, D. J.; Hass. J. R.; Schroeder, J. L.; Corbett, B. J. Anal. Chem. 1981, 53, 1755-1759. (8) HaNan, D. J.; Hass, J. R.; Wood, D. Anal. Chem. 1982, 54,332-334. (9) Burser, H. R.; Rappe, C. Anal. Chem. 1980, 52,2257-2262. ( I O ) Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50,81A-92A. (11) Yost, R. A.; Enke, C. G. Anal. Chem. 1979, 51, 1251A-1264A. (12) Harvan, D. J.; Hass, J. R.; Albro, P. W.; Friesen, M. D. Blomed. Mass Spectrom. 1980, 7 , 242-246. (13) Harvan, D. J.; Hass, J. R.; Schroeder, J. L.; Corbett, B. J. Anal. Chem. 1981, 53, 1755-1759. (14) Elder, J. F.; Cooks, R. G.; Beynon, J. H. Org. Mass Spectrom. 1978, 11 , 423-428. (15) Beynon, J. H.; Fontalne, A. E.; Lester, G. R. Int. J. Mass Spectrom. Ion Phys. 1972, 8 , 341-363. (16) Cooks, R. G.; Beynon, J. H.; Caprloll, R. M.; Lester, G. R. "Metastable Ions"; Elsevier: Amsterdam, 1973. (17) Hass, J. R.; Bursey, M. M.; Levy, L. A,; Harvan, D. J. Org. Mass Spectrom. 1979, 14,319-325. (18) Larka, E. A.; Howe, 1.; Beynon, J. H. Org. Mass Spechom. 1981, 16,
.--.
A-.R.-X A R A
(19) Fiorencio, H.; Vijfhulzen, P. C.; Heerma, W.; Dljlsstra, G. Org. Mass Spectrom. 1979, 14,198-200. (20) Harvan, D. J.; Hass, J. R.; Birch, D. J. S.; 26th Annual Conference Mass Spectrometry and Allied Topics, St. Louis, MO, 1978; pp 105-1 06. (21) Welss, M.; Karnofsky, J.; Hass, J. R.; Harvan, D. J.; 27th Annual Conference on Mass Spectrometry and Allied Topics, Seattle, WA, 1979; pp 270-271. (22) Beynon, J. H.; Cooks, R. G. J. Phys. Educ. 1974, 7 , IO. (23) Cooks, R. G. "Collision Spectroscopy"; Plenum: New York, 1978; p 7. (24) Cooks, R. G.; Beynon, J. H.; Caprlol, R. M.; Lester, G. R. "Metastable Ions"; Elsevier: Amsterdam, 1973; p 60.
RECEIVED for review July 9,1982. Accepted October 25,1982.
