R. Martin Smith Wisconsin Dspt. of Justice Crime Laboratory P.O. Box 5708
Madison. WIs. 53705
Edited by Jeanette G. Grasselli
A
m AnalysiibyMassChromatography
Identifying an arson accelerant from the ashes of a suspicious fire is a challenging task. For the past decade, most arson analysts have tried to accomplish this by comparing the gas chromatograms of heated head-space vapors or purge-and-trap extracts of questioned samples with those of known standards (I). But, as happens in many real-life chemical applications, the unknown patterns often did not match any of the standard chromatograms well enough to withstand the scrutiny of courtroom cross-examination. Although several attempts have been made to identify individual components of arson residues by techniques such as gas chromatography/ mass spectrometry (2,3),our own experience has shown that the plethora of GCiMS data produced from most m o n samples often leads to intolerably long analysis times. In addition, the informational content of this data is often dulled by the presence of many of the same hydrocarbons in nearly all petroleum-based accelerants and by the inability of even capillary GC to resolve GC peaks into individual components (4). Mass chromatography helps alleviate these problems. In this technique, 0003-2700/82/A 35 1-1399$01.00/0 0 1982 American Chemical Soclety
the data system of a GC/MS computer eration of full mass spectra for mass chromatography gives it a distinct adsystem sorts through the previously vantage over ita close relative, selected collected mass spectra from a sample for the presence of certain characteris- ion monitoring (SIM). The latter technique forces the analyst to select detic ions, then plots the intensities of . sired ions prior to the sample run, only these ions as a function of time thus precluding the possibility of find(5).The resulting mass chromatoing unanticipated compounds in the grams, which usually show significant sample or of more complete compound deconvolution of overlapping GC peaks, allow rapid detection of specific identification by spectral comparisons compounds in the sample even when at a later time. background interference is intense. For screening arson residues by mass chromatography we chose inSince many arson analyses are “one-shot” injections because of ext e n s e h s in the mass spectra of representative hydrocarbons from famitremely limited sample sizes, the gen-
Table 1. Representative Ions Normally Present in Mass Spectra of Common Accelerants C ” d
d Z
Allphatlcs Alicycli~~ and olefinics Alkylbenzenes
57, 71, 65, 99 55, 69, 83, 97 91,105,119,133
Alkylnaphthalenes Alkylslyrenes and dlhydroindenes Alkylanmracenes Alkylbiphenyls and acenaphthenes
128,142,156,170
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G&a) - I t .ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
13991
24
28
32
36
40 44
48
52 56
60
Time Figure 1. Mass chromatographic screen for various hydrocarbon families from sample 1. Note that each chromatograph has a separate intensity scale
lies of compounds normally present in common accelerants (Table I). Some ions corresponded to individually characteristic molecular ions (aromatic compounds), while others reflected common group fragmentation patterns (aliphatics, alicyclics, and alkylbenzenes). Intensities of individual ions or a sum of the intensities of up to four ions (preferable for preliminary screening) was reflected in the mass chromatogram for a particular compound or family ( 4 ) (Figure 1). For the most part, this “generic” characterization of isomeric compounds, with individual identification of compounds that were either unique for a given sample or were particularly easy to identify (e.g., benzene and naphthalene), appeared to suffice for our purposes. Our first task was to characterize as many accelerants and their evaporated residues as possible. In addition to merely detecting the presence or absence of certain groups or compounds in these samples, our data system supplied with each chromatogram a total ion count for the largest GC peak in the chromatogram. This gave l40OA
*
us a crude method of estimating the relative proportions of constituents as well, which. as it turned out, was important for differentiating between a number of these accelerants. These semiquantitative data have been condensed in Table 11. In general, the overall range and pattern of aliphatic compounds (especially if a series of n-alkanes was present), the ratio of alicyclics or olefinics to aliphatics, the relative amounts of various aromatic compounds, and the presence or absence of special groups or compounds were the most distinguishing features of these data (4). Authentic Arson Residues
As one might expect, the composition of materials obtained in the severe and complex conditions of an actual arson fire often differ suhstantially from standards evaporated in a controlled laboratory environment. Yet the usefulness of this method depended upon its ability to examine samples derived from real-life situations. Identification of specific arson accelerants from actual case samples was not always possible, hut useful chemical in-
ANALYTICAL CHEMISTRY. VOL. 54. NO. 13, NOVEMBER 1982
formation about each ssmple was ohtained hy this method. Authentic samples, collected by local law enforcement officials and transported to us sealed in unlined metal paint cans or glass canning jars, were prescreened hy conventional GC for vapor concentrations. Those lacking sufficient response for GC/MC were subjected to a purge-and-trap device ( 6 ) from which residues were extracted into carbon disulfide and evaporated to appropriate concentrations. GC/MS analyses, performed on a Du Pont DP-102 GC/MS equipped with a capillary injection system and a 30-m WCOT SP-2100 capillary column, provided the mass chromatographic data from the representative samples given in Table 111. Samples Containing Gasoline. Sample 1,the residue from a crack in the cement floor of a burned building, shows the power of this procedure. The total ion chromatogram of this sample was ambiguous. Although the overall pattern did not match that of high-boiling petroleum fractions such as fuel oil or diesel fuel, the volatility range seemed much too high for that of evaporated gasoline (Figure 2). Mass chromatographic screening for common hydrocarhon families (Figure l),and especially for individual aromatic compounds (Figure 3), showed intense concentrations of aromatics, particularly the alkylnaphthalenes (naphthalene itself was the largest peak in the total ion chromatogram). In addition, the range of aliphatic hydtocarhons and the unusually high concentrations of alkylhiphenyls and acenaphthenes (not shown in Table 111) strongly supported the presence of highly evaporated gasoline. Actually the small quantities of low-boiling aromatics and aliphatics seen in this sample (and normally absent from highly evaporated gasoline) were accounted for hy postulating a mixture of partially and highly evaporated gasoline. These conclusions had been indicated, hut not strongly supported, hy conventional GC alone. On the other hand, sample 2, charred carpeting from an office storage room, was readily identified hy conventional GC as essentially unevaporated gasoline. Mass chromatographic analysis completely confirmed this, showing patterns scarcely distinguishable from those of the standard (compare Tables I1 and 111). Suspicious Samples. Sample 3, standard charred carpeting from the same room as sample 2, shows most clearly the dangers possible when interpreting arson GC data unsupported by other analyses. While the total ion chromatogram from sample 2 exhihited all of the characteristics of gaso-
Efficient 15Nstudies by NMR Low gamma nuclei are often difficult to detect directly because of their low sensitivity, long T,’s, and unfavorable NOES. However, using the INEPT technique, signal intensity can be borrowed from the abundant coupled proton spins through a process called “magnetization transfer,” allowing their direct observation.
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ANALMiCAL CHEMISTRY. VOL. 54, NO. 13, NOVEMBER 1982
1401 A
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+++++
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Charcoal l i t e r
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Paint thinner Lighter fluid
++
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line, the one from sample 3,showing a relatively small number of moderately volatile compounds, was confusing (Figure 4). Had sample 3 been the only item of evidence submitted in the case, GC analysis alone might have misled an inexperienced arson analyst to report the presence of a probable, if unidentified, accelerant. However, two aspects of the mass chromatographic data contraindicate the presence of an arson accelerant. First, although significant amounts of alkylbenzenes and small amounts of alkylnaphthalenes were observed, this sample lacked appreciable concentrations of aliphatic hydrocarbons. Even those detected did not show a typical aliphatic pattern, but appeared instead to correlate with fragmentation of the alkylbenzenes in the sample. Second, although the intense styrene peak and the presence of other lowboiling alkylbenzenes possibly could 1402A
be construed as a synthetic solvent mixture, a more likely explanation is that all of these compounds arose from the pyrolysis of a styrene-containing polymer in the carpet or carpet backing (3,7).This is a pattern that we have seen repeatedly in samples of burned construction materials and furnishings, some of them from surprising sources. Samples 4 and 5 underscore further the dangers of unsupported GC data. Both samples gave gas chromatograms which, although not immediately identifiable as specific accelerants, contained a number of suspicious volatiles. In sample 4, unknown debris found near an automobile after a garage fire, an intense series of n-alkanes in the C 1 ~ C l range 9 suggested fuel oil or evaporated charcoal lighter fluid. The virtual absence of aromatic compounds, especially higher boiling ones, seemed to rule out fuel oil but
ANALYTICAL CKMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
was still consistent with evaporated charcoal lighter fluid. However, the standard charcoal lighter fluids we had examined had not shown the sequential n-alkane pattern observed here in either the natural or evaporated state, making this possibility less likely. Particularly baffling was identification by library search of the largest peak in the total ion chromatogram as 1,1,3-trimethyl-3-phenyldihydroindene, a type of compound whose origin we simply could not account for. Sample 5, charred pressboard shelving from a bedroom closet after a house fire, showed an intriguing series of higher molecular weight n-alkanes as well as several terpene hydrocarbons (Figure 5). Both were consistent superficially with highly evaporated lighter fluid (see Table 11). which was suspected as a possible accelerant. However, examination of individual
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ANALYTICAL, CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
1403 A
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mass spectra showed that the terpenes in this sample more closely resembled those found in turpentine than the less common terpene alcohols that had characterized evaporated lighter fluid. Since it was unlikely that terpene alcohols would have been converted to the observed compounds under the conditions of the fire, a more likely source of these compounds was the pine chips used in the pressboard. The possibility of turpentine as an accelerant could not be ruled out entirely, hut the concentrations of terpenes seen in this sample were more consistent with those arising from burned coniferous woods. The large styrene peak in sample 5 was entirely reminiscent of sample 3 and suggested the presence of a styrene-containing polymer in the debria. On the other hand, the n-alkane series matched none of the standards studied previously. The increasing peak heights at the end of the chromatogram indicated that this series extend. ed well beyond the time constraints of our analysis, putting it in the range of. a wax or similar type of product. Neither sample 4 nor sample 5 appeared to contain an accelerant. Other Samples. In contrast with the two previous samples, both of which bad contained high-boiling abphatic compounds apparently not as sociated with an accelerant, sample E the purge-and-trap residue from a sumpected arsonist’s shoes, was more easily identified as fuel oil (or a petroleum-based product of similar volatili1404A
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I Figure 2. Total ion chromatcg line: and (c) standard fuel oil
ANALYTICAL CkEMISTRY, VOL. 54, NO. 13. NOVEMBER 1982
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Figure 3. Mass chromatograms for alkylnaphthalenes and alkylbenzenes, including individual molecular ion chromatograms, from sample 1. Each chromatogam has a separate intensity scale
ty) from the combined patterns of aliphatics and aromatica. (It might he noted that fuel oil, diesel fuel, and kerosene are often 80 similar in constitution that they are barely distinguishable even as standards. We make little effort to distinguish between them in actual arson residues.) In this case the identification was particularly gratifying hecause it occurred near the detection limits of both conventional GC and GC/MS. We were reluctant to draw any conclusions from the
few bumps on our original GC screening of the sample, but had no difficulty based upon the convincing mass chromatographic data. Not all samples gave such clearly positive or negative results with this method, hut even when complete identification eluded us we often were able to define the chemical constitution of a phesihle accelerant. Sample 7, charred debris from the bedroom floor after a house fire, showed very few aromatic hydrocarbons, immediately
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suggesting charcoal lighter fluid. However, the narrow range of alkanes observed in this sample seemed to rule this out. Another possibility, a lowboiling naphtha product such as lighter fluid, was inconsistent for the same reason. In contrast, the paint thinners we had examined had shown a similar narrow range of aliphatics, but unfortunately bad also shown somewhat higher concentrations of aromatics. Clearly a product having a mineral spirits base like that found in most paint thinners was indicated. Although the observed pattern did not match any of our standards well enough for an identification, it still directed our attention to the specific types of standards needed for further comparisons.
(b) CerpetfmmSuspectArea
.
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’
Conclusion Combined GC/MS, especially as enhanced by the mass chromatographic analysis we have described, has been a valuable tool for examining arson residues. Findings that are “clearly” positive or negative by conventional GC sometimes can be deceiving. In those cases where an accelerant was indicated by GC, mass chromatography either supported this conclusion with a fairly thorough chemical analysis of the sample or eliminated the “worst possible case”-the false positive conelusion-by highlighting inconsistencies with known standards. On the other hand, some samples that were ambiguous or weak by GC alone were easily identified as common accelerants with this method. At the same time, however, we acknowledge that much work remains to he done in this area before we understand clearly the relationship between the accelerant. the substrate materials, and the effects of the fire on them.
I mple 3 and (b) sample
Acknowledgment The author expresses his deepest thanks to Michael A. Haas for preparation and prescreening of samples, to Michael J. Camp for a critical review of this work, and to Roy Shavers and Robert Warfield (University of Wisconsin-Platteville) for helping us develop and perfect our purge-and-trap apparatus.
References (1) Camp, M.J. Anal. Chem. 1980.52, 422-26 A. (2) Mach, M.A. J . Forensic Sei. 1977.22, 34%57. (3) Juhala. J. A. Arson Anal. Newsl. 1979. 4
8
12
16
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Figure 5. Total ion chromatogram for sample 5 ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
140SA
