The Analytical Approach
R. Martin Smith Wisconsin Dept. of Justice Crime Laboratory P.O. Box 5708 Madison, Wis. 53705
Edited by Jeanette G. Grasselli
Arson Analysis by Mass Chromatography 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 (1). 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 GC/MS data produced from most arson 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 351-1399$01.00/0 © 1982 American Chemical Society
the data system of a GC/MS computer system sorts through the previously collected mass spectra from a sample for the presence of certain characteristic ions, then plots the intensities of only these ions as a function of time (5). The resulting mass chromatograms, which usually show significant deconvolution of overlapping GC peaks, allow rapid detection of specific compounds in the sample even when background interference is intense. Since many arson analyses are "one-shot" injections because of extremely limited sample sizes, the gen-
eration of full mass spectra for mass chromatography gives it a distinct advantage over its close relative, selected ion monitoring (SIM). The latter technique forces the analyst to select de. sired ions prior to the sample run, thus precluding the possibility of finding unanticipated compounds in the sample or of more complete compound identification by spectral comparisons at a later time. For screening arson residues by mass chromatography we chose intense ions in the mass spectra of representative hydrocarbons from fami-
Table I. Representative Ions Normally Present in Mass Spectra of Common Accelerants Compound
Aliphatics Alicyclics and olefinics Alkylbenzenes Alkylnaphthalenes Alkylstyrenes and dihydroindenes Alkylanthracenes Alkylbiphenyls and acenaphthenes Monoterpenes (C10H16)
mix
57, 71,85,99 55, 69, 83, 97 91, 105, 119, 133 (also molecular ions 78, 92, 106, etc.) 128, 142, 156, 170 104, 118, 132, 146 178, 192, 206 154, 168, 182, 196 93, 136
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 · 1399 A
Total Ion
Aliphatics
Alicyclics
Alkylbenzenes
Naphthalenes
Styrènes
Biphenyls
Anthracenes
Terpenes
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
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 II. In general, the overall range and pattern of aliphatic compounds (especially if a series of M-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 substantially 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, but useful chemical in-
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1982
formation about each sample was obtained by 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 by 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 III. 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 hydrocarbon families (Figure 1), 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 hydrocarbons and the unusually high concentrations of alkylbiphenyls and acenaphthenes (not shown in Table III) 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 by postulating a mixture of partially and highly evaporated gasoline. These conclusions had been indicated, but not strongly supported, by conventional GC alone. On the other hand, sample 2, charred carpeting from an office storage room, was readily identified by conventional GC as essentially unevaporated gasoline. Mass chromatographic analysis completely confirmed this, showing patterns scarcely distinguishable from those of the standard (compare Tables II and III). 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 exhibited all of the characteristics of gaso-
Table II.
Standard Accelerants a Aliphatlcs
Standard
Total
n-Alkanes present
n-Alkane present in greatest intensity
Allcyclics
Low-boiling alkylbenzenes
High-boiling alkylbenzenes
Naphthalenes
Regular gasoline
++++
C5-C16+
C10
+++
+++++
+++
+++
Fuel oil
+++++
C8-C17
C12
++
+
++
+
Charcoal lighter
+++++
C8-Cl6b
Campstove fuel
+ +++
C5-C11
c8
+++++
++
+
Paint thinner
++++ +
C8-C14
C10
+++
+
++
Lighter fluid
++ + +
C7-C9
c8
+++ +
++
Styrènes
Anthracènes
Terpenes
+
++
+
+ +++
Gum turpentine
+
Evaporated gasoline
+++
C8-C19
Evaporated charcoal lighter
+ ++++
C12-C186
Evaporated campstove fuel
+ ++++
C10-C12
ct1
+++
+
Evaporated paint thinner
+ + +++
C10-C12
C11
+++
++ +
Evaporated lighter fluid
++
C10-C20+
c14
++
C12
+
+++++
++++
+
++
+
Number of +s indicates relative concentration of accelerant components determined by mass chromatography; pattern of n-alkanes observed; c Mostly terpene alcohols, not monoterpenes.
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
++
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 rc-alkanes in the C12-C19 range 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
1402 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
+ a
Condensed from Reference 4;
+++ c b
No real
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 l,l,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 rc-alkanes as well as several terpene hydrocarbons (Figure 5). Both were consistent superficially with highly evaporated lighter fluid (see Table II), which was suspected as a possible accelerant. However, examination of individual
Table III.
Authentic Arson Samples Aliphatic»
Sample
Total
(1) Cement residue (CS 2 extract) (2) Suspect carpet
++++
n-Alkanes present
n-Alkane prosent In greatest Intensity
Allcycllcs
Low-boiling alkylbenzenes
HIgh-bollIng alkylbenzenes
Naph thalenes
Styrenes
Ce-C20
C12
+
+
++
+++++ + +
C5-C13
C10
+++
++++
+++
++
+
+
++ +
tr
tr
+++
Anthracenes
++
(25 μ\. vapor) (3) Standard carpet (100 μ ι vapor)
+
(4) Garage debris (P/T)
+++++
C12-C19
C,5
(5) Closet debris (P/T)
+++++
C9-C15+
Cts
(6) Shoe extract (P/T)
++++
C10-C18
C15
++
tr
tr
++
+
+
++++
++ +
+
+++++ C e - C i 2 ++ + tr C10 (50 μ\- vapor) Number of +s indicates relative concentration of accelerant components determined by mass chromatography; tr = trace; P/T = purge-and-trap.
(7) Bedroom debris
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 al cohols 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 acceler ant could not be ruled out entirely, but the concentrations of terpenes seen in this sample were more consis tent 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 debris. On the other hand, the n-alkane series matched none of the standards stud ied 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. Nei ther sample 4 nor sample 5 appeared to contain an accelerant. Other Samples. In contrast with the two previous samples, both of which had contained high-boiling ali phatic compounds apparently not as sociated with an accelerant, sample 6, the purge-and-trap residue from a sus pected arsonist's shoes, was more easi ly identified as fuel oil (or a petro leum-based product of similar volatili-
(a)
(b)
(c)
Time Figure 2. Total ion chromatograms for (a) sample 1; (b) evaporated standard gaso line; and (c) standard fuel oil
1404 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
Total Ion
Total Naphthalenes
REAGENTGRADE SOLVENTS
Total Alkylbenzenes
Time Figure 3. Mass chromatograms for alkylnaphthalenes and alkylbenzenes, including individual molecular ion chromatograms, from sample 1. Each chromatogram has a separate intensity scale
ty) from the combined patterns of aliphatics and aromatics. (It might be noted that fuel oil, diesel fuel, and kerosene are often so 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 because 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, but even when complete identification eluded us we often were able to define the chemical constitution of a possible accelerant. Sample 7, charred debris from the bedroom floor after a house fire, showed very few aromatic hydrocarbons, immediately
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 ·
1407 A
suggesting charcoal lighter fluid. H o w ever, t h e n a r r o w r a n g e of a l k a n e s ob served in this s a m p l e seemed t o rule t h i s out. A n o t h e r possibility, a lowboiling n a p h t h a p r o d u c t such as light er fluid, was i n c o n s i s t e n t for t h e s a m e reason. In c o n t r a s t , t h e p a i n t t h i n n e r s we h a d e x a m i n e d h a d s h o w n a similar n a r r o w r a n g e of aliphatics, b u t unfor t u n a t e l y h a d also shown s o m e w h a t higher c o n c e n t r a t i o n s of a r o m a t i c s . Clearly a p r o d u c t having a m i n e r a l spirits base like t h a t found in m o s t p a i n t t h i n n e r s was indicated. Al t h o u g h t h e observed p a t t e r n did n o t m a t c h any of our s t a n d a r d s well e n o u g h for a n identification, it still di r e c t e d our a t t e n t i o n to t h e specific t y p e s of s t a n d a r d s n e e d e d for further comparisons.
(a) Standard Carpet
Olefin Cyclic
Unidentified Terpene
(b) Carpet from Suspect Area Branched Alkanes
Conclusion
Time Figure 4. Total ion chromatograms for (a) sample 3 and (b) sample 2
Charred Shelving, Purge and Trap
C o m b i n e d G C / M S , especially as en h a n c e d by t h e m a s s c h r o m a t o g r a p h i c analysis we have described, h a s been a valuable tool for e x a m i n i n g arson resi dues. F i n d i n g s t h a t a r e " c l e a r l y " posi tive or negative by c o n v e n t i o n a l G C s o m e t i m e s can be deceiving. In t h o s e cases where an accelerant was indicat ed by G C , m a s s c h r o m a t o g r a p h y ei t h e r s u p p o r t e d t h i s conclusion with a fairly t h o r o u g h chemical analysis of t h e s a m p l e or e l i m i n a t e d t h e " w o r s t possible c a s e " — t h e false positive con clusion—by highlighting inconsisten cies with k n o w n s t a n d a r d s . O n t h e o t h e r h a n d , some s a m p l e s t h a t were a m b i g u o u s or weak b y G C alone were easily identified as c o m m o n acceler a n t s with this m e t h o d . At t h e s a m e t i m e , however, we acknowledge t h a t m u c h work r e m a i n s t o be d o n e in t h i s area before we u n d e r s t a n d clearly t h e r e l a t i o n s h i p b e t w e e n t h e accelerant, t h e s u b s t r a t e materials, a n d t h e ef fects of t h e fire on t h e m .
Acknowledgment
n-Alkanes
T h e a u t h o r expresses his d e e p e s t t h a n k s to Michael A. H a a s for p r e p a ration a n d prescreening of s a m p l e s , t o Michael J. C a m p for a critical review of t h i s work, a n d t o R o y S h a v e r s a n d R o b e r t Warfield (University of W i s consin—Platteville) for h e l p i n g us d e velop a n d perfect our p u r g e - a n d - t r a p apparatus.
References
Time Figure 5. Total ion chromatogram for sample 5
(1) Camp, M. J. Anal. Chem. 1980,52, 422-26 A. (2) Mach, M. A. J. Forensic Sci. 1977,22, 348-57. (3) Juhala, J. A. Arson Anal. Newsl. 1979, 3 (4), 1-19. (4) Smith, R. M. J. Forensic Sci., in press. (5) Hites, R. Α.; Biemann, K. Anal. Chem. 1970,42,855-60. (6) Chrostowski, J. E.; Holmes, R. N. Arson Anal. Newsl. 1979, 3 (5), 1-17. (7) Jackson, M. T.; Walker, J. Q. Anal. Chem. 1971,43,74-78. ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 · 1409 A
