Report
Chemical Analysis of Fire Debris
WAS IT ARSON? L
osses as a direct result of fire are estimated at about $6 billion annually, and about 20% of all fires are believed to be of incendiary origin. The National Fire Protection Association states that in 1994, some 107,800 arsonrelatedfireswere reported, and 550 people lost their lives in these fires (1). Fiftyfive percent of arrests for arson involved juveniles 33% under the age of 15 and 7% under the age of 10. Combating arson involves individuals from a wide range of disciplines from finance to the legal profession Forensic chemists work with physical evidence to determine whether there are residual materials in thefiredebris indicating that thefirewas deliberfltely set a n d / o r arrplerated
An arson investigation usually begins right after afirehas been brought under control. Thefirechief makes a preliminary determination of the cause and ori-
Wolfgang Bertsch University of Alabama 0003-2700/96/0368-541 A/$12.00/0 © 1996 American Chemical Society
Forensic chemists use their analytical skills to determine the source of suspicious fires gin of thefireby assessing the factors thai contributed to thefire'sbehavior. A fire that appears to have multiple origins, suspicious burn patterns, an unusually high rate of spreading, or remnants of an ignition device is a primary candidate for arson. Arson investigators often have backgrounds in law enforcement, electrical engineering, or some other area of science and have had formal training in fire investigation. They interrogate victims suspected perpetrators and others who have pertinent knowledge about the source of the fire
Thefirescene is photographed, and all items removed from it are documented. Physical evidence such as charred material is collected, and a strict chain of custody is established. Thefireinvestigator weighs all of these factors and determines whether afireoriginated from natural causes or appears to be of an incendiary nature. If the physical evidence points toward afirethat has been set deliberately, the investigator assembles as much information as possible to thoroughly document the case. Indirect evidence, such as motive, opportunity, and past history of the beneficiary of thefire,is then weighed to determine whether a crime has been committed and if an insurance claim should be settled or denied. The nature of the beast Conventionalfirescan be characterized as a complex interaction between fuel, air, and heat. A source of ignition of sufficient energy is required to start the chain reaction in which a fuel is converted into gaseous products, mostly water and C02.
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explosive force. If liquid accelerants are Fire behavior can be predicted and modeled fairly accurately once the starting con- used, the available oxygen may be quickly ditions have been established. The nature consumed, leaving an excess of fuel. The of the fuel and the size, dimensions, and flames are intense, but the amount of heat ventilation of the afflicted structure are facand the rate at which it is transferred into tors in determining the velocity, duration, the matrix may be insufficient to cause and intensity of fire development pyrolysis at a level necessary to sustain Reactions are complex but distinctively combustion. different for normal fires and those that In most cases that involve arson, petrohave been accelerated by an artificial leum-based distillates such as gasoline, source of fuel, such as gasoline. In a norpaint thinners, charcoal lighter, and keromal fire, energy in the form of hot gases sene are poured along the corners of the moves upward in a room, and the fire walls and floors of a structure. Information spreads vertically if fuel is available. A is scarce on alcohol-based accelerants; it room may fill with hot gases at the ceiling, is unclear whether this lack of information which in turn may create other sources of indicates a potential weakness in analysis ignition. These gases, soot, and pyrolysis methodology or whether the use of alcoproducts ignite once the temperature inhol as an accelerant is simply underrecreases sufficiently, and the fire may ported. Solid accelerants that involve mixspread cicross the top of the room and into tures of oxidizers (chlorates or nitrates) adjacent spaces if there are no physical and combustible compounds (sugar or barriers There the layer of hot gases may starch) require analysis procedures that initiate further thermal decomposition of are entirely different from those used for other fuel sources resulting in a flashliquids over At this point temperatures rise rar> idlv and also spread downward distributThe analytical problem inc heat more evenlv Further nvrolysis The basic goal of chemical analysis of fire takes place when the oxygen content indebris a is to establish whether materials are confined space drops rapidly. present in the remnants of a fire that could
and successful collection and analysis of these residual accelerant samples is as much an art as it is a science. Sample preparation. The first step is to determine the location from which a debris sample should be taken. The human nose is the most widely used primary detector, but it has obvious shortcomings. Although several canine accelerant detection programs have been used across the country (2), dogs respond to minute traces of hydrocarbon vapor but cannot distinguish between residual accelerant vapors and those generated by burned furniture, building materials or plastics. Mechanical devices so-called "sniffers " can signal the presence of volatiles but also cannot distinguish between ovrolysis products generated by the fire and residual accelerants
Serious problems can also arise from the composition of the debris matrix. For example, fire investigators frequently sample carpet or carpet padding because these materials tend to retain liquid accelerants. Although these matrices are a logical choice from the investigator's point of view, the synthetic polymers used in these materials also tend to generate copious amounts of pyrolysate containing volatile In contrast, accelerated fires produce a have helped to start or accelerate it. At first hydrocarbons (3). it seems that a highly volatile fuel such as large amount of heat from the readily Isolating target volatiles in a reasonavailable fuel vapors within a short period gasoline would "go up in smoke" along with able quantity is another challenge. Several the structure. (This is certainly the expecta- sample preparation procedures can be oftimeand at a specific location. In extion of the arsonist.) However, rraces of the used to enrich and isolate the analyte treme cases, gases can expand so rapidly that windows or doors are blown out with fuel often remain even after an intense fire, from the matrix (4); the final choice of method depends largely on the properties and concentration of the expected accelerant If destructive testing is used, part of the sample must be saved in case addiTable 1. Accelerant classification system tional testtng ii necessary. Although ii would appear that these factors present Class Carbon Examples Dominant component number classes significant hurdles they are actually quite manageable in practice. 1-Light petroleum C 4 -C 8 Petroleum ethers, Branched alkanes distillates lighter fluid, naptha Analysis. Although the physical prop2-Gasolines C4 _ C 12 Gasoline, gasohol, Branched alkanes, erties of petroleum distillates (such as alkylbenzenes, camp stove fuels naphthalenes ignition temperature and evaporation 3-Medium petroleum &8~ C12 Paint thinner, mineral Normal alkanes, rate) vary widely, chemical composition is spirits, charcoal distillates alkylbenzenes fluid more uniform, and accelerants can be 4-Kerosene No. 1 fuel oil, jet fuel, Normal alkanes, Cg— C, 6 classified according to their carbon numinsect sprays, alkylbenzenes, ber and dominant components (Table 1). charcoal fluid naphthalene 5-Heavy petroleum Cl0~"'-' 3 No. 2 fuel oil, diesel Normal alkanes, Because of their volatility, accelerants alkylbenzenes. distillates fuel are particularly amenable to GC. Surprisnaphthalenes ingly, it may not be difficult to correctly O-Unclassified Variable Alcohols, toluene, Alkanes, xylenes, lacquer alkylbenzenes, classify a distillate, even after as much as thinner, alcohols, ketones, 90% evaporation. For example, kerosene isoparaffinic esters hydrocarbon maintains many of its characteristic chromixtures matographic features following evapora2
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Analytical Chemistry News & Features, September 1, 1996
Figure 1 . Chromatographic profiles of unevaporated and evaporated accelerants. (Top to bottom) Unevaporated gasoline, 90% evaporated gasoline, unevaporated kerosene, and 90% evaporated kerosene.
tion, whereas gasollne profiles affer evaporation show little resemblance to those before evaporation (Figure 1). Recognition of weathered accelerants thus requires a library of standards and a keen eye. A more insidious problem is the ubiquitous occurrence of materials derived from petroleum in everyday living. Petroleum-based distillates are used in products as diverse as insecticide formulas and tile glue, and the chemist seldom has detailed knowledge of the nature and history of the sample to be analyzed. False positive results are likely, especially if the sample matrix contains relatively new material that has not fully lost volatile components used in the manufacturing process. The use of control or comparison samples has been suggested to evaluate potential background interference but this at> proach has limitations because the chemist has little control over these factors and fire investigators are not always fiillv
precolumn fractionation methods not only add an extra step but also require fairly large amounts of sample, and multidimensional GC is rather cumbersome in practtce (6). Parallel-column chromatography has also been used to improve confidence in peak assignments (7), but this approach is not widely used. The most promising alternative for simplifying chromatograms is using a mass spectrometer to discriminate against pyrolytic interferences. Although the general usefulness of MS for accelerant analysis was recognized in 1982 (8), GC/MS did not quickly catch on in arson analysis because it was considered an expensive proposition that required a fair amount of expertise. It is surprising, however, that the introduction of moderately priced benchtop instruments in the early 1990s did not change analysts' attitudes significantly The major benefit of a mass spectrometer is its relatively uniform response to alkanes, alkylated benzenes, cycloalkanes, indanes, and naphthalenes. Some of these compounds do not produce strong molecular ions, but all produce an abundance of common fragmentation ions that can be used to identify accelerants. Guidelines and certification
Analysis of fire debris for the presence of accelerants is performed in government laboratories at the federal, state, and local level, as well as in commercial laboratories. Anyone with a basic background in chemistry who has access to a gas chromatograph and laboratory equipment can analyze fire debris for accelerants. Guidelines have been proposed for laboratories performing chemical and instrumental analysis of debris collected from a fire scene (9), and some professional organizations, such as the American Society of Crime Laboratory Directors, have set up aware of accreditation boards. However, participasuch potential pitfalls tion in such programs is voluntary, and esTo deal with convoluted chromatotablishing an individual as an expert in the grams, the analyst can attempt to remove chemical analysis of fire debris is done in a the offending pyrolysates by using precolcourt of law where credentials are umn fractionation methods such as acid sented on a case-by-case basis washing or solid-phase extraction, by enA series of methods for the examinahancing the separation with parallel or setion of fire debris for flammable and comquential chromatography (such as multidibustible liquid residues has been adopted mensional GC in die heartcutting mode), or by the American Society for Testing and by using instrumentation that discriminates Materials (ASTM). Individual test methagainst the interferences. Unfortunately,
ods deal with the recovery of accelerant residue from the matrix, chromatographic separation and detection, and cleanup procedures. Although minimum requirements for the identification of specific accelerants have been determined, the guidelines provide a great deal of freedom to the forensic chemist. For example, test method E1387-90 gives the analyst a choice of several GC detectors and states that "any column and conditions may be used provided that under the conditions of use the test mixture can be resolved into its component peaks." Interlaboratory testing
The Forensic Laboratory Proficiency Testing Program, a voluntary program administered by Collaborative Testing Services and affiliated with the American Society of Crime Laboratory Directors, has been evaluating forensic laboratories since 1971. Interlaboratory test samples, usually simulated arson samples, are provided annually, and the participants are asked to classify the accelerant. The selection of analytical methodology and the report format are at the discretion of the participating laboratory. Participating laboratories receive test results a few weeks after the announced deadline, and a comprehensive report describing the performance of all laborato-
Figure 2. Chromatograms of accelerants buried in the matrix. (a) Matrix (charred carpet, carpet padding, and curtain material), (b) weak simulated arson sample prepared by spiking the matrix with 80% evaporated gasoline, and (c) standard of 80% evaporated gasoline.
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ries arrives a few months later. Although it is tempting to interpret the information from the annual reports as reflecting the state of the art in the field, this projection is probably not fully justified because the studies are intended as educational, and a cautionary note attached to each report emphasizes that the summary of results is not an "overview of the quality of work performed in the profession." However, shifts in the use of analytical methodology and instrumentation can easily be extracted from these reports and they provide the only comprehensive source of information available on laboratory techniques in accelerant analysis The degree of difficulty and rate of success varied considerably between different test rounds because each round had a different focus. In some years, sample preparation technology was empha-
sized; in other years, the emphasis was on recognizing seldom encountered accelerants. In most cases, scenarios were provided, such as "a sample (container 1 containing soil) was collected from under a house that burned down." Analysts were then asked, "Does the sample contain an accelerant? If yes, how should it be classified? If there is an accelerant in container 1, does it match a liquid (container 2) found in the garage of the suspect?" Most laboratories had no difficulty with matrices containing relatively large quantities of accelerants, even those that underwent considerable weathering. However, high-boiling-range distillates such as diesel fuel were sometimes confused with kerosene. Because headspace methods often suffer from inadequate recovery of highboiling compounds, analysts sometimes misclassify the boiling-point ranges of ac-
celerants, particularly in classes 3-5, making it relatively difficult to determine the presence of accelerant mixtures such as paint thinner and diesel fuel. However, straight-run distillates such as charcoal lighter fluids, kerosene, and fuel oils do contain the diagnostic components of gasoline (alkylbenzenes) in modest quantities, and unless a significant amount of gasoline is added to the fuel oils, a chemist may ascribe the presence of alkylbenzenes as being a natural part of this particular class of accelerant A similar situation arises when turpentine is found in coniferous wood samples. Pine, which is frequently used as flooring, produces copious amounts of natural terpenes, the diagnostic compounds of the potential accelerant turpentine. Because hardwoods do not produce terpenes (10), a hardwood sample containing terpenes indicates the use of turpentine as an accelerant. When asked to determine the presence of accelerants in two boards one oak and one pine that had been spiked with turpentine most erred on the side of caution and simply reported the samples as negative for accelerants because turpentine does not produce a pattern of irreeiilar spaced peaks that can be easily recognized Fortunately is also rarely used to set fires The most serious error made in the blind tests is reporting an accelerant in a sample when none is present. For example, between 5% and 10% of fhe llboratories reported an accelerant in new and charred carpet samples when none was present. A careful evaluation of responses from the laboratories did not point to a single source of error; sample preparation procedures, instrumentation, and the experience level of the chemist did not appear to be the source of the problem. A concentrated effort must be made to eliminate such false positives.
Figure 3. Selected ion chromatograms of accelerants buried in t h e matrix. (a) Matrix, (b) simulated arson sample, and (c) gasoline standard. 544 A
Analytical Chemistry News & Features, September 1, 1996
A practical example It is difficult to quantitatively describe the effect of factors such as chromatographic resolution, ratio of pyrolysate to accelerant volatiles, mass spectral selectivity, and the method of chromatogram comparison on the outcome of an analysis. Some of these factors are within the control of the analyst, and all are related to each other but are not always easily delineated from
one another. Within limits, some of them can be traded off against each other. Chromatographic resolution must be adequate for the analyst to recognize a specific accelerant pattern. It is relatively easy to visually extract the profile of an accelerant from a debris sample if the accelerant components dominate or if the contribution of the interferences can be suppressed by the use of a specific detector, even when the efficiency of the column and the quality of the separation are relatively poor. However, the situation changes dramatically if the accelerant compounds are buried in the matrix. It is often virtually impossible to visually extract the accelerant pattern from the chromatogram of a complex spiked matrix as shown in Figure 2 for a carpet sample spiked with gasoline. In this case, the chromatograms were produced on a relatively inefficient column, but it is unlikely that the situation would improve significantly had a more efficient column been used because the chemical noise produced by the matrix overwhelms the weak from the accelerant To further complicate matters the random distribution of the alkylbenzene isomers makes identification even more difficult TTnfortiinatpIv tVipn* i
