|2a||Last Update: 12/23/99|
A variety of methods are utilized in forensic laboratories for the analysis of suspect arson samples for residual traces of volatile accelerants. The detection of arson accelerants is most commonly accomplished by using a headspace sampling technique combined with Gas Chromatography (GC) and/or Gas Chromatography/Mass Spectrometry (GC/MS). There are three basic types of headspace sampling used for sample preparation: static, passive and dynamic headspace sampling. For most organic compounds, the most sensitive of these techniques is the dynamic headspace technique.
Arson samples are normally collected at the suspect crime scene and submitted to the laboratory either in new aluminum cans or forensic evidence bags. To prepare a sample from these containers for GC analysis, the normal dynamic headspace procedure involves purging the container with an inert gas and collecting the purged volatiles on a solid sorbent material. The volatiles trapped on the solid sorbent are then eluted with an organic solvent (i.e. carbon disulfide) or alternately can be thermally desorbed onto the gas chromatographic column using the Short Path Thermal Desorption System.
The Short Path Thermal Desorption System permits the analysis of arson samples by desorbing the samples previously collected on adsorbent resins directly into the GC injection port for subsequent analysis by conventional GC detectors or via mass spectrometers. Due to its Short Path of sample flow, this new system overcomes the short comings of previous desorption systems by eliminating transfer lines (which are easily contaminated by samples) and by providing for the optimum delivery (and therefore maximum sensitivity) of samples to the GC injector via the shortest path possible.
Another technique available with the Short Path Thermal Desorption system is Direct Thermal Analysis. This technique permits samples such as arson wicks, wood or fibers to be placed directly in the sampling tube and the volatiles therein are purged directly into the GC injection port. Sample sizes of 1 mg to 500 mg can be analyzed using this technique. This technique provides for maximum sensitivity since no sample is lost in sample preparation and collection techniques and the entire sample volatiles are purged directly into the GC injection port.
The volatile residues from arson samples are collected by purging the sample container containing the suspect arson evidence with nitrogen at a flow rate of approximately 20 mL/min to 30 mL/min for 10 minutes and passing the flow through a Glass-Lined Stainless Steel (GLT) Desorption Tube packed with a porous polymer, as Tenax TA. The sample container is heated to 100 degrees C to increase the quantity of volatiles sparged from the sample. The Tenax TA is packed in amounts from 50 mg to 2.0 g. and is held in place by silanized glass wool plugs. The total volume sampled should be chosen to keep the background of the sample at a minimum, while the maximum analyte is sampled but should not exceed the breakthrough volume (i.e. the volume of air sampled at which some of the compound begins to be lost from the trap). Breakthrough volumes for a number of components in common accelerants have been tabulated by the Environmental Protection Agency (EPA).
The Scientific Instrument Services Short Path Thermal Desorber TD-1 provides a convenient and accurate means of desorbing the sample onto the chromatographic column. After the volatiles have been trapped on the Desorption Tube with sorbent, it is attached to the Short Path Thermal Desorption System. A syringe needle is also attached. It is then briefly purged with helium flow (2 to 3 minutes) before it is injected. This purging serves to remove oxygen and any water vapor that has accumulated onto the sorbent trap and thus provides for the maximum life of the capillary GC column. The Desorption Tube with needle attached is then injected into the injection port of the gas chromatograph and ballistically heated to desorb the analytes onto the column. The gas chromatographic column is held at low temperature by liquid nitrogen to cryofocus the desorbed components at the front of the column. Detection can be achieved by a variety of detectors. Due to the complexity of most of the samples, flame ionization detectors or mass spectrometers are most commonly used. To analyze the results, chromatographs of samples analyzed are compared to chromatograms of known accelerants.
TABLE Instrument Conditions
Trap: Tenax TA, mesh 60/80, 50mg., on 3 mm I.D. GLT Desorption Tubes
Purge for Sample Collection: 20mL/min. at 100 degrees C for 10 min.
Desorption Time: 4 min.
Desorption Temp.: 250 degrees C
Desorption Flow: 10 mL/min.
GC Column: 30 meter X 0.75 mm glass SPB-1 (1.0 um film thickness)
Carrier Gas: Helium 6 mL/min.
Oven Temp.: -10 degrees C (Hold 5 min.); -10 degrees C to 250 degrees C (12 C/min.); 250 degrees C (Hold 10 min.)
Detector: Flame Ionization Injector
Detector Temp: 250 degrees C
RESULTS AND DISCUSSION
Figures 1 and 2 illustrate the detection of a wide range of boiling point components present in standard accelerants which are detectable using the method described. In Figure 1, 0.1 uL of gasoline was desorbed directly using the Short Path Thermal Desorption System. It shows a wide range of components (a mixture of C3 - C12 saturated hydrocarbons and some aromatics).
Figure #1 - Gasoline 0.1 uL. Figure 2 shows the chromatographic separation resulting from the desorption of 0.05 uL of steam distilled wood turpentine, a naturally occurring background found in many wood samples.
Figure #2 - Steam Distilled Wood Turpentine .05 uL.
In order to identify accelerants, chromatographers must first analyze by GC as many potential accelerants as possible, using the resulting complex chromatograms as fingerprints. These standard chromatograms are then compared with chromatograms produced from fires of suspicious origin to identify the accelerants. The amount of accelerant present at the origin of these fires is most often times very low, so a very sensitive technique is needed to recover them. The dynamic headspace technique can fulfill this role. Figure 3 shows the chromatogram resulting from 0.05 uL/gallon of gasoline sample purged for 10 minutes at 20 mL/min. and 100 degrees C and trapped on 50mg Tenax TA. The chromatogram shows good sensitivity and recovery of the wide range of components of gasoline as well as good resolution.
Figure #3 - Gasoline .05/gal. Purge & Trap
Though gasoline has been found to be the most widely used accelerant, other heavier accelerants have been used separately or combined with it to provide a slower burning, less explosive mixture. Figure 4 illustrates the chromatogram resulting from recovering a small amount (0.05 uL/gallon) of mineral spirits by purging with Nitrogen and trapping on Tenax TA.
Figure #4 Mineral Spirits - Purge & Trap - .05uL/gal.
Often times the accelerants recovered from actual fire debris will exhibit a different chromatographic fingerprint from that produced by fresh petroleum products. The intense heat of the fire volatilizes off the lower boiling components, thus changing the ratio of the higher boiling components to the lower boiling components in the sample. A gasoline sample weathered by evaporation or partial combustion is much different from the fresh gasoline sample. Figure 5 illustrates the versatility of the Short Path Thermal Desorber by showing a chromatogram from the direct thermal analysis of a one (1) square centimeter piece of cotton cloth wick taken from a molotov cocktail. The resultant chromatogram is comparable to a chromatogram from approximately a 93% evaporated gasoline sample.
Figure #5 Wick From Molotov Cocktail - 1 square cm Direct Thermal Analysis -93% Evaporated Gasoline
The chromatograms illustrate the sensitivity and flexibility of the Short Path Thermal Desorption System which overcomes the short comings of previous desorption systems thereby fulfilling the needs of the arson analyst.
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