Supplies and Services for Mass Spectrometers, Gas Chromatographs
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2 | Last Update: 01/31/00 |
INTRODUCTION
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 headspace, passive headspace 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 shortcomings 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. The entire sample volatiles are purged directly into the GC injection port.
