The headspace sampling is followed by automated thermal desorption (ATD) sample introduction. ATD is known as a very sensitive and clean method, with the drawback that historically it was a "one shot" technique;1 once the sample is consumed, it is unavailable for re-analysis.
The novel thermal desorption technology used here includes sample re-collection, allowing laboratories to re-collect a portion of the sample so that it is available for additional analyses and archival, removing the "single shot" nature of ATD sample introduction. The following study will demonstrate sample re-collection and its effectiveness in arson investigation.Experimental
A test sample of wood was burned with gasoline as an accelerant. The sample was collected in a 0.5 L glass jar; the accelerants were extracted from the jar by purging the headspace with clean, dry air, at 50 mL/min, while heating to 80 °C for 2 min. The air samples were collected onto an ATD sample tube packed with Tenax TA adsorbent, at room temperature. A PerkinElmer TurboMatrix 650 ATD introduced the air sample into the GC–MS system. The GC–MS system used in this study was the PerkinElmer Clarus 600 T GC–MS.
Automated thermal desorption takes place in two steps — the primary and the secondary desorption. During each step the sample flow can be split; sample re-collection is activated only during the secondary desorption. The split effluent is directed to a sample tube, either the original tube or a new tube, rather than to vent. The number of cycles of sample re-collection is not limited; in methods using a high secondary split flow, duplicate data with high precision has been collected for more than 20 analyses of the same sample.
These results demonstrate that re-collection provides data consistent with the initial analysis. Beyond duplicate analysis, a re-collected sample can be stored for archival, in instances where a second analysis is required at a later date or analysed with a different split flow, achieving increased sensitivity or reduced sample size.
As with all forensic analyses, the data generated in arson investigation must be legally defensible; this creates the need for duplicate sample analysis as well as sample archival. The ability to re-collect a portion of a sample allows laboratories to perform multiple analyses of the same sample, as well as preserve a portion of the sample for archival. Thermal extraction combined with dynamic headspace and automated thermal desorption, using sample re-collection, is clearly a clean and very highly sensitive sampling technique in arson investigation.
William Goodman, PerkinElmer Inc., Waltham, Massachusetts, USA.
1. W. Bertsch and Q. Zhang, Anal Chim Acta, 236, 183–95 (1990).