Improving Sample Introduction in Arson Investigation Through Sample Re-Collection in Automated Thermal Desorption

The process of investigating a suspicious fire includes many different types of analyses. An essential step in confirming the presence of a liquid accelerant is gas chromatography (GC); detection with a mass spectrometer (MS) provides an accurate confirmation of both the presence and identity of an accelerant. In arson analysis, the sample preparation for GC–MS analysis is typically performed by headspace or solvent extraction.

Figure 1

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.

Discussion

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.

Conclusion

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.

References

1. W. Bertsch and Q. Zhang, Anal Chim Acta, 236, 183–95 (1990).

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