Improvements to sampling and instrumentation for fire debris analysis using Tenax TA® and ATD-GC-MS.

The standard practice for the separation and identification of ignitable liquids is well established in fire debris analysis. A recent study by The Netherlands Forensic Institute identified passive headspace adsorption as the predominant technique currently in use by its participating laboratories across Europe and subsequent analysis is carried out by gas the use of gas chromatography and a mass spectrometer (GC-MS). Whilst it is accepted that GC-MS is the best tool for the analysis, the passive headspace stage is not without problems.

If samples being analysed are regarded as evidence samples, then it is imperative that the sample is not subjected to any undue contamination and that the separation and identification process employed is simplistic, rugged and repeatable. Activated charcoal in a passive headspace environment is often the sorbent of choice to adsorb volatile ignitable liquids. However, activated charcoal has an affinity towards chemically charged species and therefore will adsorb these compounds in preference to long chain neutrally charged compounds, such as heavy alkanes. Ignitable liquid mixtures containing heavy alkane chains such as biodiesel fuel, fuel oil and diesel fuel will not have all of their compounds adsorbed by a charcoal strip. Also, the use of solvents to elute the analyte from the charcoal strip has been discussed many times with regard to health issues, and because it is an extra step in analyte separation from fire debris it also carries the risk of contamination.

This study has led to improvements to the passive headspace technique and also the analytical method when using an automated thermal desorber with GC-MS (ATD-GC-MS). The improved separation process still uses passive headspace diffusion in an oven, but takes advantage of the adsorption properties of Tenax TA to adsorb heavy alkanes. Tests show that oven times are reduced when compared to current techniques and there is no solvent step in the procedure, thus reducing the risk of undue contamination.