USE OF LASER-ABLATION INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY FOR FINGERPRINTING SCENE OF CRIME EVIDENCE

The requirement to uniquely characterise and compare physical evidence from crime scenes is a major task in forensic science. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was investig ated for its potential to provide data on relative trace elemental com positions to achieve this aim. Glass and steel samples were examined a s they frequently occur as physical evidence and represent two distinc tly dissimilar sample types, A fine focus Nd:YAG laser was used enabli ng specimens of approximately 50 mu m in diameter to be examined, Abla tion protocols and optimum compromise sets of laser parameters were es tablished for the analysis of both sample types using both free runnin g and Q-switched mode of laser operation. Mass spectra acquired under these conditions were reproducible and were generated in a fraction of the time required for the conventional solution analyses. Sixty-two g lass samples were examined of which thirty-one were float glasses, fou r were sheet glasses and twenty-seven were container glasses, The stee l samples examined were drillings from sixty-nine sources and included steel from safes, firearm barrels, tools, angle iron, rods and crowba rs. The LA-ICP-MS method is at present an essentially qualitative tech nique and relies on comparison of trace element assemblages or ratios, Samples can be conveniently compared by direct overlay of spectra or interpretive software can be used, Software facilitating the inter-com parison of three elements simultaneously (ternary plots) in large grou ps of samples was used to establish both the reproducibility of the 'f ingerprint' and the uniqueness of the inter-element associations. Resu lts have shown that robust analytical procedures have been developed w hich reliably discriminate both steel and glass samples and could have direct application for the examination of a wide range of other crime scene evidence.