Use of Laser Ablation Inductively Coupled Plasma Mass Spectrometry for Fingerprinting Scene of Crime Evidence

Abstract

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 investigated for its potential to provide data on relative trace elemental compositions to achieve this aim. Glass and steel samples were examined as they frequently occur as physical evidence and represent two distinctly dissimilar sample types. A fine focus Nd:YAG laser was used enabling specimens of approximately 50 µm in diameter to be examined. Ablation protocols and optimum compromise sets of laser parameters were established for the analysis of both sample types using both free running 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 glass samples were examined of which thirty-one were float glasses, four were sheet glasses and twenty-seven were container glasses. The steel samples examined were drillings from sixty-nine sources and included steel from safes, firearm barrels, tools, angle iron, rods and crowbars. The LA–ICP-MS method is at present an essentially qualitative technique 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-comparison of three elements simultaneously (ternary plots) in large groups of samples was used to establish both the reproducibility of the ‘fingerprint’ and the uniqueness of the inter-element associations. Results have shown that robust analytical procedures have been developed which reliably discriminate both steel and glass samples and could have direct application for the examination of a wide range of other crime scene evidence.