Capabilities of laser ablation—inductively coupled plasma mass spectrometry for (trace) element analysis of car paints for forensic purposes

Abstract

The capabilities of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for depth profiling analysis of car paints were explored and the use of the signal profiles thus obtained for forensic purposes—identification of the vehicle involved in a hit-and-run accident—evaluated. Depth profiling analysis of car paint samples provided detailed information on the elemental composition of the individual layers (clear coat, base coat, primer surfacer and first primer). It was, however, established that when using a quadrupole-based ICP-MS instrument, the complex matrix composition of some of the layers—organic components and high concentrations of inorganic compounds, such as kaolin (Al₂O₃.2SiO₂.2H₂O), talc (3MgO.4SiO₂.H₂O), barite (BaSO₄) and/or Fe-containing pigments—resulted in spectral interferences, significantly affecting the signal profiles obtained especially for the transition metals. Deviation of experimentally determined isotope ratios from the corresponding true values was assessed as a very useful indication of spectral overlap. Application of an inductively coupled plasma sector field-based mass spectrometer (ICP-SF-MS), operated at medium mass resolution (R = 4000), permitted most interfering ions to be identified and documented, primarily on the basis of their exact mass-to-charge ratio. Car paint fragments found at crime scenes are usually very small, such that the multi-element capabilities of LA-ICP-MS have to be exploited to the largest possible extent, allowing the measurement of as many analytes as possible, facilitating clear differentiation of the various paint sources. High mass resolution was therefore assessed as the best possible way to overcome the aforementioned spectral overlap. The signal profiles obtained via LA-ICP-MS permitted car paints of the same colour to be discriminated from one another on the basis of the elemental composition of the individual layers. Using the ratio of the signal intensity for the target elements to that of Ti—present at high concentration in the primer surfacer and the first primer in all of the car paint samples investigated—is suggested as a first step towards quantitative results, at least for these two layers.