Laser-induced breakdown spectroscopy/laser ablation coupled to inductively coupled plasma mass spectrometry (LIBS/LA-ICPMS) for the forensic screening and discrimination of lead-free solders

The tandem LIBS/LA-ICPMS technique is a desirable tool for the multi-elemental determination, characterization, and classification of alloys as forensic evidence. In this study, LIBS/LA-ICPMS is validated for the forensic evaluation of lead-free solder alloys, which form valuable evidence from post-blast crime scenes involving homemade and improvised explosive devices. LIBS/LA-ICPMS is competitive with other spectroscopic-based forensic techniques as it is in situ, analyzes samples directly, and requires minimal destruction of the exhibit. Following a one-standard calibration technique, nine major (alloying metals) and trace elements (impurities or additives) are quantified in lead-free solders. Optimizing laser parameters and using Pb as a naturally occurring internal standard are shown to compensate for mass-dependent drift and matrix effects. The quantitative results of Pb-free certified reference materials align with certificate values and with results from two techniques in a cross-validation comparison, including electrothermal vaporization-inductively coupled plasma optical emission spectrometry and neutron activation analysis. Utilizing peak ratios in a model of principal component analysis is presented to identify key compositional differences among solders and provide a visual model for solder discrimination. Outcomes of this approach demonstrate the potential for associating or discriminating lead-free solders, including different solders from the same manufacturer. Together, this technique can establish chemical concordance among known and questioned materials and offers a utilitarian approach for the forensic assessment of trace evidence.