A novel standard for forensic elemental profiling of polymers by LA-ICP-TOF-MS

Abstract

Plastic materials are often found on crime scenes in improvised explosive devices. Determining the trace-elemental composition of polymer parts may yield discrimination between samples from different sources. Although quantitative laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is well established for forensic glass analysis, the lack of sufficiently reproducible and homogeneous reference standards hampers accurate quantification of trace elements in polymers. Therefore, the present study introduces a new standard for quantitative elemental profiling of polymers. A novel approach was developed for producing polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC) standards containing 23 elements at three concentrations. The LA-ICP-TOF-MS measurements showed excellent linear response with R2 > 0.99 for almost all elements and polymer matrices. Additionally, homogeneity was significantly improved to an average of 10% within-, and 17% between-sample variation. PVC and PE matrices presented the best homogeneity which was confirmed by line ablations. The added value of the standards was demonstrated by evaluating a set of realistic forensic polymer evidence items. The between-run variation was substantially reduced from 42% in absence of calibration to 30% when correcting with an elemental internal standard and even to 24% when applying the novel standards. In addition, its discriminating power between different classes was increased, as demonstrated by ANOVA and principal component analysis. In conclusion, this study shows the feasibility of using a novel standard for quantitative analysis of chemical attribution signatures of trace elements in polymers by LA-ICP-TOF-MS. The use of the polymer standards significantly improves classification and forensic comparison of polymer-based evidence.