Reducing sample amount for forensic glass analysis using LA-ICP-TOFMS and multivariate statistics

Abstract

The forensic analysis of glass fragments is often limited by the size of objects found during investigation. Many of the glass fragments found on suspects are too small to be analysed with the established laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method, and alternative methods like refractive index measurements show significantly higher error rates. This work builds on a previous method which used resolved laser pulses and a time of flight mass spectrometer (single pulse ICP-TOFMS) to analyse glass fragments pulse by pulse and get more information from one sample while requiring 25× less material. Resolved signals of individual laser pulses are achieved by using low dispersion ablation cells, of which two were compared: The parallel flow ablation cell (PFAC) designed for fast washouts using small spot sizes and a modified version (MPFAC) specifically made for using larger spot sizes. The signal durations of each laser pulse showed a narrow Gaussian distribution for the MPFAC compared to a wide bimodal distribution of the PFAC. Due to the significantly improved aerosol transport using the MPFAC, smaller and more reproducible relative standard deviations were observed. When applying standard interval matching procedures for forensic glass fragments, the PFAC achieved a success rate of up to 95%, while the MPFAC achieved 98%, which are comparable to the established LA-ICPMS method used by forensic institutes. Using multivariate statistics for matching requires significantly higher amounts of data points than dimensions, which previous methods did not allow. Using single pulse LA-ICP-TOFMS data, it was possible to use Hotelling's T²-test for matching. Direct application of the statistical tests resulted in unusable high amounts of error rates. However, closer investigation into the resulting F-values revealed a significant difference between the values of matching pairs and mismatching pairs. Increasing the F-value threshold for matching resulted in success rates around 99% for both ablation cell designs. This work validates the value of single pulse LA-ICP-TOFMS for the analysis of small samples which are commonly found in forensic applications. Furthermore, it showed the advantages of multivariate statistics and the necessary corrections that are required when used with real case samples.