Sampling strategies for the analysis of glass fragments by LA-ICP-MS Part II: Sample size and sample shape considerations.

Glass fragments recovered from crime scenes are usually very small and therefore the amount of sample available to conduct forensic analyses is limited. Elemental analysis using conventional digestion methods consumes at least 2-3mg of glass per replicate. LA-ICP-MS requires 10,000 times less glass consumption per analysis ( approximately 280ng), and therefore the sample remains practically unaltered. Typically, the recovered fragments (unknowns) are 0.1-1mm in length, while the "known" samples are usually larger, i.e. a broken fragment from a windshield (>3mm). For bulk digestion analysis, the difference in fragment size does not present a problem for elemental comparisons - other than requiring at least 6mg for triplicate analysis - because the sample is crushed and homogenized before weighing. Laser ablation sampling results in the creation of small craters ( approximately 50mum diameter and 80mum deep) drilled into the sample due to the interaction of the laser with the glass target. This study aims to evaluate whether the quantitative elemental analysis using the LA sampling method is affected by the size of the glass fragment due to differences in heat dissipation and surface-laser interaction. The analytical method employed for the analysis of glass by LA-ICP-MS had previously shown to possess the same or better performance than dissolution ICP-MS methods in terms of accuracy, precision, limits of detection and discrimination power. A 266nm Nd:YAG laser with a flat top beam profile was used in single point mode sampling a 50mum spot size for 50s at 10Hz. Standard glass reference materials SRM 612 and SRM 610 were selected to conduct this work in order to account for different concentration ranges and different opacities of the samples. The set under study was comprised of seven fragments originating from each standard at different sizes and shapes ranging from 6 to 0.2mm length. Analysis of variance (ANOVA) followed by the Tukey's honestly significant different test (HSD) was used for data analysis. The results show that there is no significant difference in the elemental composition of different sized fragments. The conclusions, however, cannot be generalized for fragments measuring less than 0.2mmx0.1mm.