An interlaboratory study to evaluate the forensic analysis and interpretation of glass evidence

The results of an interlaboratory study by ten different laboratories using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and standard test method (ASTM E2927-16e1) for the analysis and comparison of glass evidence is reported. The aims were to evaluate the performance of the new CFGS2 calibration standard for the quantitative analysis of simulated casework samples, evaluate the comparison criterion as recommended by the ASTM E2927 method (association or non-association) and the use of a likelihood ratio (LR) as a quantitative determination of the strength of evidence found. Each laboratory calculated a LR to report the significance of glass source comparisons for a set of glass samples of known origin. Two different types of background databases were used for the calculation of the LR to evaluate the effect of the size and composition of the databases on the calculation of the LR. As expected, glass that originated from the same windowpane was found to be indistinguishable using the ASTM E2927 match criteria and result in a high LR value (strong support for an association) and glass that originated from different vehicles are distinguished (strong support for an exclusion). Glass samples that originated from different vehicles but that were the same make, model and year (or comparisons between the inner and outer pane of the same windshield) were chemically similar and reflected in a low LR. Good agreement among the laboratories was reported with < 5 % relative standard deviations (RSDs) among participants with few exceptions.