An inter-laboratory evaluation of LA-ICP-MS analysis of glass and the use of a database for the interpretation of glass evidence

Ten laboratories conducting forensic glass analysis participated in three inter-laboratory exercises to evaluate the use of a standard method (ASTM 2927-16e1) for the analysis and comparison of glass evidence using LA-ICP-MS. This study was designed to evaluate the rate of misleading evidence (ROME) when blind glass samples were distributed to the participants and asked to compare the glass samples (K vs. Q) and report their findings as they would in a case. Three different databases were used as background populations to calculate likelihood ratios (LRs) and frequency of elemental profile. The first database was composed of 420 vehicle windshield samples and the analytical data and application of this database is reported here, for the first time. The second database was provided by the BKA laboratory in Germany representing 385 casework samples including an assortment of float glass, container glass, and specialty glasses. The third background database was a combination of both databases. In the first inter-laboratory exercise, the likelihood ratio (LR) calculations result in 34/36 (94.4%) correct associations and no false inclusions for all labs. LRs in the second and third inter-laboratory exercises result in all participating laboratories correctly associating glass samples originating from the same source (57 comparisons) and all laboratories correctly discriminating glass samples from different sources (167 comparisons). The random match probability of glass samples known to originate from different glass sources was found to be similar to 0.1% and in agreement with previously reported values by other researchers.