Assessing the evidentiary value of smokeless powder comparisons

Gas chromatography-electron ionization-mass spectrometry (GC-EI-MS) and physical characteristics data for 726 smokeless reloading powders were analyzed by pairwise comparisons of samples comprising the same product and different products. Pairwise comparisons were restricted to samples having matching kernel shape, color, presence or absence of a perforation and measurements. Discrete results were analyzed for same and different products having matching chemical composition determined from a list of 13 organic components. A continuous score-based likelihood ratio was determined for same and different product comparisons using the Fisher transform of the Pearson correlation between the total ion spectra of the compared samples. Probability distributions for same product and different product comparisons appeared bimodal and were modeled with kernel density distributions. In the discrete and continuous data comparisons, the likelihood ratios for probabilities conditioned on same shape, color, presence/absence of perforation and size were found to provide relatively limited support for either the proposition of same product or different product. Further restricting the pairwise comparisons to samples belonging to the same cluster, as determined by agglomerative hierarchical cluster analysis, provided probability distributions for same product and different product comparisons that were more normal, but did not improve the resulting likelihood ratios. These results inform the forensic analyst regarding the evidentiary value of database search results and direct comparisons of recovered and control samples of smokeless powders.