Elemental and isotopic profiling of potassium perchlorate salts for forensic explosives investigations

This study explores the feasibility of using isotopic and elemental signatures for the source attribution of potassium perchlorate (KClO 4 ) salts, which can play a significant role in forensic explosives investigations. A total of 41 different reference KClO 4 batches were analysed using isotope ratio mass spectrometry and inductively coupled plasma – mass spectrometry. Rubidium was the only elemental impurity detected consistently across all reference batches. Combined with the oxygen isotopic composition, these features provide a discriminating power of 94 %. A score-based likelihood ratio (LR) system was developed also including concentrations of sodium, magnesium, chromium, and iron, which were detected in 69–98 % of the reference samples and showed significant variation between sources. The LR system employs Manhattan distance and kernel density estimation to model (dis)similarity across source comparisons. Five-fold cross validation results suggest the LR system effectively distinguishes between same-source and different-source pairs (C llr = 0.184) and can be used to report LR values of up to 74 (moderate support). Infrequently detected trace elements and chlorate concentrations allow for a more comprehensive interpretation of chemical profile analyses and should also be considered to uphold forensic integrity. A series of tests have established a reliable method for the reduction of KClO₄ to KCl for subsequent chlorine isotope ratio – mass spectrometry ( δ 37 Cl -IRMS) measurements, which could provide additional discriminating power. Further research is required to develop a scalable process that yields sufficient material for δ 37 Cl-IRMS and to ensure that the sample preparation process yields reproducible δ 37 Cl values. • Rubidium, sodium, magnesium, chromium, and iron are discriminating impurities. • Rubidium is the only element impurity consistently found in potassium perchlorate. • IRMS and ICP-MS analyses can provide moderate support of source-level hypotheses. • Chlorine isotopic composition and chlorate content can offer additional support.