Development and application of ion chromatography for analyzing explosive residues in criminal Investigations: A study of ions in ATM and residues from emulsion and pipe bomb explosions

Abstract

The criminal use of explosives, particularly in attacks on automated teller machines (ATMs), remains prevalent in many countries, including Brazil. Ion chromatography is a leading technique for analyzing residues from post-blast materials. In this study, we developed methods to analyze 8 cations and 18 anions using suppressed ion chromatography with conductivity detection. Different collection materials and procedures were also evaluated. The chromatographic methods were assessed using various metrics and demonstrated analytical performance in line with existing literature. The methods were applied to determine the ion content in three brands of swabs commonly used for sample collection. Initial results showed significant levels of various ions, which were largely removed through a simple washing process. Our findings indicate that certain swab brands of the same type perfomed better than others, and the preferred swab was used in recovery studies. We concluded that using pre-washed swabs moistened with water results in higher recovery rates and reduces interference. An ion stability test conducted over 12 weeks suggested that swabs should be stored in a refrigerator and ideally analyzed within the first week after collection to minimize analyte loss. Background samples collected from an ATM did not contain ion levels that could be mistaken for those from explosive residues. The chromatographic and sampling methods were applied to real samples from different types of explosive residues, including those from an emulsion-based explosive and a pipe bomb containing black powder substitute. The analysis successfully identified key ions: NO3⁻ and NH₄⁺ in the emulsion, and ClO₄⁻ and K⁺ in the black powder substitute, allowing us to differentiate the explosives used. This work introduces significant methodological advancements in the analysis of explosive residues, particularly through the development of an extensive chromatographic method, the mitigation of ion interferences, improved sample preservation protocols, and the successful application to real-world scenarios, distinguishing emulsion-based explosives from gunpowder.