Visualizing the Reaction Products of Sodium Rhodizonate with Lead(II) or Barium(II) Salts: New Insights into Forensic Gunshot Residue Detection

Establishing the presence of gunshot residue (GSR) is a crucial investigative insight used by forensic scientists to place a discharged firearm in the vicinity of shooters or victims in the conduct of crime scene reconstructions. The most prevalent presumptive test for identifying GSR on objects or locations is the sodium rhodizonate test, which produces a visible red or orange coloration upon reaction with lead or barium, respectively. Despite widespread use since the 1950s, the identity of the chromophores produced when sodium rhodizonate reacts with GSR metals remains poorly characterized. This work provides new insights and solid-state structures of metal rhodizonate coordination species. Additionally, a host of new and unexpected chromophore candidates were identified, providing a more complete picture of the chemical complexity underpinning GSR testing. A multi-instrumental approach was undertaken to support the characterization of five lead, four barium, and one organic dye structures with single crystal diffraction studies, thereby linking these solid-state structures to solution-state behavior. This work assigned coloration of key solution- and solid-state reaction species using UV–visible and absorbance spectroscopy, and enabled the interpretation of solution-state reaction behavior from in situ mass spectrometry analysis of the mixtures, thereby providing unprecedented insights into the chemistry that underpins the sodium rhodizonate test for GSR.