A comparison of sodium rhodizonate test solutions for improved GSR visualisation

Sodium rhodizonate is the most common chromophoric reagent used for the detection of lead in gunshot residue (GSR). When treating GSR, an acidic pre- or post-treatment is used alongside sodium rhodizonate to enhance the test's sensitivity and specificity. Widespread use of the test since the 1950s has led to several method variations being published, including the use of acidic ancillary reagents for transferring, pre-treating, or post-treating GSR. A comprehensive comparison of common reagents used in the sodium rhodizonate test was conducted here to evaluate their impact on the sensitivity, intensity, and persistence of the red development formed in the presence of lead standards. The concentration of those standards was informed by the analysis of GSR samples with ICP-OES, which allowed accurate determination of the mass of lead deposited at different shooting distances. Ancillary reagents were initially compared on Benchkote®, which prompted 2% tartaric acid, 2% sodium bitartrate, and 15% acetic acid to be selected for further investigations. An interlaboratory study was conducted to compare the performance of these three reagents when treating lead residues on fabric substrates and when used as a transfer reagent from fabric and plasterboard. The reagents were subsequently applied to simulated gunshot residue samples, to evaluate their impact on the spread, intensity, and persistence of their development. Direct treatment of substrates was found to be more sensitive than performing a transfer. Using acetic acid as the transfer reagent was found to deliver the best sensitivity on samples, but the development was weaker and persisted for a shorter time than when tartaric acid or sodium bitartrate were chosen as the ancillary reagents.