Evaluation and validation of ion mobility spectrometry for presumptive testing targeting the organic constituents of firearms discharge residue

Abstract

Firearms discharge residue (FDR) refers to both the inorganic particulates (GSR) and the organic constituents (OGSR) formed when a firearm is discharged. Currently, there are few if any viable presumptive or screening tests amenable to detection of FDR on skin. Ion mobility spectrometry (IMS) is already widely deployed in law enforcement and homeland security for use as a portable/presumptive detector for narcotics and explosives. The goal of this project was to evaluate IMS for use as a screening device to detect OGSR on hand swabs. Two instruments were thoroughly tested and figures of merit, including detection thresholds, were established. Sample stability was also characterized with significant degradation seen when samples were stored at room temperature. Based on the target compounds and media used, the stability studies suggest a holding time limit of days or a few weeks. Results showed that given proper and specialized QA/QC procedures, IMS can be successfully utilized for screening purposes without requiring modification of instrumentation. A well-suited method for daily monitoring was demonstrated through the use of control charts providing a means to track variability in peak intensity over time. Analysis of more than 200 skin swab samples demonstrated that pattern matching data analysis is preferred to peak-based methods when attempting to ascertain if a person recently fired a weapon.