Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM-EDS) is the standard for identification of gunshot residues (GSR) as it provides both morphological and elemental information. Nonetheless, recent studies have shown that false positives still occur from non-GSR particles that share similar shape and composition. Also, due to the complex thermodynamic processes occurring during a firing event, not all particles are spherical or retain the characteristic elemental profiles. Thus, there is a critical need to enhance the reliability in GSR detection. The overall goal of this project is to validate a method to characterize the organic and inorganic constituents of firearms discharge residue using a single sample and one instrument. The instrumental system proposed is liquid chromatography coupled to triple quadrupole mass spectrometry (LC/MS).
The methodology has been previously developed and refined by the researchers research group; the aim of this work is to provide the foundation needed to ensure forensic laboratories can easily adopt the procedure. LC/MS systems are widely available in forensic laboratories and as such are an ideal platform for advancing forensic firearms discharge analysis. Collection of samples from the skin of individuals applies the same general approach as is currently implemented for GSR samples with a tacky polymer used in place of carbon tape. The substrate is sequentially extracted for organic and inorganic constituents. A key advance of this procedure is the use of complexing agents and host-guest complexes to identify and quantify elements critical to the identification of inorganic GSR such as barium, antimony, lead, copper, zinc, iron, calcium, and strontium. Complexes of these elements are amenable to electrospray ionization and can be separated chromatographically on the same column as used to characterize the organic constituents. To facilitate adoption in forensic laboratories, this project will focus on three research goals. The first is validation of the method to forensic laboratory standards. Reference materials will be used to characterize aspects of the process such as extraction efficiency while a carefully designed validation study will yield qualitative and quantitative figures of merit (accuracy, repeatability, reproducibility, limit of observation, limit of quantification) along with method uncertainty estimations. Secondly, a population study will provide estimates of background and environmental levels of the characterized firearm discharge residue constituents. Specific studies will focus on expected false positive sources such as law enforcement personnel as well as expected false negatives such as might occur after hand washing. Third, the populations study will form the basis of quantitative probabilistic models for interpretation of data.
Note: This project contains a research and/or development component, as defined in applicable law," and complies with Part 200 Uniform Requirements - 2 CFR 200.210(a)(14). ca/ncf