Evaluation of X-ray microfluorescence spectrometry for the elemental analysis of firearm discharge residues

Abstract

The conventional technique employed to sample Firearm Discharge Residues (FDRs) is by pressing an adhesive coated SEM stub repeatedly over the area of interest. The sample is then analysed by Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDX). While having excellent spatial resolution, the SEM/EDX process lacks elemental sensitivity compared to other techniques such as X-ray fluorescence (XRF), where 100× to 1000× better elemental sensitivity may be achieved. The aim of this preliminary study was to evaluate relatively new technology, X-ray microfluorescence (micro-XRF), as a method of analysing FDRs, to compare the ability of micro-XRF to screen for FDR particles on SEM stubs with that of conventional SEM/EDX analysis, and to evaluate the ability of the micro-XRF to conduct on-target FDR analysis. The micro-XRF utilises a non-destructive method that can rapidly locate and analyse individual FDR particles 10 μm in diameter and larger. This analysis has the advantage that it can produce elemental maps (by X-ray mapping) directly from the target substrates, indicating the distribution of elements across a sample. Different types of ammunition were employed to include various types of primer and bullet compositions. The projectiles and primers were fired onto various target substrates at close-range and the residues analysed by micro-XRF. SEM stubs known to contain FDR particles were also analysed by micro-XRF. The results indicate that micro-XRF can effectively detect and analyse FDR particles that are larger than 10 μm in diameter. With the present instrumental configuration (50 W tube and 100 μm collimator) the micro-XRF cannot replace the SEM/EDX for the analysis of individual FDR particles less than 10 μm in diameter. It was demonstrated that micro-XRF has significant advantages for the on-target analysis of residues resulting from close range discharges (less than 30 cm). Information obtained from the micro-XRF analysis of the bullet wipe area can also give an indication of projectile composition.