The Application of Oxygen Plasma Ashing to Gunshot Residue Analysis

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Abstract
In the analysis of hand samples for barium and antimony levels indicative of gunshot residue, severe problems are encountered with swabs contaminated by blood, dirt, or grease. These problems originate from two main sources: decreased effectiveness of sample leaching and high instrumental backgrounds. When the flameless atomic absorption spectroscopy (FAAS) method developed in our laboratory [1] is used, the choice of sample collection materials is restricted to those that can be easily separated from the elements of interest. In our approach, a sample such as a cotton swab is leached with dilute nitric acid to extract any residues present. Surface contamination may make the swab impervious to solvent penetration and, therefore, inefficient leaching occurs. Contaminants such as blood, dirt, or grease on the sample can coat any primer residue particles present and prevent their dissolution in the acid, resulting in the amount of the elements of interest being lower in the solution than expected. The measured level of these elements will thus be lower than that actually present in the sample. Increasing the contact time between solvent and sample, or rigorous agitation to improve solvent attack, usually results in enhancement of the background response during the atomization phase of the AA analysis with no improvement in analytical results. This enhanced background, especially notable at the 217.6- and 217.9-nm (2176- and 2179-Å) lines used for antimony detection, may be attributed to light scattering by organic materials not completely destroyed during the drying and ashing cycle of the AA determination. Destruction of the matrix by ashing of the samples prior to analysis significantly decreases the analytical background and eliminates leaching difficulties.