Evaluation of plasma cleaning as an approach for the preparation of soil minerals for forensic comparison by photon and electron microscopy

Anna S. Duggar, Thomas A. Kubic

Abstract

Although organic material is often used for forensic analysis, a substantial portion of the data gathered for determination of common origin of forensic soil samples is the inorganic, mineralogical composition of the sample, which may be obscured by the presence of soil organic material (SOM). Traditionally, SOM is removed by acidic, alkaline, or peroxide digest, or by combustion, but these techniques risk the damage to or destruction of target minerals of interest. Low–temperature plasma ashing, on the other hand, removes organic materials by exposing them to plasma ions with high‐kinetic energy, converting organics to easily removed volatile products (CO, CO2, H2O, or methane) while avoiding the thermal alterations caused by heat combustion. This study exposed grains of known mineral types to 20 min of a low‐pressure O2 plasma generated by a 10 MHz frequency generator. Powder x‐ray diffraction was chosen as an independent method to evaluate the minerals for chemical or structural changes caused by this ashing process. Side‐by‐side comparison of before and after diffractograms revealed minimal, if any, variation in the detected 2θ and subsequently calculated d‐spacing: differences in d values were found to generally be less than 1%, and most were within Hanawalt Search Index uncertainties by no less than a full order of magnitude. Peak intensity changes were similarly minimal. This study strongly suggests that low‐temperature plasma ashing can be used for the isolation of inorganic soil material fraction for forensic soil analysis with little or no concern for potential alteration of the mineral grains.