Sample preparation and digestion considerations for determining metal deposition at small arms ranges

Determining the metal content of soil collected from small arms training ranges (SARs) is difficult and controversial because the contamination consists of fine particulates abraded from bullets and larger fragments such as intact bullets and spent shell casings. This heterogeneous distribution of materials can be difficult to sample reproducibly and difficult to prepare for analysis. Similar issues are encountered with sampling and analysing solid residues of energetic compounds for which grinding to 75 microns is necessary to achieve excellent precision. Issues to be resolved for SAR metals include the necessity of sieving, the extent of contamination from grinding, and the proper digestion procedure for efficient recovery. The work reported in this manuscript employed field and laboratory sampling approaches developed for energetics and examined a variety of laboratory sample preparation techniques for SAR metals. Application of the sampling approaches used for energetics to metals was straightforward. The US Environmental Protection Agency (USEPA) nitric acid digestion procedure is effective for the recovery of three of the most important SAR metals, copper, lead and zinc, when analysing a 2 g portion of the <2mm fraction of an unground soil sample. However, sieving the sample ignores a large portion of the total metal load and a total digestion with hydrofluoric acid is necessary to determine the concentration of all naturally-occurring metals in the sample. Finally, the USEPA-approved method for metal analysis is inappropriate for tungsten unless supplemented with phosphoric acid.