Assessing Soil Preparation Techniques to Improve Portable XRF Precision and Recovery Efficiency of Lead in Urban Soils As Compared to the EPA 3051 Total Soil Digestion Method.

As the local food movement continues to grow, interest in urban gardening activities is blossoming in an effort to provide safe, nutritious produce to inner city residents. Unfortunately, the most attractive sites for this activity (e.g. open vacant land) are typically found to possess elevated soil concentrations of some potentially toxic trace elements, such as lead (Pb) and arsenic (As). Plant uptake of these contaminants is element specific and often not enough to incur health maladies when consumed. However, the direct exposure pathways of ingestion and inhalation are still a concern. Spatial heterogeneity of contaminants makes site risk assessment difficult, increasing the need for high resolution sampling protocols. In situ concentration determinations are possible with the advent of portable x-ray florescence (XRF) analyzers, but the values measured are not always equal to 100% of that quantified by total laboratory digestion and analysis methods. A portion of this deviation may be linked to interference caused by soil properties like aggregation, organic matter, and water content. This study aimed to investigate the effects of soil preparation techniques and XRF analysis duration on the recovery of Pb and As from soils found in four urban garden test plots. Samples were each analyzed four times: after sieving to <2mm, after grinding and sieving to <0.2mm, <0.15mm and pellet form after being ground for an additional three minutes. Mortar and pestle was used to grind each sample prior to each round of XRF analysis such that all soil passed through the sieve size specified. Analysis duration was one minute and two minutes. Total element concentrations were also determined at the end via microwave nitric acid digestion (EPA 3051) followed by ICP-OES analysis. XRF recovery per degree of soil preparation and analysis duration will be compared to total digestion considering the influence of properties, such as organic matter.