Laboratory intercomparison of marine particulate digestions including Piranha: a novel chemical method for dissolution of polyethersulfone filters

The US GEOTRACES program will generate marine particulate trace metal data over spatial scales and depth resolutions never before sampled. In preparation for these analyses, we conducted a four laboratory intercomparison exercise to determine our degree of intercalibration and to examine how several total digestion procedures perform on marine particles collected on polyethersulfone (PES, Pall Supor) filters. In addition, we present a new chemical method for complete dissolution of PES filters using a combination of sulfuric acid and hydrogen peroxide called Piranha reagent that can be conducted using minimal specialized equipment. Intralaboratory subsampling variability across 142 mm particulate matter filters, for subsamples representing approximately 10 L filtered seawater, was measured at an element-dependent 1% to 9% (RSD: 1 sigma/(x) over bar, %), whereas interlaboratory variability accounted for an additional 5% to 42% variability. Lab-and element-specific trends in recoveries are discussed, though all digestion methods tested appear to completely solubilize particulate material. We recommend rigorous determination of digest-acid and/or filter process blanks, as some particulate elements (namely Pb and Zn) have natural abundances that approach blank values.