Comparison of Soil Data Obtained Using Aqua Regia Variants on 8 Standard Reference Materials

A variety of analytical protocols are employed by environmental agencies in North America, there are no consistent national or international protocols. Aqua Regia (AQ), or similar, digestions are common. To determine whether an AQ, or some other hot acid, digestion would be used to determine near-total metal and metalloid levels in soil materials for the Tri-National survey an experiment comparing five digestion protocols, Aqua Regia, Lefort (reverse AQ), 1:1 HCl-HNO3, 1:1:1 HCl-HNO3-H2O, and the HNO3-H2O2 variant of US-EPA 3050B was undertaken. Eight control reference materials, two each of soils, tills, and stream and lake sediments, were analysed, with appropriate randomization, in triplicate by the five methods. Analyses of Variance were undertaken to determine which protocols yielded similar data. These indicated that the HNO3-H2O2 variant of US-EPA 3050B generally extracts significantly less metals and metalloids, with the exception of Hf, Nb, Th and Zr, than AQ and similar digestions. For the remaining protocols, AQ and three HCl-HNO3 variants, the results are similar, even when results are statistically (p<0.05) different between protocols an inspection indicates that most differences are likely insignificant in a geochemical context. Considering the volume of AQ and US-EPA AQ-variant data existing in North America and internationally, and that the AQ-like variants yield very similar results for most elements, it is recommended that the Tri-National project employs the US-EPA 3050B AQ-variant digestion. This procedure used a 4:1 HCl-HNO3 mix rather than the 3:1 of “classical” AQ. The Geological Survey of Canada has prepared a protocol defining the procedures to be used for Tri-National analyses. These include the choices of 1 g (<63 µm) or 10 g (<2 mm) aliquots and a 1:12.5 solid:reagent ratio.