Application of a sequential extraction scheme to ten geological certified reference materials for the determination of 20 elements

Methods are described for a sequential extraction scheme to dissolve selectively elements bound in soils and sediments in the following nominal forms: (1) adsorbed, exchangeable, carbonate (AEC); (2) amorphous iron oxyhydroxide (am Fe ox), including manganese oxides; (3) crystalline iron oxides (cry Fe ox); (4) organics and sulfides; and (5) residual.(mainly silicates). This scheme has been applied in triplicate to a suite of ten international CRMs, ie., soils SRM 2709-2711 and the SO-1-4 series, marine mud MAG-1, lake sediment LKSD-4 and the till sample TILL-2. Elements determined comprise: As (by HG-quartz tube AAS, HG-QTAAS); Be, Ca, Co, Cr, Cu, Ni, P, Ti and V (by ICP-AES); Mn, Fe and Zn (by FAAS); and Cd, Ce, Li, Li, Tl Pb and U (by ICP-MS). The precision obtained is excellent, generally in the range 2-10% RSD, at concentrations 10 x higher than detection limits. Most results for the element concentrations summed over the five phases agree, within statistical uncertainties, with the recommended total values for the CRMs. Those where recoveries are significantly below 90% are for elements such as Cr and V, which are known to be present in refractory minerals and would require fusion for complete dissolution. The results presented herein for samples SO-1-4 and MAG-1 do not agree well with those recently published using a scheme purported to dissolve similar phases. This highlights the need to be more definitive in describing the nature and extent of the phases actually extracted so that comparisons can be made between different laboratories and studies.