High-precision Determination of Gold Mass Fractions in Geological Reference Materials by Internal Standardisation

Determination of gold abundances in natural rock is critical for applications, but very challenging. Here, we report a method for determining gold with a very low mass fraction (> 0.01 ng g(-1)) in rocks. The method involves Carius tube digestion with reverse aqua regia, chromatographic separation to remove most of the sample matrix and measurement by high-sensitivity ICP-MS. The mono-isotopic element gold was quantified by external calibration using an internal standardisation of gold to platinum that was precisely determined by isotope dilution. The method is robust and the obtained results are indistinguishable (< 5-10%, 2s) from those independently obtained by a standard addition technique on the same solution. The results from reference materials TDB-1 and GPt-2 are consistent with the certified values and those determined by HF-aqua regia digestion, confirming the validity of the method. TDB-1 (n = 20), GPt-2 (n = 6), BHVO-2 (n = 9) and other mafic RMs are homogenous for gold (10-20%, 2s) at the 2 g test portion level; however, sample heterogeneity affects some RMs. Gold and platinum-group elements also display different extents of sample heterogeneity for different RMs. Given the homogeneity observed for TDB-1, GPt-2 and BHOV-2, they are recommended as well-suited RMs for inter-laboratory comparison studies of gold.