Simultaneous determination of noble metals, Re, Se, As and Sb by radiochemical neutron activation analysis

Abstract

A sensitive radiochemical technique is described, which allows the simultaneous analytical determination of the platinum-group elements Pd, Pt, Ir, Os and Ru, as well as Au, Se, As, Re and Sb, in common silicate rocks. About 100 mg of finely ground powder is irradiated with different neutron fluxes, either at the SLOWPOKE reactor (University of Toronto) or at the reactor of the McMaster University, Hamilton, Ontario. After adding powdered FeNi alloy and the carrier-spike solution, the irradiated samples are fused at 1600°C in a reducing atmosphere of N 2, CO and CO 2. These conditions result in the formation of an immiscible metal liquid which separates the siderophile elements from the lithophile elements. The metal phase is dissolved in HCl and the solution is put on an ion exchange column of Srafion NMRR, which selectively absorbs the noble metals. The resin itself is counted on a sensitive intrinsic germanium detector. After a suitable delay, the resin from each sample is reirradiated and recounted to obtain the yield of the separation, which averaged 65–75% for the precious metals, Re and Sb. Recovery rates for Se and as are significantly lower, at 55% and 30%, respectively. Under optimum conditions, detection limits of 0.004 ppb for Au, 0.03 ppb for Ir and Re, 0.05 ppb for Sb, 0.2 ppb for As, 3 ppb for Pd, 1 ppb for Os, 1.5 ppb for Pt, 2 ppb for Ru and 5 ppb for Se in the silicate sample can be achieved. Precision is best for Au, followed by Ir and Re, Sb, As, Os, Pt, Pd, Ru, and Se. Accuracy, as determined by the analysis of in-house and international rock standards, is usually better than 10%. Thus, this method allows reasonably precise determinations of precious metals and associated chalcophile elements at concentration levels in the low-ppb range.