Quantifying the platinum group elements (PGEs) and gold in geological samples using cation exchange pretreatment and ultrasonic nebulization inductively coupled plasma mass spectrometry (USN-ICP-MS)

A method for analyzing low abundances of the platinum group elements (PGEs) and gold has been developed using cation exchange chromatography and ultra sonic nebulization inductively coupled plasma mass spectrometry (USN-ICP-MS ). Ultrasonic nebulization increases sensitivity over regular pneumatic neb ulization so that resolvable signals above background are possible in the f g/g range. Small sample sizes (less than or equal to 250 mg) and law volume s of reagents produce procedural blank levels below the detection limit for Ru, Pd and Ir, in the fg/g range for Ph and in the low pg/g range for Pt a nd Au. Heterogeneous PGE and Au distributions in geological samples are avo ided by a rigorous homogenization/rehomogenization procedure. The reference material UMT-1, which contains rock concentrations of the PGEs and Au at t he low ng/g levels, monitored recoverability. Interferences of HfO+ and TaO + on Ir, Pt and Au were not seen in the analyses of UMT-1 because Hf and Ta have low abundances in this reference material. However, these interferenc es are acute for silicate rocks, even after cation exchange chromatography. Isotope dilution cannot be adapted to this method for Ir and Pt because tw o interference-free isotopes are not available. An external calibration met hod has been developed for the accurate and precise determination of the PG Es and Au in rocks with low abundances of Hf and Ta (< 1 mu g/g and < 0.2 m u g/g, respectively); a standard addition method for rocks containing highe r concentrations of these elements. The work presented here demonstrates th e strengths and weaknesses of cation exchange for PGE and Au analyses in ge ological materials. It can be a useful tool in solving fundamental geologic al problems that depend on precise and accurate determination of these elem ents by taking full advantage of current technology and the sensitivity it provides. (C) 1999 Elsevier Science B.V. All rights reserved.