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 ultrasonic nebulization inductively coupled plasma mass spectrometry (USN-ICP-MS). Ultrasonic nebulization increases sensitivity over regular pneumatic nebulization so that resolvable signals above background are possible in the fg/g range. Small sample sizes (≤250 mg) and low volumes of reagents produce procedural blank levels below the detection limit for Ru, Pd and Ir, in the fg/g range for Rh and in the low pg/g range for Pt and Au. Heterogeneous PGE and Au distributions in geological samples are avoided by a rigorous homogenization/rehomogenization procedure. The reference material UMT-1, which contains rock concentrations of the PGEs and Au at the 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 interferences are acute for silicate rocks, even after cation exchange chromatography. Isotope dilution cannot be adapted to this method for Ir and Pt because two interference-free isotopes are not available. An external calibration method has been developed for the accurate and precise determination of the PGEs and Au in rocks with low abundances of Hf and Ta (<1 μg/g and <0.2 μg/g, respectively); a standard addition method for rocks containing higher concentrations of these elements. The work presented here demonstrates the strengths and weaknesses of cation exchange for PGE and Au analyses in geological materials. It can be a useful tool in solving fundamental geological problems that depend on precise and accurate determination of these elements by taking full advantage of current technology and the sensitivity it provides.