Determination of Ultra-Trace Rhodium in Geological Samples by High Radio Frequency Power Cold Plasma and Kinetic Energy Discrimination Collision Cell-Inductively Coupled Plasma-Mass Spectrometry (KED-ICP-MS)

Abstract

A novel method was developed for the determination of ultra-trace rhodium (Rh) in geological samples. Using Sb2O3 as fire assay collector, Rh in 10 to 20 g was pre-concentrated in 50 mg Sb granules. After the granules were microwave digested in aqua regia (40%, v/v), 103Rh was determined by high radio-frequency (RF) power cold plasma and kinetic energy discrimination collision cell-inductively coupled plasma-mass spectrometry (KED-ICP-MS). Four methods to eliminate the mass spectral interferences of 206Pb2+ and 40Ar63Cu to 103Rh including standard mode (STD-ICP-MS) hyphenated with equivalent concentration correction of 206Pb2+ and 40Ar63Cu, KED-ICP-MS hyphenated with equivalent concentration correction of 206Pb2+, high RF power cold plasma ICP-MS hyphenated with equivalent concentration correction of 40Ar63Cu, and high RF power cold plasma hyphenated with KED-ICP-MS were compared. The first three methods required equivalent concentration correction so the Pb and Cu content in the sample solutions were determined in advance and subtracted. On the contrary, the interferences were eliminated by high RF power cold plasma hyphenated with KED-ICP-MS. The RF power of cold plasma was studied. Moreover, the He collision gas, nebulizer flow and sampling depth were optimized. Under the optimal conditions, Rh had a linear relationship from 0.001 to 10 ng mL−1 with a correlation coefficient of 0.9999. The detection limit was 0.012 ng g−1. The method was applied to analysis of various matrix certified reference materials (CRMs) and the measured results were in good agreement with the certified values.