Sensitive and Spectral Interference-Free Determination of Rhodium by Photochemical Vapor Generation Inductively Coupled Plasma Mass Spectrometry

A sensitive method for Rh determination was developed by coupling photochemical vapor generation (PVG) for sample introduction to inductively coupled plasma mass spectrometry (ICPMS). PVG was conducted in a thin-film flow-through photoreactor operated in a flow injection mode from a photochemical medium comprising 10 M HCOOH. PVG efficiency was substantially enhanced by the addition of 10 mg L^–1 Cu²⁺ and 5 mg L^–1 Co²⁺ as mediators as well as 50 mM NaNO₃. The volatile product (likely Rh(CO)₄H) was found to be less stable when in prolonged contact with the liquid medium at the output from the photoreactor. Hence, further enhancement was achieved by introducing an Ar carrier gas near the exit of the photoreactor to minimize the interaction of volatile species with the liquid medium. Despite PVG efficiency reaching only 15%, measurement at the ultratrace level (20 ng L^–1) was characterized by very good repeatability of peak area response (2.9%) and outstanding limits of detection (13 pg L^–1, 6.5 fg absolute) using He in the collision cell. Interferences from potential coexisting metals, inorganic acids, and their anions were investigated. Accuracy was verified by analysis of OREAS 684 (Platinum Group Element Ore) and SRM 2556 (Used Auto Catalyst) following peroxide fusion for sample preparation. Application to the direct analysis of real river and lake water samples and reference materials AQUA-1 and SLRS-6 demonstrated excellent selectivity of the PVG-ICPMS methodology over conventional pneumatic nebulization-ICP(MS)/MS, the results of which were seriously biased by polyatomic interferences, especially from Sr and Cu, despite the use of various reaction/collision cell modes.