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.

Keywords:

• Geological samples
• high RF power cold plasma
• kinetic energy discrimination collision cell-inductively coupled plasma-mass spectrometry (KED-ICP-MS)
• Rh
• Sb fire assay

Disclosure statement

There are no conflicts of interest to declare.

Data availability statement

The datasets used during the current study are available from the corresponding author upon reasonable request.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [Grant Number 22106148].