Determination of Ultra-Trace Platinum, Palladium, Ruthenium, Rhodium, and Iridium in Rocks and Minerals by Inductively Coupled–Plasma Mass Spectrometry Following Nickel Sulfide Fire Assay Preconcentration and Open Mixed Acid Digestion

Abstract

Platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), iridium (Ir), and osmiun (Os) are platinum-group elements with similar physic-chemical properties, and have important applications in geochemistry and environmental chemistry. However, due to their low abundance and inhomogeneous distribution in natural ores as well as the nugget effect, the accurate determination of the platinum-group elements has been a challenge for geological analysis. In this work, self-prepared and purified sodium carbonate (NiCO₃) instead of commercial nickel oxide (NiO) was used as the fire assay collector in order to greatly reduce the reagent blank and method detection limits. In addition, the fuming time of HClO₄ was strictly controlled at 10 min and a high sensitive method was developed for the simultaneous determination of ultra-trace Pt, Pd, Ru, Rh, and Ir in minerals by inductively coupled plasma-mass spectrometry (ICP-MS) following preconcentration with the nickel sulfide fire assay. Under the optimized conditions, the linear ranges of Pt, Pd, Ru, Rh, and Ir were between 0 and 100 ng mL⁻¹, with correlation coefficients exceeding 0.9997. The detection limits were 0.015, 0.056, 0.014, 0.004, 0.012 ng mL⁻¹ (for 10 g sample) for Pt, Pd, Ru, Rh and Ir, respectively. The developed method was successfully applied to analyze Chinese Certified Reference Materials (CRMs) GBW07288, GBW07289, GBW07290, GBW07291, GBW07292, GBW07293, GBW07294, GBW07101, GBW07102 and GBW07201 and the determined values were in good agreement with the certified values. The relative standard deviations (n = 5) of Pt, Pd, Ru, Rh and Ir were between 3.42% and 6.87% for the determination of GBW07291.

Keywords:

• Inductively coupled plasma–mass spectrometry (ICP-MS)
• nickel sulfide fire assay
• Pt
• Pd
• Ru
• Rh and Ir
• purified sodium carbonate

This work was supported by the Subject of National Key Research and Development Program of China under grant number 2016YFF0201604; the Geological Survey Program of the CGS under grant number DD20189263; and the Geological Survey Program of the CGS under grant number DD20160339.