Determination of cadmium in geological samples by aerosol dilution ICP-MS after inverse aqua regia extraction

Abstract

The determination of cadmium (Cd) in geological samples by inductively coupled plasma mass spectrometry (ICP-MS) suffers from significant Mo and/or Zr based oxide interference. We have developed a valid method for Cd determination using Ar aerosol dilution ICP-MS after extraction with inverse aqua regia. Over 90% of the Zr was removed in the extraction procedure, and the residual Zr-hydroxides and Mo-oxides or hydroxides were successfully eliminated by adding an amount of Ar to the sample aerosol prior to the plasma. Compared to the conventional mode without adding Ar, the amount of oxide and hydroxide ions formed in the plasma was reduced by up to 90%. The relative yields of the interfering oxides and hydroxides were as low as 0.005% (MoOH/Mo or MoO/Mo) and 0.007% (ZrOH/Zr). Under the optimized dilution gas flow rate (0.85 L min⁻¹) and carrier gas flow rate (0.24 L min⁻¹), the limit of detection (LOD, 3σ) for ¹¹¹Cd was 1.6 ng g⁻¹. The proposed method was applied to the determination of Cd in 81 soil, sediment, and rock standard reference materials (SRMs). The results for 68 of these geological SRMs were in good agreement with the reference values. The Cd levels in 10 limestone SRMs (GSR-22, GSR-23, GSR-24, GSR-27, GSR-28, GUI-1, GUI-2, DIAN-1, DIAN-2, and DIAN-3) were reported for the first time, and reference Cd values for other 3 geological SRMs (GSD-7, GSD-19, and GSM-1) were updated by this method. The results showed that this method has great potential for Cd determination in geological samples.