Determination of Trace Cadmium in Geological Samples by Aerosol Dilution ICP-MS with Inverse Aqua Regia Extraction

Abstract:

Cadmium (Cd) is a trace element that occurs at ppb level in most terrestrial materials. The determination of Cd in geological samples by ICP-MS is subject to Zr and/or Mo based oxide/hydroxide interference. This study developed a valid method for the determination of Cd by Ar aerosol dilution ICP-MS with inverse aqua regia extraction (in a water bath at 95℃ for 2h). An Agilent 7700x ICP-MS (Agilent Technologies, USA) with an aerosol dilution system was used. The extraction procedure separated most of the Zr matrix (>90%) from the analyte, 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 operation, the amounts of oxide and hydroxide ions formed in the plasma were reduced by up to 10 times. The relative yields of the interfering oxides and hydroxides were as low as 0.012% ((⁹⁴Mo¹⁶OH⁺+⁹⁵Mo¹⁶O⁺)/(⁹⁴Mo⁺+⁹⁵Mo⁺)) and 0.007% (⁹⁴Zr¹⁶OH⁺/⁹⁴Zr⁺). 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, 3s) for ¹¹¹Cd was 1.3 ng g^-1. The accuracy of the method was assessed by using two USGS SRMs (andesite AGV-2 and basalt BCR-2). The Cd contents determined for AGV-2 and BCR-2 are 0.058±0.004μg g^-1 and 0.148±0.007μg g^-1 (N=10), which are in good agreement with the USGS reference values (0.061μg g^-1 and 0.14μg g^-1). The proposed method was also applied to determine Cd contents in 65 IGGE SRMs (28 soils, 28 sediments and 9 rocks). The measured Cd levels in these samples agree well with their certified values. The developed method shows great potential for the direct determination of trace levels of Cd in geological samples.