Determination of sub-ng g−1 Au in geological samples by ion molecule reaction ICP-MS and CH4 plasma modifier

Abstract

Direct determination of sub-ng g⁻¹ levels of gold by inductively coupled plasma mass spectrometry (ICP-MS) is complicated because of the presence of serious mass interferences, and the high first ionisation energy of Au (9.225 eV) also results in poor analytical sensitivity. A reliable method based on the combination of ion molecule reaction (IMR) ICP-MS and addition of a CH₄ plasma modifier technique was evaluated for the direct determination of Au in geological samples. The interfering ¹⁸¹Ta¹⁶O⁺ and ¹⁸⁰Hf¹⁶OH⁺ (the sample matrix source) on the mono-isotope ¹⁹⁷Au were successfully eliminated by oxidation using O₂ as the reaction gas. The deduced IMR mechanism in the reaction cell involved the oxidation of ¹⁹⁷TaO⁺ and ¹⁹⁷HfOH⁺ to the higher oxides ²¹³TaO₂⁺ and ²¹³HfO₂H⁺, ²²⁹TaO₃⁺ and ²²⁹HfO₃H⁺, and ²⁴⁵TaO₄⁺ and ²⁴⁵HfO₄H⁺, while the target Au does not react with O₂. In addition, to further improve the method's signal to background ratio (SBR), a CH₄ modifier was introduced to the ICP plasma at 2 mL min⁻¹; the poor sensitivity of Au⁺ was increased by a factor of four and the background signal (at m/z 197) was decreased to 50%. The improvement of the SBR (eight fold) was due to both the carbon enhancement effect (for Au) and the carbon competition effect (with the interfering oxide ions) in the CH₄ modified plasma. The proposed method was applied to the direct determination of Au in ten different types of geological standard reference materials (SRMs). The results of all SRMs were found to agree well with the certified values. This method has great potential for the direct determination of trace levels of Au in various geological samples.