Elimination of oxide interferences and determination of ultra-trace silver in soils by ICP-MS with ion-molecule reactions

Silver is subject to significant spectral interferences caused by high concentrations of Zr, Nb, Mo and Y in inductively coupled plasma mass spectrometry (ICP-MS) analysis for soil or sediment samples. In this study, the Zr, Nb, and Mo based oxide and/or hydroxide polyatomic interferences were successfully eliminated by ion-molecule reactions in a dynamic reaction cell (DRC). These potentially interfering ions ⁹³Nb¹⁶O+, ⁹²Zr¹⁶OH+ and ⁹²Mo¹⁶OH+ on ¹⁰⁹Ag+ were rapidly oxidized to higher oxides ⁹³NbO₂+, ⁹²ZrO₂H+/⁹²ZrO₂H+ and ⁹²MoO₂H+ by O₂ as the reaction gas in DRC. However, interfering ions ⁸⁹Y¹⁸O+ and ⁹¹Zr¹⁶O+ on ¹⁰⁷Ag cannot be removed by this method, because the reaction rates of ZrO+ and YO+ to ZrO₂+/ZrO₃+ and YO₂+/YO₃+ were too low. Under the optimized O₂ flow rate (2.4 mL min⁻¹) and DRC rejection parameter q (Rpq, 0.75), the background signal was reduced by up to 100-fold at m/z 109 and the limit of quantitation (LOQ, 10σ) for ¹⁰⁹Ag was 0.5 ng g⁻¹. The proposed method was used to determine the concentration of Ag in twenty-eight soil standard reference materials (SRMs). The accuracy of the results suggests that the method has great potential for the direct determination of trace or ultra-trace levels of Ag in various environmental samples.