Accurate determination of trace silver in geological reference materials by inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS)

Abstract

Silver (Ag) is a trace metal element in geological rocks and accurate determination of its abundance is very challenging given significant oxide and hydroxide interferences from Zr, Nb, Mo, and Y. Here, we developed a valid method based on inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS), in which NH₃ was used as the reaction gas, to determine the Ag content in geological samples. The additional quadrupole analyzer located in front of the collision/reaction cell ensured a simple and controllable reaction in the cell. The first quadrupole (Q1) was set to m/z 107 and 109 to allow Ag⁺ and its interferences to enter the reaction cell. Ag⁺ partially reacted with NH₃, forming Ag⁺ and Ag(NH₃)₂⁺ as the main products; while the major interferences, ZrO⁺ and NbO⁺, reacted strongly and were converted to ZrO(NH₃)₄⁺, ZrO(NH₃)₅⁺, and NbO(NH(NH₃)₃)⁺, respectively, and thus no longer affected the measurement of Ag. Consequently, the Ag signals could be detected by both “on-mass” and “mass-shift” methods, where the m/z of Q1–Q2 were 107–107, 109–109 and 107–141, 109–143, respectively. Under the optimized NH₃ flow rate (3.5 ml min⁻¹), the interference levels of the matrix solution (0.3 μg ml⁻¹ Zr and Mo, 0.2 μg ml⁻¹ Zn, 0.1 μg ml⁻¹ Y and Nb, and 0.05 μg ml⁻¹ Ga) in four Q1–Q2 mass pairs were negligible and comparable to background signals, and the limits of detection of geological samples were <0.53 ng g⁻¹. The Ag content of 19 geological reference materials was reported using the proposed ICP-MS/MS method, and the results were consistent with isotope dilution-ICP-MS values. Thus, the proposed method has great potential for the accurate, direct, and high-throughput measurement of Ag in geological samples.