Approaches with different reaction gases for the determination of iron in rare earth samples based on on-mass mode by inductively coupled plasma tandem mass spectrometer

The determination of iron using ICP-MS-MS is complicated due to the polyatomic spectral interferences, notably the ⁴⁰Ar¹⁶O⁺ on the abundant ⁵⁶Fe isotope. In this research, the utilization of three gases—helium, hydrogen, and methane—to accurately quantify iron was proposed using the on-mass mode by inductively coupled plasma tandem mass spectrometer. The conditions of the collision/reaction cell were optimized, particularly the gas flow rate and the hexapole bias voltage, for iron analysis. Different mechanisms, including collision and reactions like charge transfer reactions, hydrogen atom transfer reactions, and proton transfer reactions, were explored. In terms of sensitivity and background equivalent concentration (BEC), the hydrogen mode exhibited the highest sensitivity and the lowest BEC. The methane mode yields a comparable BEC to that of the hydrogen mode with a threefold reduction in sensitivity. The limit of quantification (LOQ) under the hydrogen reaction mode was as low as 0.028 μg/g.The analysis results of the national standard substance GBW07159 were in excellent agreement with the certified values, and the relative standard deviation (RSD) for n = 11 was less than 5 % in the three modes. The measured values and spiked recoveries for rare earth oxides were essentially consistent between the hydrogen and methane modes. Following validation, the hydrogen and methane reaction modes have been shown to accurately analyze iron in rare earth and rare earth oxides with high accuracy and stability, surpassing the helium mode in terms of collision mechanism.