Determination of trace calcium in high-purity rare earth oxide by triple quadrupole inductively coupled plasma mass spectrometry

To meet the higher requirements for material purity and impurity control driven by the rapid development of the rare earth market, a method for the determination of trace calcium in high-purity rare earth oxides was established using triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). To address the isobaric interference encountered in the determination of calcium by ICP-MS, CH₄ was introduced as a reaction gas in MS/MS mode. This approach utilizes the charge exchange and chain reactions between CH₄ and ⁴⁰Ar⁺ to effectively suppress argon-based polyatomic ion interferences. By optimizing key parameters such as reaction gas flow rate and hexapole bias voltage, the background equivalent concentration (BEC) for ⁴⁰Ca was reduced to 3.77 μg/L under the conditions of a CH₄ flow rate of 3.0 mL/min and a hexapole bias voltage of -2 V. The method was applied to four high-purity rare earth oxide samples (purity not less than 99.99%). The relative standard deviation (RSD, n=6) ranged from 1.3% to 3.2%, and the spike recovery rates were between 94.6% and 105.8%. The results were in good agreement with those obtained from other laboratories, indicating that the proposed method is accurate, reliable, and suitable for the determination of trace calcium in high-purity rare earth oxides.