Determination of Calcium Isotopes Using Inductive Coupled Plasma–Tandem Mass Spectrometry with Ozone-Induced Oxidation

In this study, we propose a new method for determination of calcium (Ca) isotopes using inductive coupled plasma-tandem mass spectrometry (ICP-MS/MS). The most abundant isotope of Ca, ⁴⁰Ca, has a spectrum identical with that of argon (Ar), which is often used as the carrier gas in ICP-MS analysis. The detection capability for ⁴⁰Ca is thus significantly reduced in the presence of ⁴⁰Ar. To avoid interference from ⁴⁰Ar during the ⁴⁰Ca measurements, ozone (O₃) was used as the reaction gas in the ICP-MS/MS analysis. The reaction of Ca⁺ with O₃ in the collision cell preferentially produced CaO₃⁺. In contrast, the reaction between Ar⁺ and O₃ produced ArO⁺, which further reacted with the oxidized O₃ to produce Ar⁺. Because Ar⁺ does not produced ArO₃⁺ as the main product of the reaction with O₃, the measurement of Ca⁺ as CaO₃ via O₃-induced oxidation resulted in considerably less interference from Ar⁺. The detailed mechanism of CaO₃⁺ formation was investigated via quantum chemistry calculations by using density functional theory. The interaction between Ca⁺ and O₃ yielded CaO₃⁺, which immediately dissociated into either Ca⁺ and O₃ or CaO⁺ and O₂ because the pressure in the collision cell was approximately 1 Pa in the MS/MS experiments. Consequently, CaO₃⁺ was formed by the consecutive reaction of Ca⁺ with O₃, with CaO⁺ and CaO₂⁺ as intermediates. The present method achieved the detection of less abundant Ca isotopes in certified reference materials such as ⁴³Ca, ⁴⁴Ca, ⁴⁶Ca, and ⁴⁸Ca, thereby allowing the determination of Ca isotopes with high sensitivity.