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, Ca, 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 Ca is thus significantly reduced in the presence of Ar. To avoid interference from Ar during the Ca measurements, ozone (O) was used as the reaction gas in the ICP-MS/MS analysis. The reaction of Ca with O in the collision cell preferentially produced CaO. In contrast, the reaction between Ar and O produced ArO, which further reacted with the oxidized O to produce Ar. Because Ar does not produced ArO as the main product of the reaction with O, the measurement of Ca as CaO via O-induced oxidation resulted in considerably less interference from Ar. The detailed mechanism of CaO formation was investigated via quantum chemistry calculations by using density functional theory. The interaction between Ca and O yielded CaO, which immediately dissociated into either Ca and O or CaO and O because the pressure in the collision cell was approximately 1 Pa in the MS/MS experiments. Consequently, CaO was formed by the consecutive reaction of Ca with O, with CaO and CaO as intermediates. The present method achieved the detection of less abundant Ca isotopes in certified reference materials such as Ca, Ca, Ca, and Ca, thereby allowing the determination of Ca isotopes with high sensitivity.