Determination\nof Calcium Isotopes Using Inductive\nCoupled Plasma–Tandem Mass Spectrometry with Ozone-Induced\nOxidation

In this study, we propose a new method for determination\nof calcium\n(Ca) isotopes using inductive coupled plasma–tandem mass spectrometry\n(ICP–MS/MS). The most abundant isotope of Ca, <sup>40</sup>Ca, has a spectrum identical with that of argon (Ar), which is often\nused as the carrier gas in ICP–MS analysis. The detection capability\nfor <sup>40</sup>Ca is thus significantly reduced in the presence\nof <sup>40</sup>Ar. To avoid interference from <sup>40</sup>Ar during\nthe <sup>40</sup>Ca measurements, ozone (O<sub>3</sub>) was used as\nthe reaction gas in the ICP–MS/MS analysis. The reaction of\nCa<sup>+</sup> with O<sub>3</sub> in the collision cell preferentially\nproduced CaO<sub>3</sub><sup>+</sup>. In contrast, the reaction between\nAr<sup>+</sup> and O<sub>3</sub> produced ArO<sup>+</sup>, which further\nreacted with the oxidized O<sub>3</sub> to produce Ar<sup>+</sup>.\nBecause Ar<sup>+</sup> does not produced ArO<sub>3</sub><sup>+</sup> as the main product of the reaction with O<sub>3</sub>, the measurement\nof Ca<sup>+</sup> as CaO<sub>3</sub> via O<sub>3</sub>-induced oxidation\nresulted in considerably less interference from Ar<sup>+</sup>. The\ndetailed mechanism of CaO<sub>3</sub><sup>+</sup> formation was investigated\nvia quantum chemistry calculations by using density functional theory.\nThe interaction between Ca<sup>+</sup> and O<sub>3</sub> yielded CaO<sub>3</sub><sup>+</sup>, which immediately dissociated into either Ca<sup>+</sup> and O<sub>3</sub> or CaO<sup>+</sup> and O<sub>2</sub> because\nthe pressure in the collision cell was approximately 1 Pa in the MS/MS\nexperiments. Consequently, CaO<sub>3</sub><sup>+</sup> was formed\nby the consecutive reaction of Ca<sup>+</sup> with O<sub>3</sub>,\nwith CaO<sup>+</sup> and CaO<sub>2</sub><sup>+</sup> as intermediates.\nThe present method achieved the detection of less abundant Ca isotopes\nin certified reference materials such as <sup>43</sup>Ca, <sup>44</sup>Ca, <sup>46</sup>Ca, and <sup>48</sup>Ca, thereby allowing the determination\nof Ca isotopes with high sensitivity.