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

Abstract

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, 40 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 40 Ca is thus significantly reduced in the presence of 40 Ar. To avoid interference from 40 Ar during the 40 Ca measurements, ozone (O 3 ) was used as the reaction gas in the ICP−MS/MS analysis. The reaction of Ca + with O 3 in the collision cell preferentially produced CaO 3 + . In contrast, the reaction between Ar + and O 3 produced ArO + , which further reacted with the oxidized O 3 to produce Ar + . Because Ar + does not produced ArO 3 + as the main product of the reaction with O 3 , the measurement of Ca + as CaO 3 via O 3 -induced oxidation resulted in considerably less interference from Ar + . The detailed mechanism of CaO 3 + formation was investigated via quantum chemistry calculations by using density functional theory. The interaction between Ca + and O 3 yielded CaO 3 + , which immediately dissociated into either Ca + and O 3 or CaO + and O 2 because the pressure in the collision cell was approximately 1 Pa in the MS/MS experiments. Consequently, CaO 3 + was formed by the consecutive reaction of Ca + with O 3 , with CaO + and CaO 2 + as intermediates. The present method achieved the detection of less abundant Ca isotopes in certified reference materials such as 43 Ca, 44 Ca, 46 Ca, and 48 Ca, thereby allowing the determination of Ca isotopes with high sensitivity.Calcium (Ca) is the sixth most abundant element on Earth 1 and fifth most abundant in animals. Ca plays an important role in geological and biological processes. Because Ca consists of 20 protons, which are the magic numbers of nuclei, Ca isotopes are considerably more stable than the other elements. Consequently, Ca has 26 known isotopes, ranging from 35 Ca to 60 Ca. 2 Among these, four isotopes, that is, 40 Ca (96.941%), 42 Ca (0.647%), 43 Ca (0.135%), and 44 Ca (2.086%), are known as stable isotopes. In addition, 0.004% of 46 Ca and 0.187% of 48 Ca exist as long-live radioactive isotopes. The half-life of 46 Ca and 48