Experimental Assessing about Sr Interference Effect on Ca Isotope Measurements on Thermal Ionization Mass Spectrometry

Calcium and strontium are all alkaline earth metals. Because of their similar chemical properties, it is difficult to separate strontium from calcium completely during chemical separation processes. Thus, the calcium cut after chemistry usually contains some strontium. Besides, it is known that Sr double charges, such as ⁸⁸Sr²⁺, ⁸⁶Sr²⁺, ⁸⁴Sr²⁺, could be treated as interferences to ⁴⁴Ca⁺,⁴³Ca⁺, ⁴²Ca⁺ respectively, during calcium isotope measurements on thermal ionization mass spectrometry (TIMS). To evaluate this interference effect, we designed an experiment and ran a series of samples on TIMS using single Ta filaments. IAPSO seawater standard was used in the experiment, and a certain amount of Sr standard solution was added in it to make the mixture has a Ca/Sr ratio about 5. The mixture was then divided into several equal solutions and pass through the calcium columns respectively. For each column, different percentage of Sr cut was collected together with whole Ca cut and calcium ratio was determined thereafter on TIMS using single Ta filaments. As the TIMS measurements’ result, all of those samples, although having different Ca/Sr ratios, display unique calcium isotopic compositions with IAPSO seawater standards that our laboratory monitoring for a long time within error. Based on the results, we conclude that currently, for the most geology samples, that usually contain a Ca/Sr ratio＞5, Sr interference effect on Ca measurement on TIMS using single Ta filament is very limited, which could be neglected. Besides, the chemical separation procedure for Ca could be improved, through increasing the concentration of the leaching acid, better Ca peak shape and efficiency can be reached.