MEASUREMENT OF CALCIUM ISOTOPE RATIOS USING A COLLISION CELL INTERFACED MULTI-COLLECTOR ICPMS

Calcium is composed of six isotopes, of which the major isotope ⁴⁰Ca comprises 97% of the isotope inventory. Accurate measurement of the isotopic composition of Calcium using an ICP-MC-MS, must overcome the problem of ⁴⁰Ar produced in the Argon plasma and which is isobaric with the ⁴⁰Ca. High mass resolution is impossible as the resolution required would be 170,000 and this is beyond the scope of any mass spectrometer. Alternative methods include cold plasma, which reduces the ⁴⁰Ar signal or the use of a collision cell. On the IsoProbe the addition of 1ml/min H₂ to the 2ml/min Ar collision gas, results in the complete removal of ⁴⁰Ar, this can be verified by measuring the isotopic composition of the blank. Measurement precision for the ⁴⁰Ca/⁴²Ca ratio is typically better than 0.007% for a ⁴⁰Ca ion signal of 6volts.  To get accurate isotope ratios the large mass bias must be corrected for. This can be achieved by using for example the ⁴²Ca/⁴⁴Ca isotope ratio and using an exponential fractionation correction, this approach has been widely used in TIMS. However, there still remains a correlation between the normalised ratios and ⁴²Ca/⁴⁴Ca which indicates that this method of fractionation correction is not sufficient. Mass fractionation correction procedures will be explored to improve the analysis reproducibility.