Optimisation of dynamic reaction cell (DRC)-ICP-MS for the determination of Ca-42/Ca-43 and Ca-44/Ca-43 isotope ratios in human urine

A new method for the measurement of calcium isotope ratios (Ca-42/Ca-43 and Ca-44/Ca-43) in urine using DRC-ICP-MS after oxalate precipitation is presented. The isotope ratios were measured with a precision of approximately 0.15% RSD, limited only by counting statistics in samples diluted to a total calcium concentration of 3 - 4 mg L-1. The observed mass bias was substantial, approximately 5 - 10% per u. Careful mass bias correction using matrix and concentration matched solutions was essential and necessary in order to obtain accurate results. A methane gas flow of 0.6 ml min(-1) was used as a reaction gas in the dynamic reaction cell to remove polyatomic interferences (e.g., Ar-40(+) and (ArH2+)-Ar-40). Unfortunately, other interferences are formed in the dynamic reaction cell from secondary reactions with methane adducts. An interfering ion observed at m/z = 43 with a precursor ion of m/z = 17 was removed by applying an Rpq setting of 0.4, while an interfering ion on m/z = 48 with a precursor ion of m/z = 37 could not be removed. Despite the fact that the Ar-40 interference was successfully removed by the methane reaction gas, it was not possible to measure the Ca-44/Ca-40 and Ca-42/Ca-40 ratios with a precision only limited by counting statistics, most likely due to uncertainties from the detection system when measuring high count rates. The developed method was successfully applied for the calcium isotope ratio analysis of human urine samples enriched in Ca-42 and Ca-44. It was found that an atom enrichment of 1.3% was needed in the urine samples in order to obtain reliable results.