Optimization and Application of ICPMS with Dynamic Reaction Cell for Precise Determination of ⁴⁴Ca/⁴⁰Ca Isotope Ratios (original) (raw)

2007, Analytical Chemistry

An inductively coupled plasma mass spectrometer with dynamic reaction cell (ICP-DRC-MS) was optimized for determining 44 Ca/ 40 Ca isotope ratios in aqueous solutions with respect to (i) repeatability, (ii) robustness, and (iii) stability. Ammonia as reaction gas allowed both the removal of 40 Ar + interference on 40 Ca + and collisional damping of ion density fluctuations of an ion beam extracted from an ICP. The effect of laboratory conditions as well as ICP-DRC-MS parameters such a nebulizer gas flow rate, rf power, lens potential, dwell time, or DRC parameters on precision and mass bias was studied. Precision (calculated using the "unbiased" or "n-1" method) of a single isotope ratio measurement of a 60 ng g-1 calcium solution (analysis time of 6 min) is routinely achievable in the range of 0.03-0.05%, which corresponded to the standard error of the mean value (n) 6) of 0.012-0.020%. These experimentally observed RSDs were close to theoretical precision values given by counting statistics. Accuracy of measured isotope ratios was assessed by comparative measurements of the same samples by ICP-DRC-MS and thermal ionization mass spectrometry (TIMS) by using isotope dilution with a 43 Ca-48 Ca double spike. The analysis time in both cases was 1 h per analysis (10 blocks, each 6 min). The δ 44 Ca values measured by TIMS and ICP-DRC-MS with doublespike calibration in two samples (Ca ICP standard solution and digested NIST 1486 bone meal) coincided within the obtained precision. Although the applied isotope dilution with 43 Ca-48 Ca double-spike compensates for time-dependent deviations of mass bias and allows achieving accurate results, this approach makes it necessary to measure an additional isotope pair, reducing the overall analysis time per isotope or increasing the total analysis time. Further development of external calibration by using a bracketing method would allow a wider use of ICP-DRC-MS for routine calcium isotopic measurements, but it still requires particular software or hardware improvements aimed at reliable control of environmental effects, which might influence signal stability in ICP-DRC-MS and serve as potential uncertainty sources in isotope ratio measurements. Natural variation of the isotopic composition of calcium of rocks, minerals, and biological samples represents potential interest for geochronology, climate change studies, archaeometry,