Direct determination of δ^44/42 Ca isotope ratio by ion chromatography/low-resolution multicollector ICPMS

High-precision on-line procedure for measurement of calcium isotopic ratio by coupling ion chromatography to multicollector inductively coupled plasma mass spectrometry was developed. Calcium separation from the sample matrix was achieved on an ion chromatography column-IonPac CS16-ID 3 mm connected with CERS 500 2 mm suppressor and followed by multicollector inductively coupled plasma mass spectrometry calcium isotopic ratio determination. Dry plasma mode was used with Aridus II desolvation system. To sustained samples with high level of total dissolved salts as well as account capacity of applied analytical column, the method has been optimized regarding calcium isotope ratio measurements with low-resolution mass spectrometry. Mass discrimination and instrument drift were corrected by sample-standard bracketing method using the ⁴⁴ Ca/⁴² Ca isotope ratio of SRM 915a as a standard. Good accuracy and reasonable precision of calcium isotope ratio (generally 0.20‰ [2SD]) were achieved, which are comparable to off-line Ca separation and continuous measurement. The reproducibility of the proposed analytical procedure was verified by measuring the SRM 915a standard as a sample randomly over 3 months (n = 56). Applicability of the protocol was demonstrated for matrix-rich natural water samples, coral samples, and bone standard reference materials.