Purification of Mg from low-Mg biogenic carbonates for isotope ratio determination using multiple collector ICP-MS

A technique for separation of Mg, with 100% yield from low- Mg biogenic carbonates has been developed suitable for high precision analysis of Mg isotopes by multiple- collector inductively coupled plasma mass spectrometry. Two separate stages of ion- exchange chromatography were carried out using cation exchange resin, AG50W- X12 with a Mg recovery > 99.9%. The repeatability of the Mg isotope ratio measurement using this technique including chemistry and mass spectrometry is +/- 0.14 parts per thousand on delta(26)Mg and +/- 0.09 parts per thousand on delta(25)Mg at 95% confidence. This was demonstrated using a synthetic solution ( Na : Mg : Ca : Sr = 2 : 1 : 100 : 1 in weight) over a period of six months. The robustness of the technique was further assessed by replicate analyses of three natural samples, seawater, foraminifera and dolomite. A total variation over 4.5parts per thousand on delta(26)Mg was observed and the Mg isotope composition of seawater was 2.59 +/- 0.14parts per thousand on delta(26)Mg and 1.33 +/- 0.09parts per thousand on delta(25)Mg which were the highest isotope ratios among the samples measured.