Direct determination of boron and chlorine isotopic compositions in geological materials by negative thermal-ionization mass spectrometry

Abstract

We present a new, simple technique for the rapid and precise determination of boron ( 11 B 10 B) and chlorine ( 37 Cl 35 Cl) isotopic compositions in some natural materials (e.g., seawater, hypersaline brines, groundwater and calcium carbonate). The high reactivity of B and Cl enables production of BO 2 − and Cl − ions directly from untreated solutions which are analysed by negative thermal ionization mass spectrometry. The high sensitivity of the negative ions allows trace amounts of B and Cl, with concentrations as low as 0.06 and 40 ppm, respectively, to be directly analysed in natural samples with a precision of ∼ ±2‰ for both B and Cl (at the 95% confidence limit). In the case of boron, samples were analysed with and without chemical separation and no difference in isotopic ratios was found within the experimental error. Isotope-dilution mass spectrometry using enriched 10B has also been successfully applied to the quantitative analysis of the B contents of natural samples at trace levels. These methods have been tested on a variety of geological materials including seawater, groundwater, hypersaline brines, water-soluble salts and HCl-soluble minerals and have been found to be applicable to a wide range of water salinities and chemistries. The geochemical application of B and Cl isotope systematics is discussed and illustrated using examples of terrestrial waters and evaporites from Australia, China and Israel.