Direct determination of boron and chlorine isotopic compositions in geological materials by negative thermal-ionization mass spectrometry
We present a new, simple technique for the rapid and precise determination of boron (11B 10B) and chlorine (37Cl 35Cl) 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 BO2- 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. © 1989.
Vengosh, A; Chivas, AR; McCulloch, MT
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