PRECISE BORON ISOTOPIC ANALYSIS OF NATURAL ROCK SAMPLES USING A BORON MANNITOL COMPLEX

Newly developed techniques for boron chemical separation and isotopic analysis in natural silicate rocks and waters are described. Sample dissolution and the subsequent ion-exchange chromatography were conducted using hydrofluoric and hydrochloric acids in the presence of mannitol which suppresses boron volatilization and isotopic fractionation by the formation of a boron-mannitol complex. Thermal ionization mass spectrometry using the Cs2BO2+-graphite method was employed for the determination of boron isotopic composition. No boron isotopic fractionation was observed in the course of chemical separation and mass spectrometry. In the whole analytical procedure, procedural blank and recovery yield of boron were 3-4 ng and 99 +/- 1%, respectively. The analytical precision and reproducibility of measured B-11/B-10 ratios were +/- 0.1-0.1% and +/- 0.2% for the measurements of basalt and seawater, respectively. The present method enables us to determine the isotopic composition of < 1-mu-g B in silicate samples and in natural fluids with the above-mentioned analytical errors. This method also provides a remarkable improvement in the measurement of boron concentration by isotope dilution mass spectrometry because of the achievement of complete mixing between sample and spike during sample decomposition.