Boron isotope ratio measurements by negative thermal ionization mass spectrometry method using boron-free seawater

We used boron-free seawater as an ionization promoter for negative thermal ionization mass spectrometry to obtain correct and precise boron isotope ratios. The time profiles were recorded for a whole exhaustion process with 1-10 ng boron samples. From a maximum in the initial stable stage, the boron isotope ratios were obtained with 0.05% reproducibility from 1-10 ng and 50-400 ng boron samples. The observed boron isotope ratio (42/43) is 0.24841 ± 0.0013 for NIST SRM 951, which was a little smaller than that obtained with Ca(NO3)2 as an ionization promoter (42/43=0.24900±0.00013). The negative mode results were normally larger than the positive mode results. The observed value (42/43) was corrected to an absolute value (10/11=0.2455, 0.2461) using the Rayleigh fractionation factor (1.011835). The standard 0.2473 for Na2BO2+ (86/87) was also corrected to an absolute value (10/11=0.2459) using the Rayleigh fractionation factor (1.005666). The positive and negative mode results agreed well within experimental error. (author)