Improvements of boron isotope analysis by positive thermal ionization mass spectrometry using static multicollection of Cs_2BO_2~+ ions

摘要

A static double collector system in the Finnigan-MAT 262 thermal ionization mass spectrometer was used to analyse different amounts of the boric acid standard NBS SRM 951 and natural samples. The main objectives of the study were to improve precision and accuracy of B isotope measurement, and to define the critical minimum amount of B for both standards and natural silicatic samples with which stable signals, and hence reliable measurements are obtained. A series of analyses were carried out using 1 μg, 100, 50, and 25 ng B for each NBS SRM 951 measurement, and 1 μg, 100, and 50 ng for measurement of natural silicatic samples. Stable runs could be obtained with this double collector package for both, the standard SRM 951 and for natural samples containing 100 ng of B on the filament. The internal analytical precision of measured ~(11)B/~(10)B ratios of 100 ng NBS SRM 951 boric acid is ±0.006%. The reproducibility (2σ_(mean)) is ±0.0002%, equivalent to an external uncertainty of 0.024%, both of which are more precise than previously published results from static measurements. Natural samples (two hemipelagic clays) analysed with 100 ng of B have an internal analytical precision of 0.009%-0.01% and a reproducibility (2σ_(mean)) of ±0.004%-0.007%, respectively. The external reproducibility for natural samples is ±0.11‰-0.16‰, which demonstrates that the isotope ratio of small amounts of natural samples with low B contents can be measured reliably. Although smaller B concentrations of 50 ng have been measured with higher precision than that from earlier studies, the results of this study are in favour of 100 ng B being the optimum amount for static (multicollection) positive thermal mass spectrometry measurements. Given that the availability of B is often limited (i.e. trace contents) in natural samples, the precision gained with only 100 ng B can be viewed as a significant improvement in B isotope analysis.