High precision isotope ratio measurements on boron by thermal ionization mass spectrometry using Rb₂BO₂⁺ion

Abstract

Studies were carried out to determine the 10B/11B isotope ratio by positive thermal ionization mass spectrometry (P-TIMS) analyzing boron as rubidium borate ions. Of the 36 different ionic species formed, the boron and rubidium isotope ratios were obtained from the ion intensity ratios of the most suitable ion pairs corresponding to masses 212, 213 and 215, 213 respectively. The investigations were carried out to explore the possibility of correcting the observed isotope ratio of boron by using a modified internal normalization technique based on the observed Rb isotope ratio. The method is based on the relation of isotopic fractionation of boron as a function of the rubidium (natural isotopic composition) isotopes fractionation obtained during TIMS analysis from the same filament loading. The application of the methodology to improve the precision of the observed 10B/11B isotope ratio during analysis of irradiated boron alloy samples is demonstrated. Improvement in precision from 0.25% to better than 0.05% was demonstrated using this approach.