A 10-fold improvement in the precision of boron isotopic analysis by negative thermal ionization mass spectrometry

Boron isotopes are potentially very important to cosmochemistry, geochemistry, and paleoceanography. However, the application has been hampered by the large sample required for positive thermal ionization mass spectrometry (PTIMS), and high mass fractionation for negative-TIMS (NTIMS). Running as BO₂^-, NTIMS is very sensitive and requires only nanogram sized samples, but it has rather poor precision (∼0.7-2.0 ‰) as a result of the larger mass fractionation associated with the relatively light ion. In contrast, running as the much heavier molecule of Cs₂BO₂⁺, PTIMS usually achieves better precision around 0.1-0.4 ‰. Moreover, there is a consistent 10 ‰ offset in the ¹¹B/¹⁰B ratio for NIST SRM 951 standard boric acid between the NTIMS and the certified value, but the cause of this offset is unclear. In this paper, we have adapted a technique we developed earlier to measure the ¹³⁸La/¹³⁹La using LaO^{+ 1} to improve the NTIMS technique for BO₂. We were able to correct for instrumental fractionation by measuring BO₂^- species not only at masses of 42 and 43, but also at 45, which enabled us to normalize ⁴⁵BO₂/⁴³BO₂ to an empirical ¹⁸O/¹⁶O value. We found that both I₄₅/I₄₂ = (¹¹B¹⁶O¹⁶O) and (I₄₃/I₄₂)_C = (¹¹B¹⁶O¹⁶O/ ¹⁰B¹⁶O¹⁶O) vary linearly with (I₄₅/I₄₃)_C × 0.5 = (¹¹B¹⁶O¹⁸O/¹¹B¹⁶ O¹⁶O) × 0.5 = ¹⁸O/¹⁶O. In addition, different activators and different chemical forms of B yield different slopes for the fractionation lines. After normalizing ¹¹B¹⁶O¹⁸O/¹¹ B¹⁶O¹⁶O ×0.5 to a fixed ¹⁸O/¹⁶O value, we obtained a mean ¹¹B/¹⁰B value of NIST SRM 951 that matches the NIST certified value at 4.0430 ± 0.0015 (±0.36‰, n = 11). As a result, our technique can achieve precision and accuracy comparable to that of PTIMS with only 1%o of the sample required. This new NTIMS technique for B isotopes is critical to the studies of early solids in the solar system and individual foraminifera in sediments that require both high sensitivity and precision.