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 (similar to0.7-2.0 parts per thousand) 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 parts per thousand. Moreover, there is a consistent 10 parts per thousand offset in the B-11/B-10 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 La-138/La-139 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 (BO2)-B-45 /(BO2)-B-43 to an empirical O-18/O-16 value. We found that both I-45/I-42 = ((BOO)-B-11-O-16-O-18/(BPO)-B-10-P-16-O-16) and (I-43/I-42)(C) = ((BOO)-B-11-O-16-O-16/(BOO)-B-10-O-16-O-16) vary linearly with (I-45/I-43)C x 0.5 = ((BOO)-B-11-O-16-O-18/(BPO)-B-11-P-16-O-16) x 0.5 = O-18/O-16. In addition, different activators and different chemical forms of B yield different slopes for the fractionation lines. After normalizing (BOO)-B-11-O-16-O-18/(BPO)-B-11-P-16-O-16 x 0.5 to a fixed O-18/O-16 value, we obtained a mean B-11/B-10 value of NIST SRM 951 that matches the NIST certified value at 4.0430 +/- 0.0015 (+/- 0.36parts per thousand, n = 11). As a result, our technique can achieve precision and accuracy comparable to that of PTIMS with only 1parts per thousand 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.