Fractionation Correction Methodology for Precise and Accurate Isotopic Analysis of Boron by Negative Thermal Ionization Mass Spectrometry Based on BO2- Ions and Using the 18O/16O Ratio from ReO4- for Internal Normalization

A novel approach to obtain a fractionation free B-11/B-10 isotope ratio based on oxygen isotopes determined in situ from the same filament loading by N-TIMS is described. ne method uses only a few nanograms of B to produce BO2- ions. First, the oxygen isotopes are determined at a lower filament temperature using ReO4- ions and employing Re-187/Re-185 for internal normalization. Subsequently, the filament temperature is increased to get sufficient BO2- ions and predetermined O-18/O-16 isotopes from the same filament loading is used to correct for boron mass fractionation. ne validity of the method has been demonstrated by analyzing a NIST-SRM-951 boron isotopic certified standard, two synthetic B mixtures, and two coral reference materials. An average analytical precision of 0.6 parts per thousand (n = 6) has been demonstrated. This is an important and crucial step forward in making the application of BO2- ions by N-TIMS routine in coral, foraminifera, and other samples where only limited amounts of boron are available. This new method does not require any additional effort in loading or in carrying out the mass spectrometric analysis but eliminates the need of assuming a fixed O-18/O-16 ratio and thus provides higher accuracy for applications in paleo-oceanography, geochemistry, and cosmochemistry.