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 ¹¹B/ ¹⁰B isotope ratio based on oxygen isotopes determined in situ from the same filament loading by N-TIMS is described. The method uses only a few nanograms of B to produce BO₂^- ions. First, the oxygen isotopes are determined at a lower filament temperature using ReO ₄^- ions and employing ¹⁸⁷Re/¹⁸⁵Re for internal normalization. Subsequently, the filament temperature is increased to get sufficient BO₂^- ions and predetermined ¹⁸O/¹⁶O isotopes from the same filament loading is used to correct for boron mass fractionation. The 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‰ (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 ¹⁸O/¹⁶O ratio and thus provides higher accuracy for applications in paleo-oceanography, geochemistry, and cosmochemistry.