High-precision isotopic analysis of boron by positive thermal ionization mass spectrometry with sample preheating

Abstract

We present a methodology for the precise and accurate analysis of boron isotope ratios (¹¹B/¹⁰B) by positive thermal ionization mass spectrometry (P-TIMS) using Cs₂BO₂⁺ ions. Samples in the form of caesium borate were loaded onto Ta filaments together with graphite and mannitol. The addition of mannitol to the samples is essential to suppress boron volatilization during acid treatment of the samples but is known to lower the performance of P-TIMS. Therefore, the prepared filaments were preheated in an oven at 240 °C to eliminate the mannitol thus stabilizing the chemical species of boron on the filament and increasing the ionization efficiency of Cs₂BO₂⁺, which enabled high-precision isotopic analysis of boron with small sample sizes. Analyses of NIST SRM 951 standard showed external reproducibility (2RSD) better than ±0.1‰ for 50–100 ng B and ±0.2‰ for 10 ng B. Ultrafiltration followed by cation- and anion-exchange chromatography was used to chemically separate boron from natural samples. Analyses of coral standard GSJ JCp-1 and seawater standard IRMM BCR-403 gave average δ¹¹B values of +24.25 ± 0.08‰ and +39.58 ± 0.11‰ (2SD), respectively. The P-TIMS method developed in this study is applicable to a wide field of boron isotopic research that requires high precision and accuracy, including paleo-pH studies using marine carbonate samples.