Extraction and Determination of Boron Isotopic Composition in Tourmalines
Tourmaline is an abundant boron-rich alumosilicate mineral with complex chemistry and rich in Fe, Mg, Al, Si elements. In this study, tourmaline samples were decomposed by alkali fusion method, and then the boron was separated and purified using three different procedures. It was found that both Fe3+ and Al3+ ions, rich in tourmaline samples seriously, which affected the accurate determination of boron concentration by Azomethine-H spectrophotometric method and also caused a loss of boron by specific adsorption when large amount of amorphous hydroxide precipitate formed in ion-exchange columns. The addition of small amount of EDTA can eliminate the influence, but brings serious isobaric interference on boron isotopic analysis by TIMS. Finally, a three-column ion-exchange procedure was established including the first mixed resin column, the peristaltic pump coupled boron specific resin column, and the second mixed resin column, which ensures the full recovery of boron (> 99%) from tourmaline samples with complex matrices. The PTIMS-Cs2BO2+-static double-collection method was established by selecting H3-H4 Faraday cups and optimizing parameters in Zoom Optics (Focus Quad: 15; Dispersion Quad: −85) in a Triton TI mass spectrometer. The determined average 11B/10B value of NIST SRM 951 standard boron solution was 4.05044±0.00012 (2σ, n = 8, 1 μg B), which was superior to the dynamic collection method in internal/external precision. A δ11B value of −0.3% for NIST SRM 951 through the same pretreatment procedure was obtained, indicating that there was no isotopic fractionation occurred during the extraction procedure. The comparison of boron isotopic compositions in natural samples by TIMS and MC-ICP-MS after the chemistry procedure indicated that δ11B values determined by the static PTIMS-Cs2BO2+ method were in good agreement with that by MC-ICP-MS.