Improvement on Extraction and Determination Procedures of Boron Isotopic Composition in Tourmalines

Tourmaline was decomposed by using alkali fusion,and then separated and purified by three different procedures.It is found that both Fe~(3+) and Al~(3+) ions that are rich in tourmaline samples seriously affect the accurate determination of boron concentration by azomethine-H spectrophotometric method and also cause the 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,we establish a three-column ion-exchange procedure 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(about 99%) from tourmaline samples with complex matrices.The positive thermal ionization mass spectrometry(PTIMS)-Cs_2BO_2~+ -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 ~(11)B/~(10)B value of NIST SRM 951 is ~(11)B/~(10)B=4.05044±0.00012(2σ,n = 8,1μg B),which is superior to the dynamic collection method in internal/external precision.Aδ~(11) B value of -0.3%o 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 isotopes in natural samples by TIMS and MC-ICP-MS after chemistry procedure turns out that SB values determined by the PTIMS-Cs_2BO_2~+ method are in good agreement with that by MC-ICP-MS.