Efficient separation of boron using solid-phase extraction for boron isotope analysis by MC-ICP-MS

Abstract

In nature, boron exhibits a large range of isotopic compositions (up to 150‰), which makes it a powerful geochemical tracer. Improving the separation and purification procedures of boron in complex matrices and at low boron contents is a challenge in obtaining high-precision and accurate boron isotope ratios. A method of ion exchange using solid-phase extraction (SPE) for the efficient separation of boron from samples with low boron contents was developed for determining boron isotopes by MC-ICP-MS. The results showed that the total amount of residual boron in the solution after ion exchange decreased from 60 μg at a vacuum pressure of 0.09 MPa to 0 at 0.04 MPa. When the vacuum pressure was between 0.008 and 0.04 MPa boron was not detected in the solution. The flow rate of the solution increased from 50 μL min⁻¹ at a vacuum pressure of 0.008 MPa to 1358 μL min⁻¹ at a vacuum pressure of 0.09 MPa, and the volume of eluent required increased from 300 μL at 0.008 MPa to 600 μL at 0.04 MPa. Therefore, the recommended vacuum pressure was below 0.04 MPa based on these results. The δ¹¹B values of NBS 951 after chemical treatment using SPE were between −0.25‰ and 0.26‰. Using this procedure, boron in river water, rainwater and seawater samples was separated and the B isotope compositions analyzed by MC-ICP-MS. The accuracy and precision of our method were assessed by comparing the measured δ¹¹B values of these samples with published results using positive-ion thermal-ionization mass spectrometry. The improved efficient separation of boron from samples with a low boron content using SPE is applicable to a wide range of boron isotopic separations.