Purification of boron using a combination of cationic and boron-specific resins and determination of boron isotopic composition in sediments by MC-ICP-MS

Abstract

Boron, as both a lithophilic and biophilic element with high solubility, provides profound insights into the history of the Earth and environmental changes through its isotopes present in sediments. However, there are still some challenges in the accurate and precise determination of boron isotopes in sediments, particularly in pre-processing. When using boron-specific resins for the purification and enrichment of boron, it is in an alkaline environment. During the pre-treatment process, insoluble hydroxides of Al, Fe, and rare earth elements precipitate, exhibiting strong boron adsorption in high-pH solutions. This has potential impacts on the accuracy of boron isotope measurements. This study assesses a method that uses Na₂CO₃ and K₂CO₃ for dissolution at reduced temperatures and a cationic resin combined with a boron-specific resin for the enrichment and purification of boron in sediments. All the Al, Fe, and rare earth elements, which can generate water-insoluble hydroxides, were mainly removed by the cationic resin column, and boron in the sample solutions without Al, Fe, and rare earth elements was extracted by the boron-specific resin. The boron isotope composition (δ¹¹B) was determined using MC-ICP-MS. The results indicate that this method fully decomposes and efficiently purifies the sediments. The long-term instrumental external reproducibility of δ¹¹B values for NIST SRM 951a, ERM-AE120, and ERM-AE121 were 0.01 ± 0.09‰ (n = 13), −20.28 ± 0.16‰ and 19.98 ± 0.14‰, based on the measurements using a series of international reference materials over the last three years. The measured δ¹¹B values for the standards have various matrices. The δ¹¹B values of GSR-1 (−13.14‰ ± 0.27‰), BHVO-2 (−0.66 ± 0.06‰), AGV-2 (−3.22 ± 0.14‰) and Nass-6 (39.83 ± 0.07‰) are in agreement with the published data within uncertainty. The samples from Sihailongwan Maar Lake (SMK), the Loess Plateau (Loess-1 and Loess-2), shale (SH-1 and SH-2), and basalt (BL-1 and BL-2) were treated. The δ¹¹B values for SMK, Loess-1, and Loess-2 are −10.20 ± 0.29‰, −11.06 ± 0.16‰, and −10.79 ± 0.10‰, showing a close relationship with δ¹¹B of the continental crust. The δ¹¹B values for shale (SH-1 and SH-2) are −7.06‰ ± 0.32‰ and +2.28‰ ± 0.05‰, while those for basalt (BL-1 and BL-2) are −8.78‰ ± 0.09‰ and +2.50‰ ± 0.19‰. These results indicate a significant variation in the δ¹¹B values across these samples. These findings imply that this approach is capable of efficiently enriching and purifying boron in sediments. It enables accurate and precise determination of δ¹¹B in sediments, thus offering a reliable tool for conducting boron isotope studies in sedimentary earth science.