Macro-sublimation: Purification of boron in low-concentration geological samples for isotopic determination by MC-ICPMS

Two new macro-sublimation methods allowing processing of larger sample volumes with lower concentration requirements in solution are developed and systematically assessed. Development of these methods involved the design of two new pieces of lab apparatus; 15-mL Savillex PFA vial with home-made double-layer device (0.5 mL) and an independent two-bottle device (1 mL), which allow the limits on sublimated sample volume based on currently available micro-sublimation techniques to be overcome. The double-layer devices enhance 10-fold larger sublimated volume (0.5 mL) with the constant B recovery of 100% and negligible δ¹¹B fractionation when the devices are heated at 98 °C for 18 h. Boron could be efficiently purified from various matrices (i.e., IAPSO, JCp-1, IAEA-B-8 and JB-3) with the application of the double-layer device. The double-layer device shows the potential to extract B from Ca-rich matrices without causing significant isotopic fractionations and suggests that temperature gradient between device and hotplate could be critical for B purification. The two-bottle device shows complete recovery and acceptable reproducibility when 1 mL of sample aliquot is heated at 120 °C for 24 h. In addition to purifying sufficient volume for B isotope analysis using MC-ICP-MS in one-step sublimation, macro-sublimation can avoid blank contribution from dilution with working acid and multiple applications of micro-sublimation devices. With this improved technique, accurate and high precision δ¹¹B data for low-B-content and rarely documented natural samples can be generated, such as river water and rainwater samples, which can provide data more efficiently and give better constraints on source tracing and low-temperature weathering studies.