Nanogram-scale boron isotope analysis through micro-distillation and Nu Plasma 3 MC-ICP-MS

The determination of boron isotopes (δ¹¹B) represents a powerful tool for a variety of applications such as the reconstruction of past ocean pH and atmospheric pCO₂ from the analysis of marine biogenic carbonates. In recent years, MC-ICP-MS has gained popularity over other techniques thanks to its superior sample throughput and high ionization efficiency. This study evaluates, for the first time, the performance of the Nu Instruments Plasma 3 MC-ICP-MS for measuring δ¹¹B using different sample introduction systems and detector configurations. The main goal is to provide a detailed methodology for nanogram-scale boron isotope analysis through a straightforward approach that can be easily adopted. Boron (B) purification from the carbonate matrix was performed through micro-distillation, using a temperature of 95 °C and a minimum heating duration of 15 h, allowing the full recovery of B from up to 3 mg of carbonate mass. We attained blank values (on average 14 ± 6 pg, 1 SD, n = 27) comparable to the lowest micro-distillation blanks reported in the literature. Three sample introduction systems were tested, and the 30 μL min^-1 nebuliser system outperformed the 50 and 170 μL min^-1 systems in terms of signal intensity per mass of B. Two detector configurations were used based on the total boron signal intensity achieved: (1) FC11/FC12, with two Faraday cups fitted to 10¹¹ Ω and 10¹² Ω amplifier resistors to detect ¹¹B and ¹⁰B ion beams, respectively, and (2) FC12/IC, with which we investigated, for the first time, the feasibility of combining an ion counter for detecting ¹⁰B, and a Faraday cup fitted to a 10¹² Ω amplifier for ¹¹B. The FC12/IC configuration provided accurate results compared to the use of two Faraday cups for total boron signals lower than 0.35 V (∼12 ng of B in the analysed solution). The proposed analytical procedure was validated through the analysis of several reference materials with varying boron amounts, including clam JCt-1, coral JCp-1, NIST RM 8301 Foram and Coral solutions, and boric acid ERM-AE121. Furthermore, the long-term reproducibility was assessed with two in-house standards (coral CLD-1 and foraminifera GINF-1), providing values of 25.68 ± 0.23 ‰ (2SD, n = 53; with 14-36 ng of B) and 14.90 ± 0.16 ‰ (2SD, n = 12; with 11-16 ng of B), respectively.