Research on the ocean carbon cycle is vitally important due to the projected impacts of atmospheric CO2 on global temperatures and climate change, but also on ocean chemistry. The direct influence of this CO2 rise on the seawater pH can be evaluated from the boron isotopic composition in biogenic carbonates; however, conscientious laboratory techniques and data treatment are vital in obtaining accurate and precise results. A rapid-throughput boron purification and Total Evaporation Thermal Ionization Mass Spectrometry method was developed for high accuracy and precision boron isotopic analysis for small (ng) sample sizes.
An improved microsublimation method, in which up to 20 samples can be processed simultaneously under identical temperature conditions, was developed. Several tests have confirmed the viability of this technique. First, seawater and Porites coral samples were processed with H2O2 and the results compared with those obtained using microsublimation; second, the impact of various sublimation times was evaluated; and third, quantitative recovery was assessed using standard addition.
Microsublimation provides a valid method for the quantitative recovery and separation of boron from both major elements and organic matter under low-blank conditions. The close agreement of our results with published values validates the accuracy of the measurements. The isotopic ratio for SRM 951a boric acid isotopic standard was 4.0328 ± 0.0054 (2 STD, n = 25). The reproducibility of boron isotopic composition for standards including AE121, IAEA B-1 and an in-house coral standard UM-CP1 was ±0.68‰ (2 STD, n = 15), ±1.12‰ (2 STD, n = 24), and ±1.17‰ (2 STD, n = 14), respectively. The sample sizes were <1 ng for each measurement.
The developed method for preparing and measuring boron isotopic values in a variety of carbonate materials should facilitate the reconstruction of past ocean pH conditions with decadal-scale resolution.
Liu Y.-W., Aciego S. M., Wanamaker Jr. A. D. & Sell B. K., 2013. A high-throughput system for boron microsublimation and isotope analysis by total evaporation thermal ionization mass spectrometry. Rapid Communications in Mass Spectrometry 27(15): 1705–1714. Article (subscription required).