de la Vega, Elwyn, Foster, Gavin, Martínez-Botí, Miguel, Anagnostou, Eleni , Field, Paul, Kim, Hwan, Watson, Paul and Wilson, Paul (2020) Automation of boron chromatographic purification for δ11B analysis of coral aragonite. Rapid Communications in Mass Spectrometry, 34 (11). e8762. DOI 10.1002/rcm.8762.

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Abstract

Rationale
To detect the small changes in past pH, the boron isotope ratio of coral carbonates, expressed as the δ11B value, needs to be both precise and accurate (2sd<<1‰). Boron measurements by Multi‐collector Inductively Coupled Plasma Mass Spectrometry (MC‐ICPMS) requires the boron to be carefully purified before analysis, which is time consuming, and requires specialist training. Here, we use the prepFAST‐MC that enables the automatic extraction of B (up to 25 ng load) from a CaCO3 matrix.

Methods
Samples were purified using the prepFAST‐MC automated system with a ~25‐μL column of Amberlite IRA743 resin. Boron isotope measurements were performed byMC‐ICPMS. The effect of matrix load, speed of sample load onto the column, and blank contamination were tested to evaluate the effect on the purification process. The optimised protocol was tested on various standards and samples of aragonite corals.

Results
The blank contribution for the approach is ~60 pg and is negligible given our sample size (<0.2% sample size). Efficiency of matrix removal is demonstrated with the addition of up to 1.6 mg of dissolved low‐B calcium carbonate to NIST SRM 951 with no impact on the accuracy of δ11B values. The Japanese Geological Survey Porites reference material JCp‐1, boric acid standard NIST SRM 951, and seawater, all processed on the prepFAST‐MC, give δ11B values within error of literature values (δ11BJCp‐1 = 24.31 ±0.20‰ (2sd, n=20); δ11BNIST 951 = ‐0.02 ±0.15‰ (2sd, n=13) and δ11BSeawater = 39.50 ±0.06 ‰ (2sd, n=2)). Results obtained from the coral Siderastrea siderea purified with the prepFAST‐MC show an average offset from the manual ion exchange protocols of Δδ11B=0.01 ±0.28‰ (2sd, n=12).

Conclusions
Our study demonstrates the capacity of the prepFAST‐MC to generate accurate and reproducible δ11B values for a range of material, without fractionation, with efficient matrix removal and with negligible blank contribution.

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