The effect of matrix interferences on in situ boron isotope analysis by laser ablation multi-collector inductively coupled plasma mass spectrometry

Rationale Boron isotope analysis of marine carbonates by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) offers the potential for rapid sample throughput, and the means to examine micron-scale variations in the delta B-11 signatures of fossil skeletons and shells/tests of marine organisms. Existing studies demonstrate an acceptable level of reproducibility is achievable, but also typically show a level of accuracy outside the limits required by most applications. Here we investigate matrix interference effects as a cause of inaccuracy and imprecision. Methods Analyses were performed on a standard format Thermo Scientific Neptune Plus MC-ICP mass spectrometer coupled to a New Wave Research 193 nm ArF laser ablation system. The effects of matrix interference on delta B-11 analysis were investigated through analyses of a set of reference materials with differing B/Ca ratios. Three approaches to correct for matrix-induced effects were trialled: (1) use of matrix-matched standards, (2) utilisation of the relationship between delta B-11 inaccuracy and(11)B/Ca-43, B-11/(ArCa4+)-Ar-40 or B-11/Ca-interference from three reference materials with known delta B-11 values and varying B/Ca ratios, and (3) direct characterisation of the (sloping) interference itself. Results Matrix interference from scattered Ca ions on B-10 can impede both the accuracy and the reproducibility of delta B-11 analysis by LA-MC-ICP-MS. Based on analyses of two in-house reference materials, deep sea coral PS69/3181 and inorganic calcite UWC-1, we find approach 2, following the B-11/Ca-interference relationship, gives the best mean accuracies (within 0.4 parts per thousand of solution values) and external reproducibilities (+/- 0.5 parts per thousand 2 SD for PS69/3181). This approach has been applied to analyses of an annual growth cycle of a Siderastrea siderea coral and eight Cibicidoides wuellerstorfi benthic foraminifera. Both coral and foraminifera data match solution MC-ICP-MS analyses within reported uncertainties. Conclusions LA-MC-ICP-MS can produce accurate and precise delta B-11 data to a 0.5 parts per thousand (2 sigma) level on <0.3 ng B after correction for Ca interference effects.