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 δ¹¹ B 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 δ¹¹ B 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 δ¹¹ B inaccuracy and¹¹ B/⁴³ Ca, ¹¹ B/⁴⁰ ArCa⁴⁺ or ¹¹ B/Ca_interference from three reference materials with known δ¹¹ B values and varying B/Ca ratios, and (3) direct characterisation of the (sloping) interference itself.

Results: Matrix interference from scattered Ca ions on ¹⁰ B can impede both the accuracy and the reproducibility of δ¹¹ B 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 ¹¹ B/Ca_interference relationship, gives the best mean accuracies (within 0.4‰ of solution values) and external reproducibilities (± 0.5‰ 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 δ¹¹ B data to a 0.5‰ (2σ) level on <0.3 ng B after correction for Ca interference effects.