Determining the sources of (sub)permil-level inaccuracy during laser ablation-MC-ICPMS boron isotope measurements of carbonates

Abstract

Recent developments in spatially-resolved boron isotopic analysis using laser ablation as a means of sample introduction to MC-ICPMS instruments (LA-MC-ICPMS) increasingly allow researchers to explore the spatial heterogeneity of the boron isotopic composition of a range of geochemical applications, for example in palaeoclimatology and mantle petrology. However, previous work has shown that a diffuse interference centred near ¹⁰B, when measuring samples with a calcium-rich matrix, can significantly bias especially the measurement on ¹⁰B, affecting the accuracy of boron isotope measurements. Although several correction approaches have yielded sufficiently accurate analyses of δ¹¹B in calcium carbonate, the root cause of this interference is still not fully resolved. Here, we explore the various potential sources of inaccuracy in boron isotope measurements made using (LA-)MC-ICPMS by experimenting with dry and wet plasma conditions, in both solution and laser ablation mode (in the former case, our solution (Ca–Mg)/B ratios broadly mimic those found in natural samples). In solution mode, we find that irrespective of wet or dry plasma conditions, the introduction of a Ca-containing matrix yields a baseline up to ∼4 and ∼14 times higher around m/z ≈ 10 for wet and dry plasma conditions, respectively, compared to both a Mg-only matrix and lack of matrix. In order to explore this further, we performed mass scans around m/z ≈ 10 during laser ablation of different carbonates with varying matrix [Ca]. These show that the m/z ≈ 10 interference scales linearly with a mixture of the calcium content of the analyte matrix and ⁴⁰Ar⁴⁺ ion beam intensity, as previously hypothesised. Moreover, by experimenting with different plasma loading scenarios during the ablation of CaCO₃, i.e. varying laser spot sizes, we find that permil-level inaccuracies in δ¹¹B may occur when the analyte ablated mass is significantly different than that of the standard used to calibrate instrumental mass bias. This is important given that we also show that different commonly-used reference materials ablate at very different rates, which illustrates the need for a careful standardisation approach irrespective of broader matrix effects when sub-permil level accuracy and precision are desirable when utilising LA-MC-ICPMS.