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

• Published in: Journal of analytical atomic spectrometry Vol. 39; no. 10; pp. 2409 - 2420
• Main Authors: Coenen, Douglas, Evans, David, Jurikova, Hana, Dumont, Matthew, Rae, James, Müller, Wolfgang
• Format: Journal Article
• Language: English
• Published: London Royal Society of Chemistry 02.10.2024
• Subjects: Ablation; Ablative materials; Bias; Boron; Boron isotopes; Calcium carbonate; Carbonates; Heterogeneity; Ion beams; Laser ablation; Lasers; Measuring instruments; Petrology; Plasma
• ISSN: 0267-9477; 1364-5544
• DOI: 10.1039/D4JA00154K

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 10 B, when measuring samples with a calcium-rich matrix, can significantly bias especially the measurement on 10 B, affecting the accuracy of boron isotope measurements. Although several correction approaches have yielded sufficiently accurate analyses of δ 11 B 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 40 Ar 4+ ion beam intensity, as previously hypothesised. Moreover, by experimenting with different plasma loading scenarios during the ablation of CaCO 3 , i.e. varying laser spot sizes, we find that permil-level inaccuracies in δ 11 B 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.