Matrix independent and interference free in situ boron isotope analysis by laser ablation MC-ICP-MS/MS

Abstract

The accuracy of boron isotope analysis by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), particularly when the mass bias correction utilises non-matrix-matched reference materials, is compromised by matrix- and mass-load induced biases and an interference from scattered Ca and Ar ions which can induce bias in excess of 20‰. Here we explore the first application to in situ boron isotope analysis of the Thermo Scientific Neoma MS/MS mass spectrometer, which combines a traditional MC-ICP mass spectrometer with a collision/reaction cell and pre-cell mass filtering technology. While operating in full transmission mode, i.e. without using the collision/reaction cell, the pre-cell mass filter successfully eradicates the interference from scattered ions seen on some pre-existing models of MC-ICP-MS and exhibits good analytical sensitivity (∼6-14 mV per μg per g of total boron is typically achieved here). Furthermore, when matching laser operating parameters for samples/secondary reference materials and bracketing reference materials, and limiting the mass of ablated material introduced to the plasma, matrix- and mass-load induced biases can be prevented without the need for instrument tune conditions that severely limit sensitivity. Mean values of 14 reference materials, varying in bulk chemical composition (carbonates and silicates) and boron concentration (c. 2-150 μg g^-1), are within uncertainty of reference values when instrumental mass bias is normalised using bracketing analyses of NIST SRM612 glass, demonstrating the accuracy and utility of this approach. Internal precision and external reproducibility are primarily controlled by boron signal intensity and both are typically better than 1‰ when the ¹¹B intensity is at least ∼40 mV. LA-MC-ICP-MS/MS therefore offers a new and exciting opportunity for accurate and precise matrix independent, in situ, boron isotope analysis of geological materials.

Fig. 1. (a) Mass scan across the boron isotope mass range when ablating a carbonate reference material collected on a Neptune MC-ICP mass spectrometer, showing the elevated baseline across the entire boron mass range that acts as an interference and impacts the accuracy of boron isotope measurements by LA-MC-ICP-MS. (b) A mass scan showing the well-resolved ⁴⁰Ar + Ca⁴⁺ and ¹⁰B peaks.

Fig. 2. Mean LA-MC-ICP-MS/MS δ¹¹B (± 2SD) versus reference value δ¹¹B (Table 1) for the 14 reference materials analysed in this study: (a) transect measurements, (b) spot measurements. Data label size correlates to laser beam size: smaller labels correspond to either 50 by 50 μm square (transect) or 50 μm diameter circle (spot) laser beams, and larger labels correspond to 80 by 80 μm square (transect) or 80 μm diameter circle (spot) laser beams. Solid black line represents 1 : 1 ratio.

Fig. 3. Boron isotope accuracy (Δδ¹¹B) of 14 reference materials analysed by LA-MC-ICP-MS/MS: (a) plotted against ¹¹B signal (V), (b) plotted against boron μg g⁻¹, and (c) plotted against B/Ca (μmol mol⁻¹). Solid horizontal bar represents Δδ¹¹B of 0‰, dashed horizontal bar represents Δδ¹¹B of +1‰ and −1‰ respectively. Uncertainties are the 2SD of the repeat measurements by LA-MC-ICP-MS/MS summed in quadrature with the uncertainties of the reference values presented in Table 1.

Fig. 4. Boron isotope internal precision (2 SE) of 14 reference materials analysed by LA-MC-ICP-MS/MS: (a) plotted against ¹¹B signal (V), (b) plotted against boron μg g⁻¹, and (c) plotted against B/Ca (μmol mol⁻¹). Solid black line represents 2 SE of 1‰. Dashed coloured lines show power relationship of all transect or sport data respectively, with R² included.

Fig. 5. Boron isotope external reproducibility (2SD) of 14 reference materials analysed by LA-MC-ICP-MS/MS: (a) plotted against ¹¹B signal (V), (b) plotted against boron μg g⁻¹, and (c) plotted against B/Ca (μmol mol⁻¹). Solid black line represents 2SD of 1‰. Dashed black line shows power relationship of all data, with R² included.

Fig. 6. Mass scans across the boron isotope mass range when ablating carbonate reference materials collected on a Neptune MC-ICP mass spectrometer, Neoma MC-ICP mass spectrometer, and Neoma MS/MS MC-ICP mass spectrometer. A gas blank mass scan is also shown for the Neoma MS/MS MC-ICP mass spectrometer. Data from the scans are provided in ESI Table S6.

Fig. 7. Mass scan from m/z of 9.968 to 9.992 showing the ⁴⁰Ar + Ca⁴⁺ peak when ablating NIST SRM612 or JCp-1 (transect mode, 100 μm diameter laser beam, 6 J cm⁻² laser energy density, 12 Hz repetition rate, 10 μm s⁻¹ tracking speed) on a Neptune MC-ICP mass spectrometer (dashed red line), a Neoma MC-ICP mass spectrometer (dashed blue line), and a Neoma MS/MS MC-ICP mass spectrometer (solid grey line). Note that normalising to sensitivity does not impact the overall picture.

Fig. 8. Intensity of ⁴⁰Ar²⁺ and ²⁴Mg beams when a 100 ppb Mg solution is analysed on a Neoma MS/MS MC-ICP mass spectrometer with varying pre-cell mass filter B-fields. Data are available in ESI Table S7.

Fig. 9. Boron isotope inaccuracy when measuring carbonate reference materials JCp-1 (pressed micropellet) and PS69/318-1b (calcitic coral fragment) under different plasma mass-loading scenarios, following normalisation to NIST SRM612. (a) and (b) Expressed as mass bias, calculated by subtracting the measured δ¹¹B of the carbonate reference material when analysed using the same sized laser beam to that of the bracketing NIST SRM612 analyses, from the measured δ¹¹B of the carbonate reference material when analysed using other laser beam diameters. (c) and (d) Expressed as Δδ¹¹B, calculated by subtracting the δ¹¹B reference value of the carbonate reference material in question from the measured δ¹¹B. Spot data (a and c) represent the mean of three repeat measurements (±2 SE), transect data (b and d) represent single measurements (±2 SE). Data are available in ESI Table S8.

Fig. 10. Mean δ¹¹B (a) and B/Ca (b) ±2 SD for selected reference materials following normalisation to JCp-1. Solid black line represents 1 : 1 ratio. Data are available in ESI Tables S9 and S10. Note that the large uncertainty on the inorganic carbonate crystals analysed is the result of small heterogeneity in B concentration of the target.