Introducing MC-MICAP-MS: using a N2-based plasma ion source for Sr isotope abundance ratio measurements

Abstract

High-precision metal(loid) isotope abundance ratios are a powerful research tool across various disciplines. These ratios are typically measured using multi-collector mass spectrometry with ion sources such as gas source, thermal ionization, or inductively coupled plasma (i.e., IRMS, TIMS, and MC-ICP-MS). This study presents the first integration of the recently developed microwave inductively coupled atmospheric-pressure plasma (MICAP) ion source, which sustains a plasma using N₂, with a multi-collector mass spectrometer and offers the first characterization of the resulting MC-MICAP-MS instrument for high-precision metal isotope abundance ratio measurements. The performance of the MC-MICAP-MS instrument was evaluated by measuring Sr isotope abundance ratios and directly comparing the results with those obtained using established technology (i.e., MC-ICP-MS) with an Ar-ICP as the ion source. Initial results using the MICAP ion source show that the ⁸⁷Sr/⁸⁶Sr intensity ratio precision (approx. 0.007%) and the repeatability of the ⁸⁷Sr/⁸⁶Sr intensity ratio (approx. 0.010%), as well as the intermediate precision of the conventional ⁸⁷Sr/⁸⁶Sr isotope abundance ratio (approx. 0.0013%) are fully comparable to those of conventional MC-ICP-MS systems. The instrumental isotopic fractionation (IIF) observed for the new MC-MICAP-MS instrument was predominantly mass-dependent for Sr. This allowed the successful application of common IIF correction strategies, such as internal normalisation and standard-sample bracketing, for the determination of Sr isotope abundance ratios. The conventional ⁸⁷Sr/⁸⁶Sr isotope abundance ratios and δ⁸⁸Sr/⁸⁶Sr_SRM987 values measured for various geological and biological reference materials (i.e., seawater, basalt, slate, and bone) using MC-MICAP-MS were consistent with previously reported values obtained from established technologies such as TIMS and MC-ICP-MS. Overall, this study demonstrates that MICAP is an applicable and viable alternative ion source for multi-collector mass spectrometry, maintaining both double-focusing properties and high-precision performance without compromising the accuracy and reliability of the measurement results.