Dolomite reference material synthesized by pressureless sintering for laser ablation MC-ICP-MS carbon and magnesium isotope analysis

Microanalysis of carbon and magnesium isotopes in dolomite is gaining increasing attention as a valuable tool in various fields of earth sciences. However, the matrix effect poses a major challenge for determining C and Mg isotopes using laser ablation multi-collector ICP-MS, particularly due to the wide range of chemical compositions found in natural dolomite. A preferred solution is to use a matrix-matched reference material with homogeneous isotope compositions at the micron scale. In this contribution, we established a new protocol for preparing a dolomite reference material for isotopic microanalysis through pressureless sintering. The effects of particle size and sintering temperature on the isotopic homogeneity of sintered dolomite powder pellets (SDPPs) were carefully evaluated. Our results indicate that a smaller d90 particle size improves isotope homogeneity, and SDPPs with a d90 of 3.17 mu m show the best laser ablation repeatabilities of delta 26Mg and delta 25Mg (0.11 parts per thousand and 0.09 parts per thousand) that are 2-4 times better compared to the repeatability of those with a d90 of 10.37 mu m. Higher sintering temperature enhances particle cohesion and pellet hardness, which are crucial for laser ablation performance; Vickers hardness (HV) is improved to 41.58 kgf mm-2 when SDPPs are heated up to 400 degrees C below decomposition temperature. These SDPPs show the best delta 13C, delta 26Mg and delta 25Mg homogeneity with a twice standard deviation value of 0.37 parts per thousand, 0.11 parts per thousand and 0.06 parts per thousand (n = 15), respectively. Of note, in situ carbon and magnesium isotope characterization of SDPPs using LA-MC-ICP-MS shows negligible isotope fractionation against natural dolomites. This further confirms the reliability and robustness of the proposed protocol for dolomite isotopic reference preparation via pressureless sintering. Dolomite reference material is prepared by pressureless sintering, and the powder particle size and sintering temperature are key factors for isotope homogeneity. No in situ isotope fractionation between synthetic samples and natural dolomites.