Determination of Trace Elemental Composition of CaCO3: Application to Mass Limited Abiogenic and Biogenic Carbonates

Abstract

AbstractAn improved method for accurate and precise determination of metal to calcium ratio in mass limited calcium carbonate samples has been developed. We used an Agilent®5800 ICP‐OES for major element (Na/Mg/Sr to Ca) and an Agilent®8900 ICP‐QQQ‐MS for minor and trace element (Li/B/Na/Mg/Al/Mn/Fe/Zn/Sr/Cd/Ba/U to Ca) ratio determination. We report a long‐term precision of ≤1% (1σ) by repeat analysis, spread over ≥6 analytical sessions, of multiple external standards that spanned a large range of concentration. We quasi‐quantitatively eliminated potential matrix effect from high [Ca] by concentration matching samples and standards (∆[Ca] ≤ ±5%) during both ICP‐OES and ICP‐QQQ‐MS sessions. The ICP‐OES analyses were done on 200 μl of sample at 60 ppm [Ca], consuming ∼30 μg of CaCO3 per analysis. Whereas trace element analyses by ICP‐QQQ‐MS required ∼150 μl of sample at 20 ppm [Ca], consuming ∼7.5 µg of CaCO3 per analysis. Thus, the present method allows for high precision determination of TE/Ca in <40 μg of CaCO3. We present a comprehensive approach for optimization of ICP‐OES sensitivity and stability to select elemental (Ca/Na/Mg/Sr) wavelengths with minimal interferences, high sensitivity and linearity. For choice of Ca lines, we focused on the minimization of self‐matrix effect. For the ICP‐MS method, we improved reproducibility and precision by lowering [Ca] of analyte and preconditioning of cones. Furthermore, the present method allows for precise B/Ca determination sans the use of HF matrix. In summary, we present an easily adoptable method based on readily available instrumentation for determining element‐to‐calcium ratios that is suitable for analyzing mass‐limited carbonate samples.