Characteristics of Carbon and Oxygen Isotope Standard Materials of Carbonates and Their Effect on Isotope Analysis and Standard Preservation

Carbon and oxygen isotopes in carbonates are commonly used in geology and climate studies. Reference materials for these isotopes are an important basis for analysis and data comparison. Understanding the differences in the characteristics of different standard materials is helpful for the selection and use of standards, improvement of heterogeneity, and establishment of optimal experimental conditions.

To investigate the influence of the characteristics of carbon and oxygen isotope standard materials of carbonates on isotope analysis and preservation of the standards.

In this study, X-ray diffraction (XRD), optical microscopy, and energy dispersive X-ray scanning electron microscopy (EDX-SEM) were used to analyze carbonate carbon and oxygen isotope reference materials with different properties, such as provenance, particle size, particle morphology and structure, and purity. The δ¹³C and δ¹⁸O values of some carbonate reference materials were determined by the continuous flow phosphoric acid method to explore their characteristics and their influence on isotope analysis and standard preservation.

Natural carbonate reference materials generally contain non-calcium carbonate components, such as small amounts of quartz and other particles with obvious differences in properties. There are also differences in particle size, morphology, microstructure, and mineral composition between different standard materials and between standard material particles. The δ¹³C and δ¹⁸O values of different standard materials were determined, and most results were consistent with the recommended values. The standard deviations of the δ¹³C and δ¹⁸O values of IAEA-CO-8 were larger than those of other standard materials, which is attributable to their complex composition and homogeneity. The standard deviation of the δ¹⁸O value of NBS20 was large, while the δ¹⁸O value of 7902 deviated from the recommended value. It is inferred that their powdery and fine particles are more likely to be affected by carbon dioxide and water in air.

Based on previous research as well as the current study, it is believed that under the recommended usage amount, the properties of natural carbonate reference materials have a limited impact on the accuracy of analysis. To ensure the accuracy of microanalysis and research, while selecting the standard and the particles in the standard, it is recommended to combine the characteristics of standards and materials based on the purpose of the analysis and research. At the same time, the preparation and storage of powdery and fine-grained standard materials require more attention. This study supplements information regarding the properties of carbonate carbon and oxygen isotope standard materials, which helps in the analysis and research of trace carbonate carbon and oxygen isotopes, and provides a reference for standard preservation and preparation.