Working standards and analytical framework for CSIA: isotopic composition (δ13C, δ15N, and δ34S) of 21 amino acids and 7 nucleobases

Abstract We determined the carbon and nitrogen isotope compositions of biologically important 21 amino acids and 7 nucleobases, which are used as working standards. These isotopic compositions were determined using a high-sensitivity elemental analyzer coupled to an isotope ratio mass spectrometer (nano-EA/IRMS) without chemical derivatization. The values were calibrated against internationally recognized reference materials. The δ 13 C values for amino acids range from − 33 to − 9‰, and δ 15 N values range from − 9 to + 9‰. For nucleobases, δ 13 C ranges from − 32 to − 8‰, and δ 15 N ranges from − 9 to + 6‰. Analytical precision was better than ± 0.3‰ for δ 15 N and ± 0.5‰ for δ 13 C. Determined δ 34 S values for cysteine and methionine are also reported with a precision of ± 0.2‰. Cross-validation of δ 15 N values using gas chromatography/IRMS (GC/IRMS) confirmed the consistency with nano-EA/IRMS and GC/IRMS within 0.8‰. The dataset provides a practical working standard to support compound-specific isotope analysis (CSIA), facilitating method development, calibration and data comparability in archeological, ecological, geochemical and biochemical research. In addition to reporting reference data, we outline a streamlined and standardized workflow encompassing sample treatment, compound isolation and isotope measurement. This framework aims to support standardized procedures in CSIA, accommodating both derivatized and underivatized approaches depending on analytical needs. We also present an isotope systematics that summarizes the structural and isotopic characteristics of representative carbon-, nitrogen- and sulfur-bearing compounds, including amino acids, nucleobases and other biochemically important compounds. These frameworks also support compound-level analyses of amino acids, nucleobases and related biomolecules. Together, the reference materials dataset, the standardized workflow and the isotope systematics provide a foundation for reproducible and integrated isotopic applications across broader research fields such as archeology, ecology and geochemistry in the field of Earth and planetary sciences.