Three New Calcium Formate Reference Materials for δ ¹³ C Measurements and a Redetermination of the R ( ¹³ C/ ¹² C) Ratio for VPDB Based on Proton Nuclear Magnetic Resonance Measurements

Rationale Isotope ratio mass spectrometry (IRMS) and proton nuclear magnetic resonance ( 1 H NMR) spectroscopy are independent techniques for determining the isotope ratio R ( 13 C/ 12 C) in organic compounds. However, the lack of suitable reference materials has limited intercalibration of results from these methods. Methods Three high‐purity calcium formate isotopic reference materials were developed, each ideal for accurate isotope ratio measurements by both 1 H NMR and isotope‐ratio mass spectrometry (IRMS). For each material, R ( 13 C/ 12 C) was determined by 1 H NMR, and the relative isotope abundance ( δ 13 C VPDB‐LSVEC ) by IRMS. The combined IRMS and NMR results were used to determine the R ( 13 C/ 12 C) value of Vienna Peedee belemnite (VPDB). Results The combined datasets yield a value for R ( 13 C/ 12 C) of 0.0111050 ± 0.0000047 ( k = 2) for VPDB, which is 6.7‰ lower than the commonly cited value of 0.011180, but similar to other recent determinations. These three calcium formate reference materials exhibit high accuracy across both analytical methods and provide robust tools for instrument calibration, method validation, and inter‐laboratory comparison in carbon stable isotope ratio analysis. Conclusions An accurate R ( 13 C/ 12 C) value for VPDB strengthens the link between relative ( δ 13 C VPDB‐LSVEC ) values from IRMS and R ( 13 C/ 12 C) isotope ratios from NMR and other techniques. These three calcium formate isotopic reference materials (USGS106, USGS107, and USGS108) are available from the US Geological Survey and provide reference materials for calibration, method validation, and inter‐laboratory comparison in carbon isotope ratio analysis.