Three new calcium formate reference materials for δ13C measurements and a redetermination of the R(13C/12C) ratio for VPDB based on proton nuclear magnetic resonance measurements

Rationale

Isotope ratio mass spectrometry (IRMS) and proton nuclear magnetic resonance (¹H NMR) spectroscopy are independent techniques for determining the isotope ratio R(¹³C/¹²C) 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 ¹H NMR and isotope-ratio mass spectrometry (IRMS). For each material, R(¹³C/¹²C) was determined by ¹H NMR, and the relative isotope abundance (δ¹³C_VPDB-LSVEC) by IRMS. The combined IRMS and NMR results were used to determine the R(¹³C/¹²C) value of Vienna Peedee belemnite (VPDB).

Results

The combined datasets yield a value for R(¹³C/¹²C) 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(¹³C/¹²C) value for VPDB strengthens the link between relative (δ¹³C_VPDB-LSVEC) values from IRMS and R(¹³C/¹²C) 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.