Absolute Carbon Stable Isotope Ratio in the Vienna PeedeeBelemnite Isotope Reference Determined by1H NMR Spectroscopy br

The Vienna Peedee Belemnite (VPDB) isotope reference defines the zero point of the carbon stable isotope scale thatis used to describe the relative abundance of13C and12C. An accurate and precise characterization of this isotope reference isvaluable for interlaboratory comparisons and conducting robust carbon stable isotope analyses in a vast array offields, such aschemical forensics, (bio)geochemistry, ecology, or (astro)biology. Here, we report an absolute13C/12C ratio for VPDB that has beenobtained, for thefirst time, using proton nuclear magnetic resonance spectroscopy (1H NMR). Four different NMR instrumentswere used to determine13C/12C ratios in a set of glycine reference materials from the US Geological Survey (USGS64, USGS65, andUSGS66) and a set of formate samples that were characterized by isotope ratios mass spectrometry (IRMS). Intercalibration of theNMR-derived13C/12C ratios with relative abundance (delta 13CVPDB) measurements from IRMS yields a value of 0.011100 for theabsolute13C/12C ratio in VPDB, with an expanded uncertainty of +/- 0.000026 (2 Sigma,n= 114). This is significantly different from thevalue of 0.011180 that is commonly used but falls within the range of values recently revised using IRMS and infrared absorptionmeasurements.1H NMR was found to be an effective method for measuring absolute13C/12C ratios due to its ability tosimultaneously detect signals associated with12C and13C. Results provide a new and independent measure of the carbon isotopecomposition of VPDB, improving our understanding of this important isotope reference.