Absolute Carbon Stable Isotope Ratio in the Vienna Peedee Belemnite Isotope Reference Determined by 1H NMR Spectroscopy

The Vienna Peedee Belemnite (VPDB) isotope reference defines the zero point of the carbon stable isotope scale that is used to describe the relative abundance of 13 C and 12 C. An accurate and precise characterization of this isotope reference is valuable for interlaboratory comparisons and conducting robust carbon stable isotope analyses in a vast array of fields, such as chemical forensics, (bio)geochemistry, ecology, or (astro)biology. Here, we report an absolute 13 C/ 12 C ratio for VPDB that has been obtained, for the first time, using proton nuclear magnetic resonance spectroscopy ( 1 H NMR). Four different NMR instruments were used to determine 13 C/ 12 C ratios in a set of glycine reference materials from the US Geological Survey (USGS64, USGS65, and USGS66) and a set of formate samples that were characterized by isotope ratios mass spectrometry (IRMS). Intercalibration of the NMR-derived 13 C/ 12 C ratios with relative abundance (δ 13 C VPDB ) measurements from IRMS yields a value of 0.011100 for the absolute 13 C/ 12 C ratio in VPDB, with an expanded uncertainty of ±0.000026 (2σ, n = 114). This is significantly different from the value of 0.011180 that is commonly used but falls within the range of values recently revised using IRMS and infrared absorption measurements. 1 H NMR was found to be an effective method for measuring absolute 13 C/ 12 C ratios due to its ability to simultaneously detect signals associated with 12 C and 13 C. Results provide a new and independent measure of the carbon isotope composition of VPDB, improving our understanding of this important isotope reference.