Gas chromatography-pyrolysis-isotope ratio mass spectrometry: A new method for investigating intramolecular isotopic variation in low molecular weight organic acids

A new GC-Pyrolysis-IRMS method was developed for the δ¹³C determination of carboxyl carbon in low-molecular weight organic acids. By utilizing a palladium-wire reactor at 600 °C with a helium/hydrogen reactant gas, the carboxyl carbon of low-molecular weight organic acids is pyrolytically cleaved and introduced into an IRMS for stable carbon-isotope analysis. The precision of the GC-Py-IRMS method is similar to that of more conventional, combustion-based continuous-flow IRMS techniques and interpretation of isotope-dilution experiments with acetic and octanoic acid shows that the technique is sufficiently accurate for the determination of δ¹³C values at natural abundance levels. As a demonstration of this new capability, the carboxyl carbon of low-molecular weight (LMW, C₂-C₆) organic acids generated via hydrous pyrolysis of an oil-prone source rock (the Ghareb Shale) shows δ¹³C values consistent with the hypothesis that organic acids readily undergo exchange of their carboxyl carbon with aqueous inorganic carbon.