Determination of the13C/12C Carbon Isotope Ratio in Carbonates and Bicarbonates by13C NMR Spectroscopy

This paper is the first study focused on the innovative application of13C NMR (nuclear magnetic resonance) spectroscopy to determine the bulk13C/12C carbon isotope ratio, at natural abundance, in inorganic carbonates and bicarbonates. In the past,13C NMR spectroscopy (irm-13C NMR) was mainly used to measure isotope ratio monitoring with the potential of conducting13C position-specific isotope analysis of organic molecules with high precision. The reliability of the newly developed methodology for the determination of stable carbon isotope ratio was evaluated in comparison with the method chosen in the past for these measurements, i.e., isotope ratio mass spectrometry (IRMS), with very encouraging results. We determined the13C/12C ratio of carbonates and bicarbonates (∼50−100 mg) with a precision on the order of 1 in the presence of a relaxation agent, such as Cr(acac)3, and CH313COONa as an internal standard. The method was first applied to soluble inorganic carbonates and bicarbonates and then extended to insoluble carbonates by converting them to Na2CO3, following a simple procedure and without observing isotopic fractionation. Here, we demonstrate that13C NMR spectroscopy can also be successfully adopted to characterize the13C/12C isotope ratio in inorganic carbonates and bicarbonates with applications in different fields, such as cultural heritage and geological studies.