Position-Specific Hydrogen and Carbon Isotope Fractionations of Light Hydrocarbons by Quantitative NMR

Abstract:

Traditional isotope ratio mass spectrometry methods to measure ²H/¹H and ¹³C/¹²C ratios of organic molecules only provide average isotopic values of whole molecules. During the measurement process, valuable information of position-specific isotope fractionations (PSIF) between non-equivalent H and C positions is lost, which can provide additional very useful information about the origins and history of organic molecules. Quantitative nuclear magnetic resonance (NMR) spectrometry can measure ²H and ¹³C PSIF of organic molecules without destruction. The ²H and ¹³C signals from different positions of a given molecule show up as distinctive peaks in an NMR spectrum, and their peak areas are proportional to the ²H and ¹³C populations at each position. Moreover, quantitative NMR can be applied to a wide variety of organic molecules.

We have been developing quantitative NMR methods to determine ²H and ¹³C PSIF of light hydrocarbons (propane, butane and pentane), using J-Young and custom-made high-pressure NMR cells. With careful conditioning of the NMR spectrometer (e.g. tuning, shimming) and effective ¹H -¹³C decoupling, precision of ± <10‰ (²H) and ± <1‰ (¹³C) can be readily attainable after several hours of acquisition. Measurement time depends on the relaxation time of interested nucleus and the total number of scans needed for high signal-to-noise ratios. Our data for commercial, pure hydrocarbon samples showed that ²H PSIF in the hydrocarbons can be larger than 60‰ and that ¹³C PSIF can be as large as 15‰. Comparison with theoretical calculations indicates that the PSIF patterns of some hydrocarbon samples reflect non-equilibrium processes in their productions.