H21E-1427
Position-Specific Hydrogen and Carbon Isotope Fractionations of Light Hydrocarbons by Quantitative NMR

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Changjie Liu, Texas Tech University, Department of Geosciences, Lubbock, TX, United States, Gregory P. Mcgovern, West Texas A&M University, Department of Chemistry and Physics, Canyon, TX, United States and Juske Horita, Texas Tech University, Lubbock, TX, United States
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.