Determination of Multiple Equilibrium Isotopic Fractionation Factors at Natural Abundance in Liquid-Vapor Transitions of Organic Molecules

Abstract

This work presents a simple and easy steady-state distillation method for simultaneously determining the equilibrium isotopic fractionation factor α of different isotopomers of compounds at natural abundance in liquid-vapor phase transition. The measurement of α is precise (<1%) thanks to the use of natural abundance 2H NMR for hydrogen isotope analysis, which provides the site-specific isotopic ratios of different hydrogen positions of a molecule. Moreover, for the heavy elements (C, O, and N), their site-specific isotopic ratios in different functional groups are measured precisely by isotope ratio mass spectrometry (IRMS). We studied the equilibrium isotopic fractionation factors of the major isotopomers of organic molecules containing typical functional groups. Of particular interest, the isotopic fractionation factors varied with the position in a molecule. These important results provide information on how the functional groups of a molecule contribute to the intermolecular forces in the condensed state.