A SITE-SPECIFIC AND MULTIELEMENT APPROACH TO THE DETERMINATION OF LIQUID-VAPOR ISOTOPE FRACTIONATION PARAMETERS - THE CASE OF ALCOHOLS

Isotope fractionation phenomena occurring at the natural abundance level in the course of liquid-vapor transformation have been investigated by using the SNIF-NMR method (site-specific natural isotope fractionation studied by NMR) which has a unique capability of providing simultaneous access to fractionation parameters associated with different molecular isotopomers. This new appproach has been combined with the determination of overall carbon and hydrogen fractionation effects by isotope ratio mass spectrometry (IRMS). The results of distillation and evaporation experiments of alcohols performed in technical conditions of practical interest have been analyzed according to the Rayleigh-type model. In order to check the performance of the column, unit fractionation factors were measured beforehand for water and for the hydroxylic sites of methanol and ethanol for which liquid-vapor equilibrium constants were already known. Inverse isotope effects are determined in distillation experiments for the overall carbon isotope ratio and for the site-specific hydrogen isotope ratios associated with the methyl and methylene sites of methanol and ethanol. In contrast, normal isotope effects are produced by distillation for the hydroxylic sites and by evaporation for all the isotopic ratios. The effective fractionation factors obtained from least-squares treatment of the data have been analyzed in terms of liquid-vapor equilibrium fractionation factors and in terms of kinetic evaporative fractionation factors associated either with the whole set of isotopomers or with the individual isotopic species. Suitable conditions for an accurate determination of the relative values of these parameters are provided by the proposed approach which gives access to several fractionation factors in a one-pot experiment. From a practical point of view, the unit fractionation factors determined in this work provide a quantitative basis for correcting the isotope ratio of the ethanol isotopic probe for physical effects due to distillation and evaporation. © 1990 American Chemical Society.