Accurate Method for the Determination of Intramolecular ~(13)C Isotope Composition of Ethanol from Aqueous Solutions

摘要

A new method, combining headspace solid phase microextraction (HS-SPME) with an online pyrolysis system coupled with isotope ratio mass spectrometry (IRMS), is developed for the determination of the intramolecular ~(13)C isotope composition of ethanol in aqueous solutions. The δ~(13)C values of the pyrolytic fragments (CO, CH_4, C_2H_4) are shown to be highly reproducible (sd <0.4‰). Furthermore, using 14 ethanol samples of known intramolecular isotope distribution, the CO and CH_4 fragments are shown to arise solely from the methylene (CH_2OH) and methyl (CH_3) carbon atom positions of the original ethanol, respectively. Although the different steps (extraction and pyrolysis) fractionate between ~(12)C and ~(13)C, the isotopic fractionation is reproducible (sd <0.4‰), allowing correcting factors to be applied in order to back-calculate the original δ~(13)C_(CH2OH) and δ~(13)C_(CH3) values of ethanol. The method thus allows the determination of the isotope composition of ethanol at the intramolecular and molecular levels, within a single run and a short experimental time (30 min), and with a very easy sample preparation. The method is then applied to alcoholic beverages to show its potential for authentication purposes.