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

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 ¹³C isotope composition of ethanol in aqueous solutions. The δ¹³C values of the pyrolytic fragments (CO, CH₄, C₂H₄) are shown to be highly reproducible (sd <0.4‰). Furthermore, using 14 ethanol samples of known intramolecular isotope distribution, the CO and CH₄ fragments are shown to arise solely from the methylene (CH₂OH) and methyl (CH₃) carbon atom positions of the original ethanol, respectively. Although the different steps (extraction and pyrolysis) fractionate between ¹²C and ¹³C, the isotopic fractionation is reproducible (sd <0.4‰), allowing correcting factors to be applied in order to back-calculate the original δ¹³C_CH2OH and δ¹³C_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