Gas chromatography/isotope ratio mass spectrometry: Analysis of methanol, ethanol and acetic acid by direct injection of aqueous alcoholic and acetic acid samples

RATIONALE: Methanol, ethanol, and acetic acid are not easily extracted from aqueous samples and are susceptible to isotope fractionation in gas chromatography/isotope ratio mass spectrometry (GC/IRMS) analysis. Developing a direct dilution GC/IRMS method for aqueous samples, by adjusting the sample concentrations in common solvents to be similar to each other and using a fixed GC split ratio, is very convenient and important because any linearity effects caused by amount-dependent isotope fractionation can be avoided. METHODS: The suitability of acetonitrile and acetone solvents for the GC/IRMS analysis of pure methanol, ethanol and acetic acid, and commercial liquor and vinegar samples was evaluated using n-hexane and water as control solvents. All the solvents including water were separated from the analyte on a HP-INNOWAX column and were diverted away from the combustion interface. The influence of liquor matrix on the ethanol GC/IRMS analyses was evaluated by adding pure ethanol to liquor samples. RESULTS: Acetonitrile and acetone gave similar delta C-13 values for pure ethanol and pure acetic acid to those obtained in water and n-hexane, and also gave similar delta C-13 values of ethanol in liquor and acetic acid in white vinegar to that obtained in water. For methanol analysis, acetonitrile and refined acetone gave similar delta C-13 values to that obtained in water, but n-hexane was not a suitable solvent. In addition, isotopic fractionation caused by solvent and solute interactions was observed. CONCLUSIONS: We recommend using acetonitrile for the GC/IRMS analysis of aqueous alcoholic samples, and acetone for the analysis of aqueous acetic acid samples. This direct dilution method can provide high accurate and precise GC/IRMS analysis of the relative changes in delta C-13 values of methanol, ethanol, and acetic acid. Copyright (C) 2014 John Wiley & Sons, Ltd.