Determination of hydrogen, carbon and oxygen isotope ratios of ethanol in aqueous solution at millimole levels

Abstract

Three stable isotope ratios, D/H, (13)C/(12)C and (18)O/(16)O, are measurable in ethanol, an important organic compound that is used as a material for food and beverages, fuel and chemical feedstock, and as a substance related to metabolism. We developed a simple and rapid method of measurement of three isotope ratios of ethanol in aqueous solution at millimole levels using gas chromatography-high-temperature conversion or combustion-isotope ratio mass spectrometry (GC-TC/C-IRMS) combined with solid-phase microextraction (SPME). Using this method, the delta value for ethanol was determined in 30 min for deltaD and delta(13)C, and in 75 min for delta(18)O with precisions of +/-9 per thousand, +/-0.3 per thousand and +/-0.7 per thousand, respectively, for deltaD, delta(13)C, and delta(18)O. An advantage of this process is that it requires no distillation for ethanol purification. The method is useful for small quantities of analyte with low ethanol concentrations, which is expected for environmental and metabolic studies.

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

“…Polyethylene glycol stationary phases are commonly used in GC/IRMS analyses for their selectivity in separating polar compounds such as ethanol and acetic acid . In our study, water was used as the GC/IRMS injection solvent.…”

“…Methanol, ethanol, and acetic acid are miscible with water, which means they are not easily extracted from aqueous samples using common organic solvents. To avoid using chemical derivatization during sample preparation, many methods using solid‐phase microextraction (SPME) sample preparation techniques have been developed for gas chromatography (GC), GC/mass spectrometry (GC/MS), and GC/isotope ratio mass spectrometry (GC/IRMS) for the analysis of these types of compounds. Direct injection of aqueous or water‐rich samples of these compounds into GC, GC/MS, or GC/IRMS instruments has also been reported.…”

“…GC/IRMS is predominantly used for compound‐specific isotope analysis (CSIA) in complex mixtures because of the high separation efficiency of GC. SPME GC/IRMS methods have also been developed for the isotope ratio analysis of ethanol and acetic acid in aqueous matrices and methanol in air samples.…”

“…Methanol, ethanol, and acetic acid are important industrial chemicals and they are frequently analyzed for their stable isotope ratio signatures. For example, knowledge of the stable isotope ratios of ethanol and acetic acid can contribute to the authentication of liquors and vinegar, and thus the identification of fraud, in the food industry .…”

“…Therefore, a GC/IRMS procedure for the dilution of methanol with appropriate solvents before determination is required so that additional analysis on another elemental analyzer/IRMS (EA/IRMS) or LC/IRMS instrument is not necessary. In the reported GC/IRMS methods for the isotope ratio analysis of methanol, ethanol and acetic acid, no applications have been reported and validated that involve dilution with water‐miscible organic solvents (e.g. acetonitrile and acetone) for the GC/IRMS determination of the carbon isotope ratios of pure methanol, ethanol in liquors and acetic acid in vinegars.…”