Ai Guomin, Sun Tong, Dong Xiuzhu
State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, No.1 West Beichen Road, Chaoyang District, Beijing, 100101, China.
Rapid Commun Mass Spectrom. 2014 Aug 15;28(15):1674-82. doi: 10.1002/rcm.6948.
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.
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.
Acetonitrile and acetone gave similar δ(13) C values for pure ethanol and pure acetic acid to those obtained in water and n-hexane, and also gave similar δ(13) C 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 δ(13) C 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.
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 δ(13) C values of methanol, ethanol, and acetic acid.
甲醇、乙醇和乙酸不易从水性样品中萃取出来，并且在气相色谱/同位素比率质谱（GC/IRMS）分析中易发生同位素分馏。通过将常见溶剂中的样品浓度调整为彼此相似，并使用固定的GC分流比，开发一种用于水性样品的直接稀释GC/IRMS方法非常方便且重要，因为可以避免由量依赖性同位素分馏引起的任何线性效应。
以正己烷和水作为对照溶剂，评估了乙腈和丙酮溶剂对纯甲醇、乙醇和乙酸以及市售酒类和醋类样品进行GC/IRMS分析的适用性。所有溶剂（包括水）在HP-INNOWAX柱上与分析物分离，并从燃烧接口分流出去。通过向酒类样品中添加纯乙醇来评估酒类基质对乙醇GC/IRMS分析的影响。
乙腈和丙酮对于纯乙醇和纯乙酸给出的δ(13)C值与在水和正己烷中获得的值相似，并且对于酒类中的乙醇和白醋中的乙酸给出的δ(13)C值也与在水中获得的值相似。对于甲醇分析，乙腈和精制丙酮给出的δ(13)C值与在水中获得的值相似，但正己烷不是合适的溶剂。此外，观察到了由溶剂和溶质相互作用引起的同位素分馏。
我们建议使用乙腈对含酒精的水性样品进行GC/IRMS分析，使用丙酮对含乙酸的水性样品进行分析。这种直接稀释方法可以对甲醇、乙醇和乙酸的δ(13)C值的相对变化提供高精度和高准确度的GC/IRMS分析。
