title: Gas chromatography/isotope ratio mass spectrometry: analysis of methanol, ethanol and acetic acid by direct injection of aqueous alcoholic and acetic acid samples.
abstract: BACKGROUND
  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 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.
  
  
  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 δ(13) C values of methanol, ethanol, and acetic acid.
authors: State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, No.1 West Beichen Road, Chaoyang District, Beijing, 100101, China.
authors: Guomin
authors: G
authors: Ai
authors: Tong
authors: T
authors: Sun
authors: Xiuzhu
authors: X
authors: Dong
mesh_terms: D019342:Acetic Acid / Q000032:analysis* / Q000737:chemistry; D000431:Ethanol / Q000032:analysis* / Q000737:chemistry; D017022:Flow Injection Analysis / Q000379:methods; D008401:Gas Chromatography-Mass Spectrometry; D059625:Liquid-Liquid Extraction / Q000379:methods*; D000432:Methanol / Q000032:analysis* / Q000737:chemistry; D012997:Solvents / Q000032:analysis / Q000737:chemistry
chemical_list: D012997:Solvents; D000431:Ethanol; D019342:Acetic Acid; D000432:Methanol