Static headspace GC-MS&FID analysis of volatile fatty acids

Volatile fatty acids (VFAs), also known as short-chain fatty acids, are aliphatic monocarboxylic acids containing from 2 to 7 carbon atoms. They are key intermediates and metabolites in many biological processes and their presence in a sample often indicates bacterial activity. VFAs are found in a wide range of matrices, i.e., wastewater, landfill leachates, human and animal fluids, food, and environmental systems. Therefore, their quantification and speciation is important in the analysis of such natural samples. 

Experimental

VFAs are analyzed using the  GC-MS&FID, which consists of a 5973C single-quadrupole mass spectrometer coupled with a  7890B gas chromatograph (Agilent). The mass spectrometer is operated in EI mode using helium as a carrier gas. Analytes are separated on the DBWAXUI capillary column (30m × 0.25mm, 0.25µm,  Agilent) using helium as a carrier gas. The gas chromatograph is equipped with a 2-way splitter based on capillary flow technology, which allows splitting column effluent between FID and MS detectors. The system is equipped with a CTC PAL autosampler, which is operated in a static headspace sampling option.

Sample preparation.  WFAs are converted into neutral, unionized compounds by the addition of non-volatile strong, concentrated inorganic acid namely, ortho-phosphoric acid, followed by spiking with the known amount of internal standard (2-ethyl-butanoic acid). By heating a headspace vial before sampling, concentrations of VFAs increase in the gas phase, improving the sensitivity of the analysis.

Calibration curves acquired for propanoic, butanoic, isobutyric and isovaleric acids in GC-MS mode.