Isotope-labeling in situ derivatization and HS-SPME arrow GC-MS/MS for simultaneous determination of fatty acids and fatty acid methyl esters in aqueous matrices

Fatty acids (FAs) and fatty acid methyl esters (FAMEs) co-occur in many samples, and analysis of both substance classes is frequently of high interest. To this end, this study introduces the first method for simultaneous determination of FAs and FAMEs including fully automated solvent-free solid-phase microextraction (SPME) arrow headspace extraction combined with isotope-labeling in situ FA derivatization with deuterated methanol (CD3OD). By using the chromatographic isotope effect (Delta Rt = 0.03 min) and the + 3 m/z mass shift, FAs can be selectively differentiated from the FAMEs during gas chromatography tandem-mass spectrometry (GC-MS/MS) operated in the multiple reaction monitoring (MRM) aquisition mode. Additionally, an approach is presented to predict the retention times of deuterated compounds. Optimization of the derivatization conditions was accomplished by design of experiments and found to be 20 min, 50 degrees C, 4 v/v% CD3OD, and pH 2.1. During method validation, FAs and FAMEs were calibrated in different concentration ranges by standard addition in five real matrices and ultrapure water leading to good linearities and method detection limits for FAs ranging from 1-30 mu g L-1 and for FAMEs from 0.003-0.72 mu g L-1. FAs and FAMEs were detected in real samples from surface water, wastewater treatment plant effluent, and three different bioreactor samples and could be quantified in concentrations ranging from 2-1056 mu g L-1 for FAs and 0.01-14 mu g L-1 for FAMEs.

Automated summary

This study introduces a method for simultaneous determination of fatty acids and fatty acid methyl esters using automated solvent-free solid-phase microextraction and isotope labeling in situ derivatization. Gas chromatography tandem-mass spectrometry with the chromatographic isotope effect and mass shift allows for selective differentiation of these two substance classes. The optimized derivatization conditions and standard addition calibration method provide accurate quantification of fatty acids and fatty acid methyl esters in different samples.