Enrichment and quantification of monoacylglycerols and free fatty acids by solid phase extraction and liquid chromatography-mass spectrometry

Quantification of monoacylglycerols (MAG) and free fatty acids (FA) is of interest in biological systems, in food, cosmetic and pharmaceutical products. This manuscript describes and validates a reversed phase liquid chromatography-tandem mass spectrometry based approach for simultaneous quantification of these analytes in fats and oils. Purification and concentration of MAG/FA were performed using cation exchange solid phase extraction, which allowed elimination of the abundant triacylglycerols. Following cleanup and concentration, the analytes were separated and detected with the aid of volatile ammonium-formate buffer. MAG were detected in positive ion mode, while FA were detected in negative ion mode. The method was validated by the method of standard additions and using stable isotope labeled internal standards. The results confirm the feasibility of quantifying these two classes of analytes simultaneously without any chemical derivatization. The obtained main quantitative features include: (1) lower limits of quantification 1-30 ppm for MAG analytes, (2) lower limits of quantification 90-300 ppm for FA analytes, (3) averaged inter-batch precision 6%, and (4) averaged bias -0.2% for MAG and 0.5% for FA. Various animal fat and vegetable oil samples were characterized for their MAG/FA profile indicating the usefulness of the method to address quality and authenticity of fats and oils.