Diels-Alder derivatization for sensitive detection and characterization of conjugated linoleic acids using LC/ESI-MS/MS.

The utility of Diels-Alder derivatization with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) for liquid chromatography/electrospray ionization tandem mass spectrometry of conjugated linoleic acids (CLAs) was examined. PTAD rapidly reacted with the CLAs, and the resulting derivatives were highly responsive in electrospray ionization mass spectrometry operating in the positive-ion mode. The derivatives produced characteristic product ions during tandem mass spectrometry, which enabled the sensitive detection [limit of detection 18 fmol (signal-to-noise ratio of 5)] and the identification of the conjugated diene position. The PTAD derivatization also significantly increased the reversed-phase liquid chromatography separation selectivity for the most biologically active CLA isomers: cis-9,trans-11-CLA and trans-10,cis-12-CLA. The PTAD derivatization was applied to analyses of food and biological samples; the major CLAs in milk and beef fat samples were successfully identified, and trace amounts of CLAs in human saliva were detected with a simple pretreatment and short analysis time.