Copper soaps of fatty acids are selectively transferred to an organic phase during partition chromatography.  Extraction of FFAs with alcoholic KOH gives an acid degree  Solvent is initially used to extract the lipid fraction from the cheese, followed by the preparation of methyl esters of fatty acids. For separation of lipids (standards of triolein and diacylphosphatidylcholines as well as three cheese samples) by solid-phase extraction using a self-packed column filled with partly deactivated silica. Non-halogenated solvents are used for the elution of the lipid classes. Cyclohexane/ethyl acetate (1:1, v/v) are served for the elution of neutral lipids, while polar lipids are eluted with three solvents (ethyl acetate/methanol, methanol, and methanol/water) into one fraction. The separated lipid fractions are  transesterified and the individual fatty acids are  quantified by using gas chromatography coupled to electron ionization mass spectrometry (GC/EI-MS) in the selected ion monitoring (SIM) mode. The recovery rate for standard phosphatidylcholines can be obtained approximately 90% and cross-contamination from neutral lipids is  negligible. . Quantitative amounts of individual fatty acids in the phospholipid fractioncan be detremind as 

Τhank you very much for your detailed answer. I think, it is exactly what i need.

Do you know if these assay could be use by FID detector?

You should be able to just direct methylate the fatty acids with sodium methoxide. See: Long,A.R., Massie,S.J. and Tyznik,W.J., J. Food Sci., 53, 940-942 (1988).

I would suggest the direct derivatization (ethyl esterification) approach using Ethyl chloroformate (ECF). we have optimized such a method for free fatty acids quantification in milk. with the direct derivatization (esterification), we ensure higher recovery of free fatty acids, in milk or cheese or any other matrices, especially the volatile and water soluble short chain free fatty acids. for more details you can have a look at our method: http://www.sciencedirect.com/science/article/pii/S0958694613001489