Determination of ester carbonyl 18O/16O ratios in phospholipids by gas chromatography-mass spectrometry.

A method is described for the quantitation of 18O enrichment in phospholipid acyl groups. Methyl esters are prepared by transesterification with sodium hydroxide in methanol and are hydrogenated prior to gas chromatography-mass spectrometry (GC-MS). Estimation of 18O content of the resulting esters is made using the m/z 74 and m/z 76 fragments, which contain both the ester oxygens. The accuracy of the method is demonstrated with synthetic methyl esters and phosphatidylcholines containing a known amount of 18O.