Differently Saturated Fatty Acids Can Be Differentiated by 31P NMR Subsequent to Derivatization with 2-Chloro-4,4,5,5-tetramethyldioxaphospholane: A Cautionary Note

The analysis of free fatty acid (FFA) mixtures is a very important but, even nowadays, challenging task. This particularly applies as the so far most commonly used technique gas chromatography/mass spectrometry (GC/MS)-is tedious and time-consuming. It has been convincingly shown (Spyros, A.; Dais, P. J. Agric. Food Chem. 2000, 48, 802-5) that FFA may be analyzed by P-31 NMR subsequent to derivatization with 2-chloro-4,4,5,5-tetramethyldioxaphospholane (CTDP). However, it was also indicated that differently unsaturated FFAs result in the same P-31 NMR chemical shift and cannot be differentiated. Therefore, only the overall fatty acid content of a sample can be determined by the CTDP assay. In contrast, we will show here by using high field NMR (600 MHz spectrometer, i.e, 242 884 MHz for P-31) that the CTDP assay may be used to differentiate FFAs that have pronounced differences in their double bond contents: saturated fatty acids (16:0), moderately unsaturated (18:1, 18:2), highly unsaturated (20:4), and extremely unsaturated fatty acids (22:6) result in slightly different chemical shifts. The same applies for oxidized fatty acids. Finally, it will also be shown that the CTDP derivatization products decompose in a time dependent manner. Therefore, all investigations must adhere to a strict time regime.