Isotope-label is a good approach to isolate the contributions from different lipid compositions and investigate the effects of individual components. For instance, the ester carbonyl groups of PC can be isotope-edited by 13 C, which gives rise to a red-shifted carbonyl stretch 40 cm -1 away from the carbonyl stretch of unlabeled PS . As a result, the effects of PC or PS can be studied separately, though the method is limited to investigating species with relatively high concentrations.