Thus, this spectral region can be used to track phase transitions in model bilayers. It is important to note that probing intrinsic vibrational modes yields an ensemble-averaged view across oscillators. For instance, the hydrogen-bond number obtained from FTIR spectrum of POPS:DMPC represents the average number for all ester groups in both lipid species, and hydrogen bonds formed specifically with POPS cannot be differentiated from those with DMPC.