Fig. 4.

Hierarchical lipid annotation. Annotation levels of PE 16:0/18:1(9Z) are related to mass spectrometric data. Annotation at lipid species level requires precursor mass and LCF ion: Without separation of isobaric PE O-35:1 ([M − H]⁻m/z 716.5600) by HRMS, annotation of PE 34:1 ([M − H]⁻m/z 716.5236) is based on (∗) the assumption that only even carbon number fatty acyls are present. Molecular lipid species annotation needs the LCF and both molecular lipid fragments (molecular lipid species-specific fragment, here fatty acyl product ions) but does not specify sn-positions PE of 16:0_18:1. Annotation at sn-position level PE 16:0/18:1 demands further data, for example, the application of fatty acyl ion ratios (98, 99) or fragmentation models (60).