For the evaluation of in-source fragmentation (ISF), MS/MS spectra of the standards obtained from intact M+H + or M+NH4 + precursors were manually extracted in XCalibur QualBrowser (ver. 4.0.27.13, Thermo Fisher Scientific). As they were obtained under rather mild conditions (NCE 20%), the most abundant fragment peak was considered as a candidate to assess ISF. Next, the averaged accurate mass of the fragment across collected MS 2 spectra was extracted from the full scan TIC trace under 5 ppm mass tolerance value and the resulting chromatographic peak was considered if co-eluting with the selected ion chromatogram of the parent ion. Finally, fragment-to-precursor ion peak area ratios were used as a measure of ISF. For all other measurements, the raw data (Thermo .RAW format) were deconvoluted with the open-source MS-DIAL package (v4.92) using the parameters described in the Table . Lipid annotation was conducted on bond type level as recommended by Liebisch et al. . The annotated data were manually checked to correct automatic integration and remove unreliable hits demonstrating either unsatisfactory chromatographic performance (massively distorted, smeared, splitted peaks) or non-informative fragmentation spectra. Intensity drift along the analytical sequence was corrected using LOESS regression for each feature via pooled QC data (tricube kernel, initial span 0.75). Peak areas were taken as a measure of concentration and areas of all adducts were summed for each annotated lipid to provide final abundance values .