The Need for Commercially Available Defined Oxidized (Phospho)lipids

Phospholipids are essential components of cell membranes and organelles. Furthermore, they are present in body fluids, particularly in the blood, where lipids are made "water-soluble" in the form of lipoproteins to enable their transport in an aqueous environment. The ubiquitous occurrence of lipids led to thedevelopment of different analytical "lipidomics" techniques, whereby the majority of these methods are based on mass spectrometry (MS). Although there was an enormous progress in this field, lipid analysis is still considered to be challenging due to the extreme structural variability of lipids which leads to the appearance of hundreds of different lipid species in a typical biological sample . This is due to the different headgroups such as phosphorylcholine or -ethanolamine, the different fatty acyl residues ranging from saturated residues such as myristic acid (14:0) up to highly unsaturated residues such as docosahexaenoic acid (22:6) and the linkage types (acyl-acyl-, alkylacyl-, and alkenyl-acyl).

All these structural aspects cannot be assessed in a single MS experiment using the m/z ratios only. Either LC separation priorto MS, sophisticated MS/MS techniques and/or additional methods such as ionmobility spectroscopy is mandatory. It must also be emphasized that quantitative data can only be (if at all) obtained when suitable lipid standards are used: one stableisotope- labelled (deuterated or 13C-labelled) standard per lipid class is normally needed. These standards are also useful to correct losses of dedicated lipid classes upon the extraction process which is necessary for the enrichment of lipids as well as the removal of salts and other contaminations. Although such isotope-labelled standards are quite expensive, they are nowadays commercially available from many companies. Unfortunately, the situation is much more difficult when oxidized lipids are of interest.