Comparison of electrospray ionization-lithium adduct formation and atmospheric pressure chemical ionization for lipid...

We previously published a normal-phase liquid chromatography (NPLC) method for separating thirty classes of lipids. The non-polar solvents used required coupling with APCI-MS or APPI-MS ionization sources, which cause significant fragmentation and do not always allow the observation of pseudo-molecular ions. This is particularly true for molecular species such as sterol esters (SE), triacylglycerols (TG), and acylsterol glucosides (ASG). Additionally, the ionization of monoacylglycerols (MG) and lysophosphatidylcholines (LPC) is low, which can create challenges in analysis. In this study, we enabled the hyphenation of NPLC with an ESI ionization source via a post-column addition of lithium chloride (LiCl) in water-isopropanol. Optimization was performed using a mixture of standards comprising 21 lipid classes distributed over the entire chromatogram and then applied to wheat and soya lipid extracts. Data from NPLC-ESI⁺-MS-lithium adduct formation were compared with NPLC-APCI⁺-MS. As a result, the intensities of free fatty acids, MG, and LPC ESI⁺-MS were significantly improved. For low- and medium-polarity lipids, the formation of lithium adducts provided access to molecular species. MS² and MS³ fragmentation provide structural information to identify the nature of fatty acids and the mass of sterol nuclei. However, using ESI⁺-MS, squalene (SQ), cholesterol (Chol), and acyl-monogalactosyldiacylglycerol (acyl-MGDG) were not observed, and some phospholipids (PL) show the coexistence of several adducts, making data processing complex. This work evidences the complementarity of the two approaches.