Charge Inversion of Mono- and Dianions to Cations via Triply Charged Metal Complexes: Application to Lipid Mixtures

Electrospray ionization (ESI) of mixtures can give rise to ions with different masses and charges with overlapping mass-to-charge (m/z) ratios. Such a scenario can be particularly problematic for the detection of low-abundance species in the presence of more highly abundant mixture components. For example, negative mode ESI of polar lipid extracts can result in highly abundant singly charged glyerophospholipids (GPLs), such as phosphatidylethanolamines (PE) and phosphatidylglycerols (PG), that can obscure much less abundant cardiolipins (CLs), which are complex phospholipids with masses roughly double those of GPLs that mostly form doubly charged anions. Despite their low relative abundance, CLs are lipidome components that perform vital biological functions. To facilitate the study of CLs in lipid mixtures without resorting to offline or online separations, we have developed a gas-phase approach employing ion/ion reactions to charge invert anionic lipid species using a trivalent metal-complex. Specifically, ytterbium(III) is shown to readily complex with three neutral ligands, N,N,N′,N′-tetra-2-ethylhexyl diglycolamide (TEHDGA), to form [Yb(TEHDGA3)]3+ using ESI. Herein, we describe pilot studies to evaluate [Yb(TEHDGA)3]3+ as an ion/ion reagent to allow for chemical separation of doubly and singly charged anions, using lipid mixtures as examples, without neutralizing ions of either charge state.