Sample preparation for the analysis of glycerophospholipids by matrix-assisted positive and negative ion 252Cf plasma desorption time-of-flight mass spectrometry

Abstract

A new sample preparation technique for the analysis of 13 different classes of glycerophospholipids by positive- and negative-ion 252 Cf plasma desorption time-of-flight mass spectrometry is described. Until recently only the direct deposition and the electrospray deposition technique, both yielding mass spectra of poor quality, were applied. The introduction of the bifunctional small molecule matrix 3-(3-pyridyl) acrylic acid (PAA) as sample matrix brings the advantage of selective analyte adsorption from chloroform solution (in two cases also from pure aqueous solution: phosphatidylinositol-4-phosphate and phosphatidylinositol-4,5-diphosphate) onto the matrix surface allowing a novel two-step rinsing procedure. Prior to this rinsing procedure, abundant alkali-cationised molecules and an extensive number of diagnostic fragment ions are observed for these compounds. During the first rinsing step excess of alkali cations is removed by cleaning the matrix surface with up to a few mL of water. After the second rinsing step, where chloroform is used, the intensity of (de)protonated molecular ions and adduct ions with the matrix increases significantly. Most of the fragment ions exhibit a significant decrease in relative abundance or even disappear due to lowering the internal energy of desorbed/ionised molecules by removal of non-adsorbed analyte layers from the matrix surface. In cases where analyte molecules do not form protonated and deprotonated molecular ions ([M + H] + or [M ‐ H] ‐ ), respectively, intense peaks due to adduct ions ([M + PAA + H] + and [M + PAA ‐ H] ‐ ) between matrix (PAA) and analyte (M) molecules are observed. These adduct ions most likely originate via a common mechanism independent of the chemical nature of the glycerophospholipid. Choline-containing phospholipids, which do not form [M‐H] ‐ ions, most likely form ion-pairs with the matrix already in the condensed phase yielding [M + PAA ‐ H] ‐ ions in the negative-ion mass spectra. For the nitrogen-free phospholipids (e.g. phosphatidylinositol), which do not form [M + H] + ions, ion-pair formation between lipid and matrix in the condensed phase is also assumed to be the major process of [M + PAA + H] + ion formation in the postive-ion mode. Applying this sample preparation technique to the analysis of normal phase