2-(2-aminoethylamino)-5-nitropyridine as a basic matrix for negative-mode matrix-assisted laser desorption/ionization analysis of phospholipids.

Fast and easy analysis of phospholipids (PLs) by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been well demonstrated. However, when using common organic matrices, such as 2,5-dihydroxybenzoic acid (DHB), the detection of most PL classes in positive-ion mode is difficult when PLs containing zwitterionic groups, such as phosphatidylcholines (PCs) and sphingomyelins (SMs) are present. To reduce this limitation, 2-(2-aminoethyloamino)-5-nitropyridine (AAN), a basic compound, was evaluated as an alternative matrix. Negative-ion spectra showed enhanced detection of phosphatidyl ethanolamines (PEs), phosphatidyl serines (PSs), phosphatidyl glycerols (PGs), and phosphatidyl inositols (PIs) in simple mixtures and in a crude methanolic soybean extract. The relative ionization efficiency (RIE) was highest for PIs and lowest for PGs, PSs, and PEs. Compared to DHB and para-nitroaniline, AAN resulted in greater sensitivity for the detection of PL classes in the negative mode. Indeed, the S/N ratio was nearly an order of magnitude higher than that reported for similar PI concentrations but with DHB. MALDI spots produced with AAN were homogeneous thus allowing automation and improved reproducibility. Positive-mode traces could also be acquired with AAN as the matrix, but with lower sensitivity than in the negative mode.