1,8-Bis(dimethylamino)naphthalene/9-aminoacridine: A new binary matrix for lipid fingerprinting of intact bacteria by matrix assisted laser desorption ionization mass spectrometry

The effectiveness of a novel binary matrix composed of 1,8-bis(dimethylamino)naphthalene (DMAN;proton sponge) and 9-aminoacridine (9AA) for the direct lipid analysis of whole bacterial cells by matrixassisted laser desorption ionization mass spectrometry (MALDI MS) is demonstrated. Deprotonated ana-lyte signals nearly free of matrix-related ions were observed in negative ion mode. The effect of themost important factors (laser energy, pulse voltage, DMAN/9AA ratio, analyte/matrix ratio) was investi-gated using a Box–Behnken response surface design followed by multi-response optimization in order tosimultaneously maximize signal-to-noise (S/N) ratio and resolution. The chemical surface compositionof single or mixed matrices was explored by X-ray photoelectron spectroscopy (XPS). Moreover, XPSimaging was used to map the spatial distribution of a model phospholipid in single or binary matrices.The DMAN/9AA binary matrix was then successfully applied to the analysis of intact Gram positive(Lactobacillus sanfranciscensis) or Gram negative (Escherichia coli) microorganisms. About fifty majormembrane components (free fatty acids, mono-, di- and tri-glycerides, phospholipids, glycolipids andcardiolipins) were quickly and easily detected over a mass range spanning from ca. 200 to ca. 1600 m/z.Moreover, mass spectra with improved S/N ratio (compared to single matrices), reduced chemical noiseand no formation of matrix-clusters were invariably obtained demonstrating the potential of this binarymatrix to improve sensitivity.