Alternative organic and inorganic matrices for in-vitro and in-situ MALDI TOF MS detection of low molecular weight analytes

Phosphatidylcholines and sphingomyelins are important components of biomembranes. Due to their permanent positive charge, they are easily ionized by MALDI and produce intense peaks in positive-ion mass spectra. However, the ionization efficiencies (IE) of other phospholipid (PL) classes are much lower and result in poor sensitivity in their analysis by MALDI-MS. To reduce this limitation, the detection of PLs as negative ions was pursued with the use of 2-(2-aminoethyloamino)-5-nitropyridine (AAN), a basic compound (pKa ∼ 10.5). As Chapter II reveals, AAN produced homogeneous spots and enhanced the sensitivity of detection (nearly a ten-fold improvement compared to a commonly used matrix, dihydroxybenzoic acid) for phosphatidyl-ethanolamines (PEs), -serines (PSs), -glycerols (PGs) and -inositols (PIs) in simple mixtures and in a crude soybean extract. The IE was highest for PIs and decreased for PGs, PSs, PEs.;Micro- and nanoparticles of Fe2O3 enabled ionization and desorption but the sensitivity was not as good as that obtained with TiO 2. There was less fragmentation and the cleavage occurred primarily at the ester linkages connecting to the acyl chains.;To reduce theses undesirable effects, nanoparticles of TiO2 and Fe2O3 were synthesized in the presence of organic acids of different length to provide a protective coating. Unfortunately, the modified nanoparticles did not enable the efficient ionization of PLs. It is proposed that the organic barrier prevented effective ionic transfer to the PL headgroups. Further understanding of the physicochemical steps involved in ionization could lead to the rational design of new and selective matrices.;To reduce spectral complexity in the MS analysis of small molecules (less than 1000 Da), microparticles and nanoparticles of two inorganic oxides with different isoelectric points (IEPs)---TiO2 (IEP ∼3-6) and Fe2O3(IEP ∼6-8)---were evaluated. Titania nanoparticles resulted in extensive cleavage of the polar moieties attached to the phosphate groups in in-vitro studies. However, and unlike microparticles, their small size (∼100 nm) enabled uniform coverage of plant tissues for in-situ chemical mapping. Indeed, PLs and triacylglycerols were detected in soybean slices with minimal fragmentation. Anthocyanins and flavonols could be readily mapped in a bicolored section of a rose petal. Importantly, no matrix-related peaks were present, in the spectra of either nano- or microparticles.