In matrix-assisted laser desorption/ionization (MALDI) mass spectrometric (MS) imaging of biological tissues, an organic matrix is deposited onto a sample and co-crystallized with the peptides and proteins that are extracted from the tissue. Thus, analytes are incorporated into matrix crystals, making them accessible to the matrix-assisted desorption/ionization process. However, crystal dimensions can limit the spatial resolution of these imaging experiments and hinder observation of small features in the tissue. The addition of differing amounts of triethylamine (TEA) to solutions of common MALDI matrices-α-cyano-4-hydroxycinnamic acid (CHCA), 2,5-dihydroxybenzoic acid, and sinapinic acid-results in the formation of liquid matrices that may alleviate these problems. Liquid matrices form optically transparent, viscous films that do not crystallize, even when kept open to air for several days at room temperature or placed in the vacuum of the mass spectrometer. These matrices allow peptides to be characterized using MALDI MS down to the femtomole level. Although signal intensity is reduced in liquid matrices as compared to standard crystalline matrices, a phenomenon especially evident for higher molecular mass analytes, peptides are still detected even when TEA comprises 2/3 (v/v) of the MALDI matrix mixture. Transparent matrices demonstrate excellent repeatability and sample stability. Post-source decay sequencing of peptides with a CHCA/TEA matrix produces intense peptide fragment signals in the low molecular mass region, and rat pituitary blots yield high-quality spectra using these mixtures. Overall, TEA-based transparent MALDI matrices are promising for coupling MALDI MS and light microscopy.
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