Dried small-molecule MALDI

The sample preparation procedures for the direct analysis of small molecules in tissue have been described by several papers [120-124], Tissues (brain, heart, lung, kidney, liver, etc.), were immediately frozen and stored at -80 °C after harvest. The frozen tissues were subsequently cut into serial 10-20 pm thick section which was typically prepared by cryosectioning on a microtome at a temperature of -20 °C. The adjacent sections were gently mounted onto a conductive surface, MALDI imaging target plate or glass slides. These plates were desiccated under low vacuum for a short period of time until dry, then robotically or manually coated with the... 

MALDI is achieved in two steps. In the first step, the compound to be analysed is dissolved in a solvent containing in solution small organic molecules, called the matrix. These molecules must have a strong absorption at the laser wavelength. This mixture is dried before analysis and any liquid solvent used in the preparation of the solution is removed. The result is a solid solution deposit of analyte-doped matrix crystals. The analyte molecules are embedded throughout the matrix so that they are completely isolated from one another. 

Consider the analysis of proteins in a mixture The active surface for the capture of target proteins can take the form of various chromatographic surfaces, such as those modified chemically with cationic, anionic, hydrophobic, or hydrophilic groups or with metal ions (Figure 2.13). Active spots can also be prepared by biochemical modifications with antibodies, receptors, DNA molecules, or enzymes. A small volume of the crude sample (e.g., serum, urine, or plasma) is applied directly to the spot and washed with an appropriate solvent to remove unretained proteins and other constituents of the biological matrix. The MALDI matrix is applied to the dried surface, and bound species are analyzed as usual. Protein chips with multiple active spots can be created for high-throughput analysis. 

This results in co-crystaUization of matrix and sample and therefore higher sample concentration focused at the distinct hydrophilic anchor points [16]. One of the most commonly used matrices is a saturated solution of a-cyano-4-hydroxy-cinnamic acid in 97% acetone/3% water containing a small amoimt of TFA to support the ionization of the sample molecules in the MALDI source. The matrix is directed to the AnchorChip target and this is then dried. The sample fractionation after HPLC can be performed automatically using special sample fractionation systems (e.g., Probot, LC Packings Dionex). The target is introduced to a special desk and can be adjusted in the three XYZ planes so that the small nano-LC volume is placed directly onto the matrix. Using this set-up, fractions of volume less than 40 nL can be collected automatically. After collection, the individual fractions are recrystallized from 0.01% TFA, 30% acetone, and 60% ethanol in water and analyzed. 

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