MALDI sublimation

Matrix selection and optimization of the sample preparation protocol are the most important steps in the analysis because the quality of the results depends on good sample preparation. However, the preparation procedures are still empirical. The MALDI matrix selection is based on the laser wavelength used. In addition, the most effective matrix is strongly related to the class of analyte and may differ for analytes that have apparently similar structures. The MALDI matrices must meet a number of requirements simultaneously. These are strong absorbance at the laser wavelength, low enough mass to be sublimable,... 

MALDI represents an important technical development on FAB. In MALDI, the analyte molecule of interest is dispersed in a crystalline, not liquid, matrix and the beam of fast atoms is replaced by laser irradiation. The analyte is pre-mixed with a matrix material and crystals are then placed on a solid surface (Figure 9.5). Nanosecond laser pulses sublime the matrix... 

Sections, 10-12 p thick, were cut from flash-frozen tissue, thaw-mounted on MALDI plates, and dried in a desiccator prior to deposition of the matrix. Sublimation of 2,5-dihydroxy-benzoic acid over -4 min resulted in a homogenous 5-10 p coating. Alternatively, the tissue was dry-coated with matrix that was passed through a 20 p sieve for 20 min. Both methods were stable in the vacuum for 24 h, although back-sublimation is a risk that can create artificial analyte concentration gradients. 

Most matrices rapidly vaporize at moderate temperatures, a few hundred degrees Celsius is typical. The energy required from the laser varies with the initial sample temperature, " consistent with a requirement for reaching a characteristic minimal temperature to induce sufficient material ablation. The necessary laser-deposited energy density must be more associated with phase explosion than simple vaporization, since the heat of matrix sublimation is not correlated with MALDI efficacy. Smaller or defect-rich crystals are more easily vaporized than larger, more ideal crystals, leading to quahtative changes in the sample as a function of number of laser shots. ... 

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