Laser desorption ionization other considerations

Atmospheric pressure matrix-assisted laser desorpt-ion/ionization (ap-MALDI) A variant of the conventional MALDI technique is ap-MALDI in which the sample is ionized outside the vacuum system and ions are captured through a small orifice, usually into an ion trap mass spectrometer. Its advantage appears to be a considerable amount of rapid collisional cooling of the ions by the high atmospheric pressure, leading to stabilization of sensitive compounds. On the other hand, some compounds, such as carbohydrates, appear to suffer increased fragmentation under these conditions. 

What, then, is so special about the chemical matrix in MALDI Some of its important features, such as the absorption of the laser energy, are easily understood, but rather surprisingly the overall process of the desorption and ionization has not yet been fully described, almost 30 years after its invention. Considerable progress regarding the mechanism of analyte desorption and protonation was recently achieved [24, 25]. Meanwhile, the search for better (i.e., more sensitive) matrices does not remain completely empirical, as some of the critical parameters for efficient analyte protonation (see Section 1.5) are uncovered, although other aspects such as prediction and targeted manipulation of the matrix morphology remain [26]. 

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