Principles of MALDI

Although lasers had been applied in the early 1980s for desorbing analytes from e.g. metal surfaces, ionization efficiency of this direct desorption approach was only poor and strong fragmentation was observed. Even worse, laser desorption was mainly restricted to molecules up to a mass of @1000 Da, thus being incompatible with most applications in bioanalysis. In 1988, Karas and Hillenkamp [1] introduced matrix-assisted laser desorption/ionization (MALDI) as a new ionization technique which turned out to be ideally suited for bioanalytical mass spectrometry. In MALDI, the analytes are co-crystallized in an organic matrix, which mostly consists of small organic molecules that are able to absorb light at a characteristic wavelength (Fig. 8.1). Ideally, this absorption maximum is compatible with the laser wavelength used for desorption. The matrix-to-analyte ratio is predominantly in the range of 1000 to 10000.

After matrix and sample have been deposited onto the MALDI target, the solvent is evaporated, and the crystalline surface is desorbed by nanosecond laser pulses with energies of 106-107 W cm~2 (Fig. 8.2). Thus, matrix molecules absorb the laser energy, and in a complex series of electronic excitation, relaxation and rapid thermal extension, parts of the crystalline surface evaporate. During this process, both matrix and analyte molecules are transferred into the gas phase. Provided that the laser energy was not too high, analyte molecules, e.g. proteins, peptides etc., are ionized without showing significant fragmentation.

As the laser pulse is in the nanosecond range, a fast mass spectrometer has to be coupled in series. In most cases, MALDI is connected to a time-of-flight (TOF) mass spectrometer with which m/z ratios are determined by precisely measuring the time an ion needs to pass from the ion source to the detector. Besides its abil-

Fig. 8.1 An overview on commonly used MALDI matrices. Depending on the analytes that have to be investigated and depending on the provided laser wavelength, the appropriate matrix has to be selected. The absorption maxima of the respective compounds are given in brackets.
Analyte And Matrix

Fig. 8.2 Principle of the MALDI process. Initially, analyte and matrix are co-crystallized. After evaporation of the solvent, a nanosecond laser pulse is directed onto the crystalline surface, and both matrix and analyte molecules are desorbed. A complex reaction cascade leads to the formation of charged analyte molecules that reach the mass spectrometer without significant fragmentation.

Fig. 8.2 Principle of the MALDI process. Initially, analyte and matrix are co-crystallized. After evaporation of the solvent, a nanosecond laser pulse is directed onto the crystalline surface, and both matrix and analyte molecules are desorbed. A complex reaction cascade leads to the formation of charged analyte molecules that reach the mass spectrometer without significant fragmentation.

ity for fast measurements, TOF-MS offers advantageous ion transmission, which allows to also detect low ion fluxes originating from the MALDI source.

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