Page 353 - Tandem Techniques

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Matrix Assisted Laser Desorption/Ionization (MALDI)

This type of ionization system can provide spectra of substances of extremely high molecular weight.
The laser usually employed, is the Nd-Yg-Laser, having a wavelength of 286 nm, and a pulse width of
about 10 ns. The sample is dispersed in a suitable involatile liquid to prevent decomposition and allow
the surface to be continually renewed. A number of different liquids have been employed, including
glycerol and nicotinic acid. Nicotinic acid is particularly useful as it absorbs very strongly at 286 nm the
wavelength of the laser light. A diagram showing the basic layout of the matrix assisted laser
desorption/ionization source and mass spectrometer is shown in Figure 9.6.

Figure 9.6
Diagram of the Basic Matrix Assisted
Laser Desorption Ionization Source

Light from the laser is arranged to strike the surface at a 45° angle, and the ions emitted are collimated,
by a three element Einzel lens, through an ion deflector, to the time of flight mass spectrometer. The
ions are accelerated by an appropriate voltage, and then allowed to drift to an ion reflector and then
back to the electron multiplier. This system was employed by Karas and Hillenkamp [2] to obtain the
mass spectrum of a