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by an expanding argon stream from a 100 µm orifice, through a skimmer, into the accelerating section
of the mass spectrometer. In this section, the molecules are again subjected to a high-energy laser light
beam, this time from a dye laser, which produces ions by photo-ionization. The ions formed are
immediately accelerated through the region into a drift section, where they are deflected by an ion
reflector, to an electron multiplier detector. It is seen that to desorb and ionize a sample from a liquid
chromatograph, a fairly complicated transport system would again be necessary.

Laser desorption can also be used with conventional electron impact and high-pressure chemical
ionization sources, and also in conjunction with other ionization modes. A diagram of a sample probe
that can be used with an electron impact ion source is shown in Figure 9.5.

Figure 9.5
The Laser Desorption Sample Probe for
Use with Electron Impact Ionization Sources

The laser light is focused by means of suitable lenses, and a optical fiber pipe, though the probe and
onto a glass window at the end of the probe. The sample is placed next to the glass window and the
desorbed molecules pass along the hollow tube to be ionized by electron impact. This type of
desorption causes very little thermal degradation, and has been used satisfactorily for measuring the
spectra of labile materials, such as certain antibiotics.