Page 355 - Tandem Techniques

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is mounted on an insulated probe, which can be introduced into the ion source by means of the standard
vacuum lock. About 2 mm from the probe tip is a counter electrode, which is held at a potential of
about 10,000 V relative to the probe.

Figure 9.8
Diagram of a Field Desorption Source
The ions produced from the emitter are accelerated toward the counter electrode, pass through a hole
into the focusing section, and then into the mass spectrometer analyzer. The preparation of the emitter
surface is both tedious and time consuming. The most commonly used surface consists of carbon micro
needles that are formed on tungsten wire in a vacuum. One substance that is used for this purpose is
benzonitrile. The material is placed on the wire and the temperature adjusted to gently pyrolyse the
material, producing carbon spots along the wire. A high voltage is then applied and the pyrolysis
continued, and carbon needles are formed that grow from the original carbonaceous deposits on the
wire. The sample is then coated on the carbon surface and in an electric field, the high potential gradient
close to the needle points causes ions to be emitted from the sample. Field desorption ionization also
provides spectra of high molecular weight materials but, due to the nature of the probe surface, the
technique would be very difficult to use with tandem instruments. Even with the use of a transport
system, the need for such a carefully prepared