Page 119 - Tandem Techniques

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It follows that for a given system, the mass of the ion is directly proportional to the square of the transit
time to the sensor. The sample is volatilized into the space between the first and second electrodes and a
burst of electrons, over about a microsecond period, is allowed to produce ions. The extraction voltage,
(E) is then applied for another short time period which, as those further from the second electrode will
experience a greater force than those closer to the second electrode, will focus the ions.

Figure 3.36
The Time of Flight Mass Spectrometer

After focusing, the accelerating potential (V) is applied for a much shorter period than that used for ion
production (ca 100 nsec) so that all the ions in the source are accelerated almost simultaneously. The
ions then pass through the third electrode into the drift zone and are then collected by the sensor
electrode. The particular advantage of the time of flight mass spectrometer from the point of view of
tandem systems lies in the fact that it is directly and simply compatible with direct desorption from a
surface. Consequently, it can be employed with laser desorption and plasma desorption techniques
which will be discussed in a later chapter.