Page 348 - Tandem Techniques

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However, if the sample is dissolved or dispersed in a liquid matrix such as glycerol, the liquid surface is
continuously renewed and an intense primary beam can be safely used. This method of ionization has
become a standard process for obtaining the mass spectra of very polar or labile substances. The
temperature of the glycerol can be quite critical. At -20°C the glycerol becomes too viscous, and cannot
dissipate the energy sufficiently rapidly, and at 40°C the vapor pressure is too high and the glycerol
evaporates in the high vacuum of the source. Optimum sensitivity appears to be realized when the
glycerol is about 25°C. It is clear that this form of ionization would need to be employed with a
transport interface if used in a tandem instrument associated with a liquid chromatograph.

Plasma Desorption Mass Spectrometry

Plasma desorption is achieved by the use of a radioactive source, the fission particles being used to
ionize the sample. Employing Cf as the source the technique can be used with the time of flight mass
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spectrometer in a rather clever manner. Cf has a half life of 2.7 years and decays giving an alpha
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particle and two charged fission fragments simultaneously emitted in opposite directions. Typically a
pair of fission fragments might be Te and 142 Ba with energies of 104 and 79 MeV respectively. A
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diagram of the plasma desorption apparatus is shown in Figure 9.2.
The sample is placed on a thin aluminum sheet or a sheet of aluminized polyester film, and the
aluminum is connected to a high positive potential (assuming the sample ions will be positively
charged). When fission occurs, one particle strikes the sample and produces ions, while the other,
emitted in the opposite direction, is sensed by the trigger detector and starts the time of flight
measurement. The ionized sample molecule or fragment is accelerated to its characteristic velocity,
passes through the drift region of the spectrometer and is finally sensed by the stop sensor which arrests
the time measurement.

The yield of sample ions is very low and so a large number of spectra needs to be collected, which may
take several minutes. It is apparent that this long sampling time would prohibit the use of the plasma
desorption technique as a tandem instrument in real time.