5.2.1.1 Ionization Techniques for Solid Non-Volatile Analytes
Field Desorption: Field Desorption (FD) techniques are relatively simple and do not require analyte separation in a GC since only one compound is introduced into the MS at a time. As noted in the heading above, compounds analyzed by this technique tend to be non-volatile, have high molecular weights, and degrade at higher temperatures. Analytes are introduced to the system on a probe made of carbon fibers that has been lightly coated with pure analyte. A high current is applied between the probe and an adjacent electrode. The current moves the ionized analyte towards the end of the carbon fibers by charge attraction, where the molecules are ionized into the vapor (plasma) phase. Then they enter the mass analyzer and then the detector. The breaking of bonds within the analyte (fragmentation) is rare in FD techniques, thus the spectrum only contains the molecular ion. Many older inexpensive bench-top systems used to come with a direct probe build into EI systems. However, this feature has been removed due to the high number of service calls to clean out the MS units when too much analyte was placed on the probe. Service technicians refer to these analyte-rich probes as having “peanut butter” placed on them.
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