9-2 FI/FD Operation
Introduction
In field ionization (FI/FD), sample molecules pass close to the tips of a mass of
carbon micro-needles on the emitter. Extraction rods maintained at high
voltage produce a very high electric field around the needles. This causes
ionization by quantum tunnelling of a valence electron. The process is very
'soft' often producing spectra with little or no fragmentation. The GCT
Premier has a dedicated outer and inner source for FI and FD operation.
Preparing the instrument for FI/FD Operation
Choosing an emitter for the FI/FD inner source
FI emitter
Six FI emitters are supplied with the instrument (available from Carbotec
[http://www.carbotec.com]). The emitters are made from 5-µm diameter
tungsten wire. Emitters are also available from Linden (http://linden.com).
The advantages of 5 µm diameter emitters in FI are:
• High field strength at minimum extraction voltage.
• Minimum surface area into which analyte can condense.
• Very fast heating and cooling characteristics. This ensures fast recovery
of sensitivity after flash off.
FD emitter
FD or all-round emitters give improved performance in FD mode of operation.
These emitters have a 10 µm diameter wire and have longer and more highly
branched carbon dendrite micro needles. Absolute sensitivity is significantly
lower than when using the 5-µm diameter FI emitter.
An emitter with a higher surface area, such as a 10 µm emitter, that can
partially act as a sample reservoir without completely suppressing sensitivity
is required in FD mode.
See also: Changing an emitter on page 10-33.
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