Capillary Chromatography - The Flame Ionization Detector
Detectors for Use with Capillary Columns
Detectors for use with capillary columns must have high sensitivities due to the limited sample size that can be used whith such columns. In addition, due to the very small peak volumes produced by the column, the sensing volume must also be extremely small. As the flame ionization detector (FID) is mass sensitive as opposed to concentration sensitive (see book 4 of this series for the meaning of mass and concentration sensitivity) the dilution by hydrogen does not effect the detector response. Thus, the FID has both the high sensitivity and the small sensor volume that is necessary and is, consequently, ideal for use with capillary columns. The nitrogen phosphorous detector (NPD) is also appropriate for capillary columns (the function of which is very similar to that of the FID) for the same reason. The micro-argon detector employs a scavenger flow which also, in effect, reduces the sensor volume, so this detector can also be used with capillary columns. The FID and the NPD detector will be described here. For a more detailed treatment of GC detectors see book 4 of this series.
The Flame Ionization Detector
The FID, invented by Harley and Pretorious (6), and separately by McWilliams and Dewer (7), evolved from the Heat of Combustion Detector developed by Scott (8). The FID operates in the following manner. Hydrogen is mixed with the column exit gas (which may be helium, hydrogen or any other appropriate gas) and then burnt at a small jet which is situated inside a cylindrical electrode system.
Figure 13. The Flame Ionization Detector