Gas Chromatography Detectors

Despite they re being many GC detectors to choose from, the majority of GC separations are monitored by the flame ionization detector (FID), the nitrogen phosphorus detector (NPD), the electron capture detector (ECD) or the katharometer detector. However, the latter (the katharometer) is almost exclusively used in gas analysis and rarely used in chiral chromatography. The FID is the most popular GC detector and is used in probably 90% of all chiral analyses. However, before describing the construction and function of the (FID), (NPD) and the (ECD), the pertinent detector specifications will be considered.

Detector Specifications

There are many different detector specifications, all of which are important under certain circumstances, but only a few are pertinent to chiral chromatography. Detectors are only of peripheral interest to the subject of this book, and, consequently, only certain detector specifications will be considered here, a more detailed discussion is given in Chromatography Detectors (35).

Detector Linearity

If a solute enters the detector at concentration (c), A linear detector will give the following response

Unfortunately, no practical detector can be precisely linear although the response can be made to tend to true linearity by appropriate design of the sensor and associated electronics. It is clear that some measure of detector linearity is needed.

An alternative method for defining linearity is to assume, that for a nearly linear detector, the response could be expressed by the following simple relationship,

(30)

Consequently, for a truly linear detector the response index (a) will be unity and, if the response is not linear, the numerical value of (a) will provide a measure of the proximity of the detector response to true linearity.