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Study Notes: GC Detectors

The detector is the part of the apparatus that converts the signal from the bands eluting off the column into a readable form via a chart recorder. The characteristics of an ideal detector include the following.

  • Adequate sensitivity (ie sufficiently low limit of detection).
  • Adequate linear range (the ratio of the largest to the smallest concentration within which the detector is linear).
  • Good stability and reproducibility (two consecutive samples should have identical or very similar chromatograms).
  • Temperature range from room temperature to at least 400°C.
  • Short response time independent of flow rate.
  • Similar response to all analytes.
  • Non-destructive of sample.
  • High reliability and ease of use - virtually foolproof is the ‘industry norm’.

Detectors can be classified into two general types:

Universal Detectors for detection of a wide range of analytes:

  • Flame ionisation detector (FID) (destructive).
  • Thermal conductivity detector (TCD) (non-destructive).

Selective/Specific Detectors for detection of particular types of analytes, and include:

  • Electro capture detector (ECD).
  • Photoionisation detector (PID).
  • Flame photometric detector (FPD).

The most widely used detector is FID, which, because of its high sensitivity, makes it very useful with capillary columns (able to detect analytes in the small quantities of samples analysed by such columns). In contrast, the relatively low sensitivity of the TCD means that it is not useful for this application.

A diagram showing the essential parts of an FID including the flame, gases and electrodes.

Components of the FID Detector

FID can detect most organic compounds, but is insensitive to water. This means that that the technique is very useful for the analysis of aqueous solutions or water-contaminated samples.

Some characteristics of detectors (notice the variation in sensitivity) are shown in the following.

Detector
Selectivity
Application
Sensitivity
TCD Organics and various inorganics Wide range of sample types including gases µg levels
FID Organics Wide range of sample types for example, beer alcohol analysis ng levels
ECD Halogens, peroxides, quinones, nitro groups Trace level halogenated pesticides, food residues, pharmaceuticals & environmental samples pg to ng levels
PID Aliphatics, aromatics, phenols, olefins Environmental applications (soil, water) pg to µg levels
FPD Phosphorus, sulfur Pesticides P: pg to ng levels
S: pg to ng levels

µg = microgram = 10-6 g
ng = nanogram = 10-9 g
pg = picogram = 10-12 g

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