Analytical Chemistry

VDU screen via suitable electronic amplifying circuitry where the data are presented in the form of an
elution profile. Although there are a dozen or more types of detector available for gas chromatography,
only those based on thermal conductivity, flame ionization, electron-capture and perhaps flame emission
and electrolytic conductivity are widely used. The interfacing of gas chromatographs with infrared and
mass spectrometers, so-called 'hyphenated' techniques, is described on p. 114 et seq. Some detector
characteristics are summarized in Table 4.11.

Table 4.11 Gas chromatography detector characteristics

Detector Minimum detectable

quantity (gs–^1 )

Linear range Temperature

limit (°C)

Remarks

Thermal conductivity

(TCD)

10 –^9104450 non-destructive, temperature and flow

sensitive

Flame ionization (FID) 10 – 12 107 400 destructive, excellent stability

Electron capture (ECD) 10 –^13102 – 103 350 non-destructive, easily contaminated,

temperature-sensitive

Phosphorus 10 –^14105 400? similar to flame ionization

Nitrogen 10 –^13105 400? similar to flame ionization

Flame photometric

(FPD)

(P compounds) 10 –^12104 ~250 signal approximately

(S compounds) 10 –^10 — ~250 proportional to square of S concn.

Thermal Conductivity Detector (TCD)

This detector is based on the principle that a hot body loses heat at a rate which depends on the thermal
conductivity and therefore the composition of the surrounding gas. Sometimes called a katharometer, it
consists of two heated filaments of a metal which has a high coefficient of resistance, e.g. platinum, and
which form two arms of a Wheatstone bridge circuit. The two filaments are situated in separate channels
in a heated metal block (Figure 4.19). Pure carrier gas flows through one channel and the effluent from
the column through the other. The rate of heat loss from each filament determines its temperature and
therefore its resistance. A change in thermal conductivity of the gas flowing through the sample channel
arising from elution of a sample component alters the temperature and hence the resistance of the
filament in that channel and this produces an out-of-