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Lecture 4. Detectors for GC.

Thermal conductivity detector (TCD).

% TCD detector consists of an electrically-heated wire or thermistor &tungsten-rhenium wire'. The temperature of the sensing element depends on the thermalconductivity of the gas flowing around it. Changes in thermal conductivity#

such as when organic molecules displace some of the carrier gas# cause atemperature rise in the element which is sensed as a change in resistance.

Response is universal and proportional to concentration.

,est gases for TCD: or e# because of highest thermal conductivity &/.+0/and /.+1+ 23&45m5s'# respectively# for (/./1'. or helium and hydrogen thetemperature conductivity lowers when solute is eluted.

The sensitivity is inversely proportional to flow rate: The detector is moresensitive at lower flow rates.

Detection limit: 1// pg3ml.6inear response range: 7 +/8.

To increase sensitivity the temperature of the bloc9 should be maintained atlowest temperature that allows all the solutes remain gaseous.

ilament

Gasflow

outGasflowin

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Lecture 4. Detectors for GC.

Flame-Ionization Detector! (FID).

The eluate is burned in a mi"ture of hydrogen and air. Carbon atoms &e"cept-C and -C' produce C radical# which are thought to produce C