The Thermal Conductivity Detector (TCD) is one of the more widely used detectors for gas chromatography because it is simple in construction, rugged, versatile, sensitive, relatively linear over a wide range, nondestructive to the sample, and inexpensive. It is because of these qualities that it continues to be widely used.
 
The TCD consists of a block (usually metallic) containing a cavity through which the gas flows. A heated element (resistance wire or thermistor) is positioned in the flow path of the gas. For practical considerations a differential method is usually used that requires two cavities and two heated elements. Only carrier gas passes through one cavity and the column effluent through the other. For process analysis, a reference and sample gas are used. 
  • Precise matching of filaments and thermistors over an extended current-resistance curve and under controlled heat dissipation conditions ensure superior detector performance
  • Rigid gas and electrical leakage tests and are run on specified zero gas
  • Micro-cells and GADEs are tested for signal output
  • Flow-through, semi-diffusion and diffusion geometries
  • Detector Sensitivity: depends on several factors. Most basic is the difference in thermal conductivity (or other characteristics) between the background (carrier, reference) gas and the sample.
  • Sensitivity of hotwire thermal conductivity detectors (TCD) depends on the internal geometry, type of detector elements, and the temperature differential (Dt) between filaments and cell block.
  • Thermistor detectors are at their best in sub-ambient and ambient operation.
  • Gas density detectors (GADE) vary in sensitivity directly with the difference in molecular weight, as well as with the number and type of sensor elements.
Thermal Conductivity Detectors

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Additional tidbits of information

Zero Stability

Drift and noise are affected by numerous variables. GOW-MAC quality control procedures ensure superior detector performance, by precise matching of filaments and thermistors over an extended current-resistance curve and under controlled heat dissipation conditions. Compatible filament pairs are installed symmetrically in a cell block. Under conditions of constant cell current, cell block temperature, carrier gas flow and pressure, negligible drift and noise will be noted on a 1 mV potentiometric recorder. The signal can be amplified many times under such precisely controlled conditions.

Oxidation and Corrosion Resistance

Thermal conductivity detectors (TCD) can be supplied with gold-sheathed tungsten, rhenium-tungsten, nickel, or other special filaments installed in cell blocks of Monel, etc.

Gas Density Detectors (GADE) with elements in the inert reference stream are not exposed to the sample and are capable of long service life with highly corrosive samples.

Detector Sensitivity

Sensitivity of hot wire thermal conductivity detectors (TCD) depends on the internal geometry, type of detector elements, and the temperature differential (Dt) between filaments and cell block. Thermistor detectors are at their best in sub-ambient and ambient operation. Experiments with filament and 8K thermistor detectors of identical geometry demonstrate that greater signal is obtained from the hot wires at cell temperatures of 75 °C and above.

Technical Literature

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