Thermal conductivity detector

The thermal conductivity detector or katharometer is used in gas chromatography and is one of the first used. It has a wide application and its use is based on the difference in thermal conductivity of the carrier gas when the analyte also circulates.

This type of detector is also called catharometer. The sensor of a katharometer consists of an electrically heated element (resistance). This resistance, for a constant electrical potential, has a temperature that depends on the surrounding gas. The resistor can be a thin wire of platinum, gold or tungsten, or a semiconductor thermistor. The basic difference between metal detectors and the semiconductor thermistor is that the latter has a negative temperature coefficient, in other words, its resistance decreases as the temperature increases.

Operation

Two pairs of elements or sensors are used, one of them in the column effluent flow and the other in the gas stream prior to the sample injection chamber (clean gas). In the electrical diagram they are shown as sample and reference, respectively. Through this electrical assembly, it is possible to compensate for the effect of changes in pressure, flow and electrical power, measuring only the changes in the conductivity of the gas.

Currently, single-filament sensors are also used, which lack baseline drift, equilibrate quickly and are very sensitive. The operation of this sensor consists of a 5 μl chamber containing a small filament. The reference gas and the column outlet gas are passed alternately over it, with a frequency of 10 Hz, obtaining an electrical signal of 10 Hz whose amplitude depends on the difference between the thermal conductivity of the reference gas and the exit. Another advantage of using a 10 Hz signal is that thermal noise is eliminated from the system.

The gases used as carriers make it easy to distinguish when the gas carries analyte, because the conductivities of hydrogen and helium are 6 to 10 times greater than most organic compounds. This effect does not occur in other carrier gases such as nitrogen, which is why the use of this detector is limited to the use of hydrogen or helium as a carrier gas.

Advantages of this detector:

• Simplicity.
• Wide linear dynamic range, 10⁵ units.
• Universal response to organic and inorganic compounds.
• Non-destructive detector.

Disadvantages:

• Relatively low sensitivity, 10^-8 g of solute/ml of carrier gas.
• Impossibility of using it in capillary columns (small output channel).