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High Amplitude Arbitrary/Function Generator Simplifies Measurement in Automotive, Semiconductor, Scientific and Industrial Applications Application Note

Figure 10. Thermal Conductivity Detector and Wheatstone bridge.

Characterization of Gas Chromatography Amplifiers

Gas chromatography is a technique for separating and analyzing the existence and concentration of chemicals in a complex sample. It involves the analyte being vaporized and injected into a continuous flow of an inert carrier gas, most commonly helium for its high thermal conduction. To detect the characteristics of the sample in the carrier gas, various detector types are available, each with particular advantages and disadvantages.

temperature controlled. Measurements with a TCD are made by measuring changes of the heated elements' resistance caused by temperature variations during the flow of the analyte gas.

The heated elements are either filaments or thermistors. The resistance of filaments increases with temperature (positive coefficient of resistance), and the resistance of thermistors decreases with rising temperature (negative coefficient of resistance). The choice of heating element depends on the temperature inside the cell and properties of the measured substance.

One of the most common types of gas chromatography detectors is the Thermal Conductivity Detector (TCD). Although more sensitive and specialized detectors are available, TCDs continue to be popular due to their simple construction, ruggedness, versatility, sensitivity, linearity, and low cost.

A TCD consists of a sample cell and a reference cell. The sample cell is used for characterizing the analyte. The refer- ence cell contains only carrier gas. Each cell contains a heated element positioned in the flow path of the gas and is

When only carrier gas is present in the cells, thermal energy flows at a stable rate from the heated element to the detector body. When an analyte is introduced into the sample cell, the thermal conductivity inside the cell changes, the heated element warms up and changes its resistance. The heated elements of sample and reference cell are often incorporated into the arms of a Wheatstone bridge circuit (see Figure 10). In this configuration, variations in resistance of the heated element inside the sample cell changes the output voltage of the bridge.

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