The Thermal Conductivity Detector

Jan 01, 2006
Volume 24, Issue 1, pg 38–45

Thermal conductivity detectors have been in use since before the beginning of gas chromatography. Essential for fixed-gas detection — no substitute has the same ease of use and stability — thermal conductivity detectors also are employed when the auxiliary or combustion gases required by flame ionization or other detectors are unsafe or impractical. Although they cannot match the sensitivity of ionization detectors, thermal conductivity detectors are the third most used detector, surpassed only by flame ionization and bench-top mass-spectrometry detectors. This month's installment of "GC Connections" takes a look at the operating principles and inner workings of the thermal conductivity detectors.

John V. Hinshaw
Gas chromatography (GC) detectors can be classified into two broad categories: bulk-property and chemical-specific. Flame ionization detectors (1) fall into the chemical-specific class: molecules that contain carbon-hydrogen bonds are ionized and the resulting current is amplified to produce a signal, while other chemical classes produce little or no response. Thermal conductivity detectors, on the other hand, do not interact with solutes chemically but instead respond to changes in a bulk physical property, namely, the thermal conductivity of pure carrier gas compared with the conductivity of the solute–carrier-gas mixture passing through the detector reference and sample cells, respectively. Thermal conductivity detection (TCD) responds in proportion to the concentration in the sample cells of any gas that has a thermal conductivity different than that of pure carrier gas. The sensitivity of TCD response to various solutes is dictated by the solutes' thermal conductivities relative to the carrier: solutes with thermal conductivities close to that of the carrier gas will elicit small responses and those that differ more from the carrier gas in their thermal conductivities will generate larger sensitivities. This makes TCD respond universally without dependence upon specific chemical elements or structures, beyond any indirect effects on solutes' thermal conductivity.

Thermal conductivity detectors comprise one or more active thermal-sensing elements in two gas streams: the reference stream contains pure carrier gas and the sample stream contains the column effluent. The thermal elements' temperatures are nearly the same with pure carrier gas flowing in both the sample and the reference streams. The thermal conductivity of the gas in the sample cell changes as peaks are eluted from the column, while that of the reference stream remains constant. The temperature of the affected sample-sensing element changes in response, while the reference side stays the same and the resulting imbalance changes the circuit output level. Figure 1 shows a typical chromatogram of gases using a thermistor bead thermal conductivity detector.

Thermal Conductivity

The coefficient of thermal conductivity λ determines the rate of heat transfer, or flux, through a pure gas or gas mixture that spans a temperature gradient. Thermal conductivity is defined in terms of the heat flux qz and thermal gradient dT/dz by the following equation:

lorem ipsum