Thermal conductivity detector
The thermal conductivity detector (TCD TCD abbreviated or after the English phrase Thermal Conductivity Detector ) is one of the main detector in gas chromatography, which is used primarily for the detection and quantification of permanent gases, carbon dioxide, sulfur dioxide and noble gases. The measuring principle is based on the continuous measurement of the heat conductivity difference between the sample gas flow with respect to a reference gas stream.
Historical
Even the pioneers of gas chromatography ( eg Erika Cremer ) used in the late 1940s in the experimental setup detectors based on thermal conductivity measurements. The then Katharometers devices mentioned were originally developed for the measurement of carbon dioxide in flue gases. The implementation of the measuring principle specifically designed for use in gas chromatography was first described in 1954 and continued to develop until today.
Principle of operation
A thermal conductivity detector is composed of a thermostated metal block having two identically constructed measuring cells. One of these cells is traversed by the gas to be analyzed, and the other measuring cell is permanently flowed through by pure gas and is used for comparison measurements. In both cells is in each case a heating wire (also called the filament ) made from platinum, tungsten, nickel or alloys thereof, which is heated to a higher temperature than the surrounding him detector block. Therefore, there takes place a continuous flow of heat from the heating wires of the surrounding gas flows to the detector block of the thermal conductivity (and hence of the composition), depending of the gases. Therefore, changes in the composition of the measured gas temperature cause changes in the flow cell and thus a change in the electrical resistance in the heat wires. Since measuring and reference cells are combined to form a Wheatstone bridge circuit, the temperature difference of the heating wires can be measured as a voltage difference and record.
In a gas chromatography system complex mixtures of substances are separated in time because of different properties of the individual substances by means of a carrier gas stream in a separation column. The use of the TCDs as a detector in the gas chromatography of the outlet of the separation column used is associated with one of the measured cells. As the reference used for the chromatographic separation of carrier gas is passed through the other cell.
Flows through the measuring cell of pure carrier gas, the thermal conductivities in the measurement and reference cell are equal, and no signal will be measured. However, the carrier gas in the gas flow analysis, an analyte added, then the thermal conductivity of this gas mixture the pure carrier gas, which is recorded as a signal different. The thus measured signal is the sample concentration in the carrier gas proportional.
Designs
Classic is the detector of a stainless steel block with the two measuring cells, glands for the supply and discharge of the measurement and the reference gas and the electrical connections for the filaments. The internal volumes of the cells were measured in the usual carrier gas flows of the packed glass or metal columns that were once used in gas chromatography as a separation column (inner diameter: 2-4 mm) adjusted. The flow rates for columns of this type are approximately 20-60 ml / min.
Today's standard micro packed capillary columns or capillary columns (inner diameter: 0:18 to 0:32 mm), which are operated at flow rates of 1-2 ml / min, (possibly with appropriate adapters ) can still be connected and used, if at front of the detector input a T-piece, an additional gas flow is fed, the so-called make-up gas. Thereby, the flow velocity is increased in the cell to maintain the required time resolution. However, as the concentration of the analyte will be reduced in the gas flow and reduces the sensitivity of the detection method.
As an alternative to the two -flow measuring cells, a variant has been developed in which only one cell is used which is passed through the quick change of the measurement gas and the reference gas. Both streams of gas are hereby switched through a valve.
Since recently were built with the help of the microchip technology and miniaturized detectors ( micro - TCDs ), which are ideally suited for use with capillary columns, because they work with lower gas flow rates. They differ in the detection sensitivity is not of the classic design, but allow a change of damaged filaments no longer.
Area of application
All substances that pass through the measuring cell a thermal conductivity detector, leading to a detector signal. Therefore, the TCD is one of the universal detectors. It is used both for the qualitative detection of the analyte as well as to quantify individual materials.
Qualitative analyzes
Qualitative analysis refers to the unambiguous identification of analytes sought in a sample. In gas chromatography this is done by comparing the retention time of the test substance with a known reference, the so-called standard. To obtain a clear evidence is excluded that under the present conditions other substances with the same retention time the detector happen ( co- elution) and thus lead to a false positive result. The thermal conductivity detector as the sole measurement system can not achieve this. It was only by the additional use of an alternative detection technique (eg, mass spectrometry) can co- elution be safely ruled out.
The sensitivity of the detector is dependent on many factors. Essential is the temperature difference between the filament and the housing: the detector is more sensitive, the greater the temperature gradient within the detector. However, a chosen too low temperature of the detector block cause fluctuations in ambient temperature affect the readings, or analytes ( such as moisture ) can affect the thermal conductivity and thus the measurement result by condensation in the measuring cell. However, the temperature of the filaments is too high, their life and the risk of burning the heating wires decreases increases. Generally temperatures of 80-120 ° C for the detector block, and 150-250 ° C are used for the filaments.
Quantitative analyzes
In general, the TCD is not only used for the identification of the analyte, but also to quantify the constituents of a sample. This is possible because the measured detector signal is proportional to the sample concentration in the carrier gas. In order to derive reliable concentration data from the measured values , a calibration for each substance should be made which is to be determined. For this purpose, first a series of samples are measured with known concentrations and their outcomes determined from a mathematical function with the help of which may also be the conversion of measured values of unknown samples into the appropriate concentrations. In general, the measurement values are approximated by a straight line. This so-called linear range of the detector comprises about 5 orders of magnitude.
With organic compounds ( such as hydrocarbons ), the thermal conductivities are very similar and very different from the carrier gases commonly used. In analytical tasks of this type of TCD can also be used without calibration, because the concentration of a single analyte can be estimated from the ratio of the sum of all analytes.
The current technically achievable detection limit is about 1 ppm per substance in the gas analysis ( which is about 5-50 ng ), ie due to the over other detectors relatively low detection sensitivity of the TCD is not suitable for trace analysis.
Alternative universal detectors to the TCD for more sensitive analyzes of the pulsed helium photoionization detector (PDD ) and the ion mobility spectrometer, with which detection limits in the ppb range are achievable.
Detected analytes
Although the thermal conductivity detector is universal, it is used because of the relatively low sensitivity of most frequently for the analysis of permanent gases and noble gases. This, as well as the nitrogen, carbon and sulfur oxides can be inexpensively detected only with this type of detector.
Limitations arise only when the thermal conductivity of the carrier gas and analyte are not significantly different (as in argon and nitrogen) or the thermal conductivity of the mixture is not linear over the range of composition changes (such as hydrogen and helium). In practice, is then switch to an alternate carrier gas.
Assay mixture containing corrosive substances (for example, hydrogen chloride, hydrogen cyanide ), the life of the filament may be adversely affected.
Coupling options
Since the TCD does not destroy the analytes in the detection, it is a further detector may still be followed in order to obtain additional specific information on the detected substances. For this, a flame ionization detector (FID) or electron capture detection (ECD) is typically used. This type of coupling is also referred to as a tandem - detection.