Thermal conductivity detector
The thermal conductivity detector (abbreviated to TCD or TCD after the English phrase Thermal Conductivity Detector ) is one of the most important detectors in the gas chromatography , which in particular for the detection and quantification of permanent gases , carbon dioxide , sulfur dioxide and noble gases is used. The measuring principle is based on the continuous measurement of the thermal conductivity difference of the sample gas flow compared to a reference gas flow .
Historical
Even the pioneers of gas chromatography (e.g. Erika Cremer ) used detectors based on thermal conductivity measurements in their experimental setups in the late 1940s. The devices called Katharometers at that time were originally developed for measuring carbon dioxide in combustion exhaust gases. The implementation of the measuring principle specifically for use in gas chromatography systems was first described in 1954 and has been continuously developed to this day.
Working principle
A thermal conductivity detector consists of a thermostatically controlled metal block with two identical measuring cells. One of these cells is flowed through by the gas to be analyzed, the other measuring cell is continuously flowed through by pure gas and is used for comparison measurement. In both cells there is a heating wire (also called filament) made of platinum , tungsten , nickel or their alloys , which is heated to a higher temperature than the detector block surrounding it. There is therefore a continuous flow of heat from the heating wires through the enveloping gas flows to the detector block, which is dependent on the thermal conductivity (and thus on the composition) of the gases. Changes in the composition of the measuring gas therefore cause temperature changes in the measuring cell and thus a change in the electrical resistance in the heating wires. Since the measuring and reference cells are connected to a Wheatstone bridge circuit , the temperature differences between the heating wires can be measured and recorded as a voltage difference.
In a gas chromatography system, complex substance mixtures are temporally separated due to the different properties of the individual substances with the aid of a carrier gas flow in a separation column . When using the TCD as a detector in gas chromatography, the outlet of the separation column used is linked to one of the measuring cells. The carrier gas used for the chromatographic separation is passed through the other measuring cell as a reference.
If pure carrier gas flows through the measuring cell, the thermal conductivity in the measuring and reference cell is the same and no signal is measured. However, if an analyte is added to the carrier gas in the analysis gas flow, the thermal conductivity of this gas mixture differs from the pure carrier gas, which is recorded as a signal. The signal measured in this way is proportional to the sample concentration in the carrier gas .
Designs
Classically, the detector consists of a stainless steel block with the two measuring cells, screw connections for the inlet and outlet lines for the measuring and reference gas and the electrical connections for the filaments. The internal volumes of the measuring cells were adapted to the usual carrier gas flows of the packed glass or metal columns (internal diameter: 2-4 mm) used as separation columns in gas chromatography at that time . The flow rates for columns of this type are around 20–60 ml / min.
The micro-packed capillary columns or capillary columns (internal diameter: 0.18–0.32 mm) that are customary today and which are operated at flow rates of approx. 1–2 ml / min can still be connected and used (if necessary with appropriate adapters) if An additional gas flow is fed in via a T-piece in front of the detector entrance, the so-called make-up gas. This increases the flow rate in the measuring cell in order to maintain the necessary time resolution. However, this also reduces the concentration of the analytes in the gas flow and reduces the sensitivity of the detection method.
As an alternative to the two flow-through measuring cells, a variant was developed in which only one measuring cell is used, through which the measuring gas and reference gas flow rapidly. Both gas flows are switched over here via a valve .
More recently, miniaturized detectors (micro-TCDs) have also been built with the help of microchip technology , which are ideally suited for operation with capillary columns since they manage with lower gas flow rates. In terms of detection sensitivity, they do not differ from the classic design, but they no longer allow damaged filaments to be changed.
Application area
All substances that pass the measuring cell of a thermal conductivity detector lead to a detector signal. The TCD is therefore one of the universal detectors. It is used both for the qualitative detection of analytes and for the quantification of individual substances.
Qualitative analysis
Qualitative analysis is the clear identification of a sought analyte in a sample. In gas chromatography, this is done by comparing the retention time of the substance to be examined with a known reference, the so-called standard. In order to obtain clear evidence, it must be ruled out that other substances with the same retention time pass the detector under the present conditions ( co-elution ) and thus lead to a false positive result. This cannot be achieved with the thermal conductivity detector as the sole measuring system. Co-elution can only be reliably excluded by using an alternative detection technique (e.g. mass spectrometry ).
The sensitivity of the detector depends on many factors. The temperature difference between the filaments and the housing is essential: the greater the temperature gradient within the detector, the more sensitive it is. However, if the temperature of the detector block is selected too low, fluctuations in the ambient temperature can affect the measured values, or analytes (e.g. moisture) can influence the thermal conductivity and thus the measurement result due to condensation in the measuring cell. If, on the other hand, the temperature of the filaments is set too high, their service life is reduced and the risk of the heating wires burning through increases. Typically temperatures of 80-120 ° C for the detector block and 150-250 ° C for the filaments are used.
Quantitative analysis
As a rule, the TCD is not only used to identify analytes, but above all to quantify the components of a sample. This is possible because the measured detector signal is proportional to the sample concentration in the carrier gas . In order to be able to derive reliable concentration information from the measured values, a calibration must be carried out for each substance that is to be determined. For this purpose, a number of samples with known concentrations are first measured and a mathematical function is determined from the results, with the help of which the measured values of unknown samples can also be converted into the associated concentrations. As a rule, the measured values are approximated by a straight line. This so-called linear range of the detector comprises about 5 powers of ten .
In the case of organic compounds (such as hydrocarbons ), the thermal conductivities are very similar and very different from the carrier gases commonly used. For analytical tasks of this kind, the TCD can also be used without calibration, because the concentration of an individual analyte can be estimated from the ratio to the sum of all analytes.
The currently technically achievable detection limit is approx. 1 ppm per substance in the analysis gas (this corresponds to approx. 5–50 ng ), i. H. Due to the comparatively low detection sensitivity compared to other detectors, the TCD is not suitable for trace analysis .
Alternative universal detectors to the TCD for more sensitive analyzes are the pulsed helium photoionization detector (PDD) and the ion mobility spectrometer , with which detection limits in the ppb range can be achieved.
Detected analytes
Although the thermal conductivity detector is universal, it is most often used for the analysis of permanent gases and noble gases due to its comparatively low sensitivity . These, but also the nitrogen , carbon and sulfur oxides can only be detected cost-effectively with this type of detector.
Restrictions only arise if the thermal conductivities of the carrier gas and analyte do not differ significantly (as in the case of argon and nitrogen ) or if the change in the thermal conductivity of the mixture changes the sign across the composition range (as in the case of hydrogen and helium ). In practice, an alternative carrier gas is used.
If the analysis mixture contains corrosive substances (e.g. hydrogen chloride , hydrogen cyanide ), the service life of the filaments can be impaired.
Coupling options
Since the TCD does not destroy the analytes during detection, a further detector can be connected downstream to obtain additional specific information about the substances detected. A flame ionization detector (FID) or an electron capture detector (ECD) is usually used for this. This type of coupling is also known as tandem detection.
See also
literature
- Bruno Kolb: Gas Chromatography in Pictures . 2nd Edition. Viley VCH-Verlag, Weinheim 2003, ISBN 3-527-30687-0
- Dean Rood: Troubleshooting in Capillary Gas Chromatography . Hüthig Buch Verlag, Heidelberg 1991, ISBN 3-7785-2104-7
Web links
- For a review of TCDs in gas chromatography from LCGC North America (English).
Individual evidence
- ↑ Experimental set-up by Erika Cremer. Archived from the original on March 26, 2013 ; Retrieved September 20, 2009 .
- ^ NH Ray: Gas chromatography. I. The separation and estimation of volatile organic compounds by gas-liquid partition chromatography. In: Journal of Applied Chemistry. 4, 1954, p. 21, doi : 10.1002 / jctb.5010040106 .
- ↑ The Katharometer Detector. Retrieved September 20, 2009 .
- ^ Bruno Kolb: Gas chromatography in pictures . 2nd Edition. Viley VCH-Verlag, Weinheim 2003, ISBN 3-527-30687-0 , p. 182.
- ↑ Peter J. Baugh (Ed.): Gaschromatographie. Vieweg Verlag, Braunschweig 1997, ISBN 3-528-06657-1 , p. 53 ( limited preview in the Google book search).
- ^ Dean Rood: Troubleshooting in Capillary Gas Chromatography. , Hüthig Buch Verlag, Heidelberg 1991, ISBN 3-7785-2104-7 , p. 29
- ^ Dean Rood: The troubleshooting and maintenance guide for gas chromatographers. Viley-VCH Verlag, Weinheim 2007, ISBN 978-3-527-31373-0 , pp. 164–170 ( limited preview in the Google book search).
- ↑ Clarke C. Minter: Thermal conductivity of binary mixtures of gases. I. Hydrogen-helium mixtures . In: The Journal of Physical Chemistry . tape 72 , no. 6 , p. 1924–1926 , doi : 10.1021 / j100852a011 ( acs.org [PDF; accessed March 15, 2018]).