The Thermal Conductivity Detector

Bulk property detectors continuously monitor some physical property of the column eluent. The presence of a solute modifies that property and provides an output that can be recorded. Bulk property detectors have limited sensitivity due to the fact that changes in ambient conditions, temperature, pressure, etc. provide signals commensurate to that from the presence of a solute. 

Kourilova, K. Slais and M. Krejci, Inst. Anal. Chem. 

---

Alternatively, gas chromatography may be used Fig. XVII-5 shows a schematic readout of the thermal conductivity detector, the areas under the peaks giving the amount adsorbed or desorbed. 

By far the most used detector is the thermal conductivity detector (TCD). Detectors like the TCD are called bulk-property detectors, in that the response is to a property of the overall material flowing through the detector, in this case the thermal conductivity of the stream, which includes the carrier gas (mobile phase) and any material that may be traveling with it. The principle behind a TCD is that a hot body loses heat at a rate that depends on the... 

For measuring the inert species, some of which are present in the majority of gases, the thermal-conductivity detector (TCD) is often the detector of choice for gas analyses. Since the TCD is a concentration detector and its sensitivity is lower than that of mass-flow detectors such as the flame-ionization detector (FID), relatively high concentrations of compounds in the carrier gas are needed. This means that packed columns, with their high loadability, are still quite popular for such analyses. 

Thermal conductivity detector. The most important of the bulk physical property detectors is the thermal conductivity detector (TCD) which is a universal, non-destructive, concentration-sensitive detector. The TCD was one of the earliest routine detectors and thermal conductivity cells or katharometers are still widely used in gas chromatography. These detectors employ a heated metal filament or a thermistor (a semiconductor of fused metal oxides) to sense changes in the thermal conductivity of the carrier gas stream. Helium and hydrogen are the best carrier gases to use in conjunction with this type of detector since their thermal conductivities are much higher than any other gases on safety grounds helium is preferred because of its inertness. 

Thermal Conductivity Detector In the thermal conductivity detector (TCD), the temperature of a hot filament changes when the analyte dilutes the carrier gas. With a constant flow of helium carrier gas, the filament temperature will remain constant, but as compounds with different thermal conductivities elute, the different gas compositions cause heat to be conducted away from the filament at different rates, which in turn causes a change in the filament temperature and electrical resistance. The TCD is truly a universal detector and can detect water, air, hydrogen, carbon monoxide, nitrogen, sulfur dioxide, and many other compounds. For most organic molecules, the sensitivity of the TCD detector is low compared to that of the FID, but for the compounds for which the FID produces little or no signal, the TCD detector is a good alternative. 

There are several types of detectors, devices that can tell when a sample is passing by them. They detect the presence of a sample and convert it to an electrical signal that s turned into a GC peak (Fig. 109) on the chart recorder. The most common type is the thermal conductivity detector. Sometimes called hot-wire detectors, these devices are very similar to the filaments you... 

Detectors range from the universal, but less sensitive, to the very sensitive but limited to a particular class of compounds. The thermal conductivity detector (TCD) is the least sensitive but responds to all classes of compounds. Another common detector is the flame ionization detector (FID), which is very sensitive but can only detect organic compounds. Another common and very sensitive detector is called electron capture. This detector is particularly sensitive to halogenated compounds, which can be particularly important when analyzing pollutants such as dichlorodiphenyltrichloroethane (DDT) and polychlorobiphenyl (PCB) compounds. Chapter 13 provides more specific information about chromatographic methods applied to soil analysis. 

The thermal conductivity detector (TCD) is a classical detector for both packed and capillary columns. A schematic representation of a modern... 

In the chromatographic column the combustion gases are separated so that they can be detected in sequence by the thermal conductivity detector (TCD). The TCD output signal is proportional to the concentration of the elements. 

In addition to the analytical columns (columns used mainly for analytical work), so-called preparative columns may also be encountered. Preparative columns are used when the purpose of the experiment is to prepare a pure sample of a particular substance (from a mixture containing the substance) by GC for use in other laboratory work. The procedure for this involves the individual condensation of the mixture components of interest in a cold trap as they pass from the detector and as their peak is being traced on the recorder. While analytical columns can be suitable for this, the amount of pure substance generated is typically very small, since what is being collected is only a fraction of the extremely small volume injected. Thus, columns with very large diameters (on the order of inches) and capable of very large injection volumes (on the order of milliliters) are manufactured for the preparative work. Also, the detector used must not destroy the sample, like the flame ionization detector (Section 12.6) does, for example. Thus, the thermal conductivity detector (Section 12.6) is used most often with preparative gas chromatography. 

The thermal conductivity detector (TCD) operates on the principle that gases eluting from the column have thermal conductivities different from that of the carrier gas, which is usually helium. Present in the flow channel at the end of the column is a hot filament, hot because it has an electrical current passing through it. This filament is cooled to an equilibrium temperature by the flowing helium, but it is cooled differently by the mixture components as they elute, since their thermal conductivities are different from... 

The thermal conductivity detector is universal (detects everything) and nondestructive (can be used with preparative GC), but it does not detect very small concentrations, compared to other detectors. 

The problems with this approach are 1) without comparing the peaks to a standard or a set of standards, it is not known whether the result is a weight, volume, or mole percent, and 2) the instrument detector does not respond to all components equally. For example, not all components will have the same thermal conductivity, and thus the thermal conductivity detector will not give equal sized peaks for equal concentrations of any two components. Thus, the sum of all four peaks would be a meaningless quantity, and the size of peak B by itself would not represent the correct fraction of the total. 

The thermal conductivity detector, or katharometer, was the first ever detector employed for GLC and is still being used today be virtue of its versatility, stability, simplicity and above all the low-cost. 

The thermal conductivity detector (TCD) is a universal detector that is nondestructive, which is a major advantage for preparative work (Dybowski and Kaiser, 2002). However, it is not sensitive enough for many of the analyses discussed later. This detector operates on the principle that a hot body loses heat at a rate dependent on the composition of the material surrounding it (Burtis et ah, 1987). In a TCD, two filaments are heated, one in carrier gas, and the other in the column effluent. The voltages required to maintain the filament at a constant temperature are measured and compared. When compounds elute from the column the voltage of the sample filament is different from that of the filament in carrier gas and is recorded as a peak (Burtis et al., 1987). 

Samples are injected into the vaporizer by a metering pump or manually with septum injection the manual injection procedure is intended for method development. The sample gas mixture then passes through the chromatographic column where the sample compounds separate. Fractions pass through the thermal conductivity detector and then to a condenser collection manifold where up to five fractions can be collected. Complete control of the system is achieved via a mini-computer. 

To understand the effect of the carrier gas on the response of the thermal conductivity detector, consider the steady-state condition that... 

The thermal conductivity detector used in the continuous flow method can sense signals corresponding to less than 0.001 cm of adsorption with 1 % accuracy, causing it to be considerably more sensitive to small amounts of adsorption than the volumetric or gravimetric methods. 

Response to organic compounds is proportional to solute mass over seven orders of magnitude. The detection limit is 100 times smaller than that of the thermal conductivity detector (Table 24-5) and is reduced by 50% when N2 carrier gas is used instead of He. For open tubular columns, N2 makeup gas is added to the H2 or He eluate before it enters the detector. The flame ionization detector is sensitive enough for narrow-bore columns. It responds to most hydrocarbons and is insensitive to nonhydrocarbons such as H2, He, N2, 02, CO, C02, H2Q, NH NO, H2S, and SiF4. 

The most general purpose detector for open tubular chromatography is a mass spectrometer. Flame ionization is probably the most popular detector, but it mainly responds to hydrocarbons and Table 24-5 shows that it is not as sensitive as electron capture, nitrogen-phosphorus, or chemiluminescence detectors. The flame ionization detector requires the sample to contain SlO ppm of each analyte for split injection. The thermal conductivity detector responds to all classes of compounds, but it is not sensitive enough for high-resolution, narrow-bore, open tubular columns. 

If qualitative information is required to identify eluates, then mass spectral or infrared detectors are good choices. The infrared detector, like the thermal conductivity detector, is not sensitive enough for high-resolution, narrow-bore, open tubular columns. 

Direction of the gas chromatographic effluent into a vessel containing activated carbon attached to an automatic recording electromicrobalance is the basis for the device known as the Brunei mass detector (47). This is an absolute analytical method and requires no calibration, and in fact, can be used to calibrate other detectors which have unpredictable responses. The sensitivity of the detector is in the same range as the thermal conductivity detector. 

Cg portion and then switching to the TCD for carbon dioxide, ethane, oxygen (if present), nitrogen, and methane, a complete analysis is shown in Figure 6.30. The signal was automatically switched to the thermal conductivity detector at 8 min. 

Virtually every conceivable means of detecting gases and vapors has been exploited in designing GC detectors, and over one hundred have been described. The two most popular ones, the thermal conductivity detector (TCD) and the flame ionization detector (FID), will be described in some detail. They are classified (according to the criteria in Chapter 7) and compared in Table 6. 

Phillips and Timms [599] described a less general method. They converted germanium and silicon in alloys into hydrides and further into chlorides by contact with gold trichloride. They performed GC on a column packed with 13% of silicone 702 on Celite with the use of a gas-density balance for detection. Juvet and Fischer [600] developed a special reactor coupled directly to the chromatographic column, in which they fluorinated metals in alloys, carbides, oxides, sulphides and salts. In these samples, they determined quantitatively uranium, sulphur, selenium, technetium, tungsten, molybdenum, rhenium, silicon, boron, osmium, vanadium, iridium and platinum as fluorides. They performed the analysis on a PTFE column packed with 15% of Kel-F oil No. 10 on Chromosorb T. Prior to analysis the column was conditioned with fluorine and chlorine trifluoride in order to remove moisture and reactive organic compounds. The thermal conductivity detector was equipped with nickel-coated filaments resistant to corrosion with metal fluorides. Fig. 5.34 illustrates the analysis of tungsten, rhenium and osmium fluorides by this method. 

---