The Thermal Conductivity Detector TCD

Universal (except for H2 and He) non-destructive concentration detector no auxiliary (aux.) gas works better with a parallel column insensitive limited dynamic range. 

The TCD was the first widely commercially available GC detector, in the era when all the columns were packed, and samples were neat (i.e., not diluted solutions) mixtures to 

Note the application of the TCD for the detection of fixed gases in the chromatogram of Fig. 12.4. These are generally not seen by other detectors (except hyphenated GC-MS). Note the low sensitivity signal for H2, whose thermal conductivity is the only one to closely match that of the He carrier gas used. If only H2 were being measured, it would be better to use N2 as the carrier. This would yield a peak signal in the negative direction. In Fig. 12.5, note that the detector is described as a p,-TCD. This must employ miniaturized cells to be compatible with the low carrier flows of the open-tubular 

PLOT column and the need for small detector cell volumes to avoid extra-column band broadening. 

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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. 

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. 

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 (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). 

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. 

Many GC detectors exist, but not all are suitable for phytochemicals. The thermal conductivity detector (TCD) is considered a universal detector and is appropriate for most analytes as long as the thermal conductivity of the carrier gas is different from that of the analytes. During the early development phase of GC, TCD was an easy choice because thermal conductivity measuring devices were already in use (Colon and Baird, 2004). Ionization detection arrived with its improved trace determinations and replaced TCD in many applications. While TCD is still used for some food applications (Allegro et ak, 1997 Sun et ak, 2007) and in the past was used for phenolic acids (Blakely, 1966), currently it is not generally used for phytochemicals. Rather, the flame ionization detector (FID) is better-suited due to its selectivity for organic compounds and superior measuring ability for trace measurements. 

Other detectors in use include the thermal conductivity detector (TCD) and the phosphorus-nitrogen detector (PND) which is much used in toxicology because of its selectivity for nitrogen-containing compounds. It is a feature of gas-liquid chromatography that spectrophotometric detectors can be coupled readily to the outflow for detection this includes IR, NMR and, particularly, mass spectrometers, in combined GC-MS analysers. Spectroscopic analysis allows structural information to be... 

The thermal conductivity detector (TCD) is based on changes in the thermal conductivity of the gas stream brought about by the presence of separated sample molecules. The detector elements are two electrically heated platinum wires, one in a chamber through which only the carrier gas flows (the reference detector cell), and the other in a chamber that takes the gas flow from the column (the sample detector cell). In the presence of a constant gas flow, the temperature of the wires (and therefore their electrical resistance) is dependent on the thermal conductivity of the gas. Analytes in the gas stream are detected by temperature-dependent changes in resistance based on the thermal conductivity of each separated molecule the size of the signal is directly related to concentration of the analyte. 

In addition to the thermal conductivity detector (TCD), flame photometric (FPD) and atomic absorption spectrometric (AAS) detectors, which offer high sensitivity and the advantage of selectivity, are also suitable for use with inorganic substances (Table 1.2). By-products from chemical reactions of the analytical process will not be detected. 

In order to obtain a complete separation of the different compounds, a two level heating and cooling program has been established in the oven zone, coupled with column isolation. Runtime of the chromatographic method is 23.4 minutes. The flame ionisation detector (FID) is used to analyse hydrocarbons (CH4 and C3H ). The thermal conductivity detector (TCD) is used to analyse H2. CO. CO2, N2 and high concentration of hydrocarbons (CH4 and C3Hg). The polarity and sensitivity of the thermal conductivity detector have been set according to (he conductivity response of the... 

Detectors may be classified on the basis of selectivity. A universal detector responds to all compounds in the mobile phase except carrier gas. A selective detector responds only to a related group of substances, and a specific detector responds to a single chemical compound. Most common GC detectors fall into the selective designation. Examples include flame ionization detector (FID), ECD, flame photometric detector (FPD), and thermoionic ionization detector. The common GC detector that has a truly universal response is the thermal conductivity detector (TCD). Mass spectrometer is another commercial detector with either universal or quasi-universal response capabilities. 

The katharometer detector [sometimes spelled cath-erometer and often referred to as the thermal conductivity detector (TCD) or the hot-wire detector (HWD)] is the oldest commercially available gas chromatographic (GC) detector still in common use. Compared with other GC detectors, it is a relatively insensitive detector and has survived largely as a result of its almost universal response. In particular, it is sensitive to the permanent gases to which few other detectors have a significant response. Despite its relatively low sensitivity, the frequent need for permanent gas analysis in many industries probably accounts for it still being the fourth most commonly used GC detector. It is simple in design and requires minimal electronic support and, as a consequence, is also relatively inexpensive compared with other detectors. 

A method to circumvent this dilemma was sought by Ferng (11). An extensive and detailed study of static sorption methodology was first conducted to provide a basis of reproducible data for starches of different macromolecular structure. This was followed by studies of sorption isotherms by IGC with different GC conditions including zero loads with empty and supposedly inert support material (diatomaceous earth). The data showed that the response of the thermal conductivity detector (TCD) to controlled chromatographic conditions of temperature, flow rate, and partial water vapor... 

The thermal conductivity detector (TCD), which was one of the earliest detectors for gas chromatography, still finds wide application. This device consists of an electrically heated source whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas. The heated element may be a fine platinum, gold, or tungsten wire (Figure 31 -9a) or, alternatively, a small thermistor. The electrical resistance of this element depends on the thermal conductivity of the gas. Twin detectors are ordinarily used, one located ahead of the sample injection chamber and the other immediately beyond the column alternatively, the gas stream can be split. The detectors are incorporated into two arms of a simple bridge circuit (see Figure 31 -9) such that the thennal conductivity of the carrier gas is... 

The use of capillary columns is a relatively recent advance in chromatography when compared with the thermal conductivity detector (TCD). The TCD is well established and is among the most commonly used detectors in gas chromatography. Some of the advantages of the TCD Include the simplicity, stability, and universal nature of the detector. 

Thermal Conductivity Detector, The thermal conductivity detector (TCD) is a simple universal detector (see Fig. 22.8) that produces a large signal requiring no amplification. The detector cell has either two or four filaments arranged in a Wheatstone bridge circuit (Fig. 22.9). In the four-filament model, two filaments in... 

The most common are the flame ionization detector (FID) and the thermal conductivity detector (TCD). Both detectors are sensitive to a wide range of components, and both work over a wide range of concentrations. While TCDs are essentially universal and can be used to detect any component other than the carrier gas, FlDs are sensitive primarily to hydrocarbons and are more sensitive to them than TCDs. 

The adsorption of 2-CEES on nanocrystalhne NaZSM-5 and silicalite was monitored using a flow reactor apparatus and the thermal conductivity detector (TCD) of a gas chromatograph. During the initial adsorption period, the total amount of 2-CEES adsorbed on silicalite (ZSM-5) at room temperature was measured and quantified as listed in Table 1. 3.7 and 4.5 mmol/g of 2-CEES were adsorbed on silicalite (25 run) and NaZSM-5 (15nm), respectively. The desorption during a room temperature helium purge was measured and was foimd to be 1.3 and 2.6 mmol/g for silicalite (23 run) and NaZSM-5 (15 nm), respectively. The amoimt of 2-CEES desorbed during the room temperature... 

Obviously, the chromatographic principles are the same in process and laboratory GCs and they are built up in a very similar way. Standard detectors are in each case the thermal conductivity detector (TCD), which is a universal detector for all components, and the flame ionization detector (HD), which is a specific detector for hydrocarbons. To detect sulfur gases selective detectors like an electrochemical detector, chemiluminescence detector and, most important, flame photometric detector (FPD) are used. Gaseous fuels hke natural gas, synthetic gases, and blends are complex mixtures that cannot completely be separated in a single column. Two or more different columns must be combined. To monitor the fuel quality a quasi-continuous analysis is necessary this means that very short cycle times must be realized. To do so, high-boiling components are removed... 

Among the variety of detectors, only the thermal conductivity detector (TCD) and the flame ionization detector (FID) are in broad use in PGC. The flame photometric detector, typically used for measuring trace sulfur containing species, and the photoionization detectoi predominately used in environmental monitoring, also see some usage. The variety of detector types available for PGC tends to be limited because of the requirements for robustness and sensitivity to a variety of stream components. In addition, many PGC detectors are not optimized for use with capillary columns. 

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