Thermal conductivity detectors applications

As the vapor leaves the tube, the compounds in the sample are detected by a device such as a thermal conductivity detector. This instrument continuously measures the thermal conductivity (the ability to conduct heat) of the carrier gas, which changes when a solute is present. The detection techniques are very sensitive, allowing tiny amounts of solutes to be detected. Many environmental monitoring and forensic applications have been developed. 

The following table provides guidance in the selection of hot wires for use in thermal conductivity detectors (TCDs).13 This information is applicable to the operation of packed and open tubular columns. Some of the entries in this table deal with analytes, and others deal with solutions that might be used to clean the TCD cell. 

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. 

The flame ionization detector (FID) is mostly used as detector in steroid analyses. For very low concentrations of steroids, the application of electron capture detection (BCD) is needed. Thermal conductivity detectors (TCD)... 

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... 

Using a 10-p sample and a thermal conductivity detector, phenyl silicon trifluoride can be accurately determined in phenyl silicon dichloride down to 0.01%. The same procedure is applicable to the separation of high boiling chlorosilanes, which are difficult to separate otherwise, and also achieved the separation of various cyanopropylmethyl-chlorosilanes. 

Thermal conductivity detectors have been discussed in detail by Ingraham (107), who also described their application to thermodynamic and kinetic measurements. In this same book. Lodding (4) describes the gas density detector as well as several ionization detectors, such as the argon ionization detector, the electron capture detector, and others. Flame ionization detectors have been described in detail by Brody and Chaney (108) and Johnson (109). The latter also discusses other types of detectors. Malone and McFad-den (110) described many different types of special identification detectors, such as those listed in Table 8.3. Numerous texts on gas chromatography describe a wide variety of detectors, many of them useful in EGD and EGA. 

No detectors for LC are as universally applicable as the name ionization and thermal conductivity detectors for gas chromatography described in Section 27B-4. GC detectors were specifically developed to mca.surc small concentrations of analytes in flowing gas streams. On the other hand, L.C detectors have often been traditional analytical instrumenl.s adapted with flow cells to measure low concentrations of solutes in liquid streams. A major challenge in the development of LC has been in adapting and improving such devices." ... 

The aldehyde functional groups can be tested by classical wet methods. Individual aldehydes can be analyzed by various instrumental techniques such as GC, HPLC, GC/MS, colorimetry, polarography, and FTIR. Of these, GC, GC/MS, and HPLC are referred to here because of their versatility and wide application. Although an FID is commonly employed in GC, a thermal conductivity detector can also be suitable, especially for lower aliphatic aldehydes. The advantage of FID is that aqueous samples can be injected straight into the column. 

A simple, universal detector, the katharometer, or thermal conductivity detector (TCD), applicable to chromatography with analytes eluting in gas phase, was available to provide an electrical signal which could be displayed as a chromatogram. 

The flame ionization detector (FID) is the detector most often used in steroid analyses. For very low concentrations of steroids, the application of ECD is needed. Thermal conductivity detectors (TCDs) cannot be used in the analysis of steroids because of their very low sensitivity. For steroidal alkaloids, a nitrogen-specific detector (NPD) has also been used. By the use of dual detector systems (e.g., FID and NPD), closely related nitrogen-containing and non-nitrogen-containing steroids can be easily differentiated. The application of MS as detector was already discussed in a previous entry in this encyclopedia." By using a GC/MS system, the identity of the peak(s) can be determined in an undisputed manner. ... 

With new technology of Micro Injectors (Ml) and Thermal Conductivity Detectors (TCD) with cell volumes of 1-5 nanoliters ( ), the use of very fast ALOT columns came within the reach of practical application. 

This most useful technique has been applied to the liquid PEG 200, 300, and 400. Fletcher and Persinger [131] studied this application using PEG 200, 300, and 400 and reported the determination of response factors relative to ethylene glycol for conversion of peak areas to weight percent when a thermal conductivity detector was used. 

Another important instrument required in modem LC is a sensitive or selective detector for continuous monitoring of the column effluent. In GC. the differences in physical properties of the mobile phase (carrier gas) and the sample are great enough for universal detectors with good sensitivity to be used (e.g., flame ionization detector, thermal conductivity detector, - Gas Chromatography). The problem in LC is that the physical properties of the mobile phase and the sample are often very similar, which makes the use of a universal detector impossible. Nevertheless, presently available LC detectors are very sensitive, are generally selective, and have a relatively wide range of applications (see Table 1). 

Petroleum Industry. The petroleum companies were among the first to make widespread use of GC. The technique was successfully used to separate and determine the many components in petroleum products. One of the earlier publications concerning the response of thermal conductivity detectors to concentration resulted from research in the petroleum field (15). The application of GC to the petroleum field is discussed in Chapter 13. 

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