Thermal conductivity detector response characteristics

Classical detectors are important for the consideration of GC-MS coupling if an additional specific means of detection is to be introduced parallel to MS. The classical detectors can be grouped by the response characteristics into the concentration dependant detectors (analyte concentration in the carrier gas) such as the thermal conductivity detector (TCD), and the mass-flow-dependent detectors such as FID, NPD as is the MS (Hill, 1992). A change of make-up gases do not affect the signal in mass flow dependant detectors. 

Figure 15.6 is a plot of the thermal conductivity of mixtures of helium and nitrogen obtained on an apparatus similar to that described in the next section. Characteristically, the thermal conductivity of most mixtures does not vary linearly with concentration. The slope of the curve at any point determines the value of A/c and, therefore, the detector response. Figure 15.6 also illustrates that the greater the difference between thermal conductivities of the adsorbate and carrier gas, the higher will be the slope and therefore the detector response. 

The thermal conductivity (TC) detector is the best known of a class of detectors known as bulk property detectors. These are sensitive to some overall property of the effluent. Often the measured property is a physical parameter, rather than a chemical one. The distinguishing characteristic of a bulk property detector is that there is a significant response in the absence of sample. A bulk property detector, when sample comes through, measures a change of property from the baseline value, A to A+... 

The identification of the chemical forms of an element has become an important and challenging research area in environmental and biomedical studies. Two complementary techniques are necessary for trace element speciation. One provides an efficient and reliable separation procedure, and the other provides adequate detection and quantitation [4]. In its various analytical manifestations, chromatography is a powerful tool for the separation of a vast variety of chemical species. Some popular chromatographic detectors, such flame ionization (FID) and thermal conductivity (TCD) detectors are bulk-property detectors, responding to changes produced by eluates in a characteristic mobile-phase physical property [5]. These detectors are effectively universal, but they provide little specific information about the nature of the separated chemical species. Atomic spectroscopy offers the possibility of selectively detecting a wide rang of metals and nonmetals. The use of detectors responsive only to selected elements in a multicomponent mixture drastically reduces the constraints placed on the separation step, as only those components in the mixture which contain the element of interest will be detected... 

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