Thermal conductivity detector katharometer

TCD Thermal conductivity detector (katharometer) Differences in thermal conductance of analyte vs. carrier gas... 

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. 

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 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. 

Gas chromatography (g.c.) has proven to be particularly useful for the analysis of phosgene in a wide range of concentrations. A katharometer (thermal conductivity) detector is most frequently employed for routine use (down to about 200 p.p.m. [1339]), but very low concentrations in air, in the p.p.b. [563,598,1039,1663,1887] or even sub-p.p.b. range [448,1886] have been analysed using an electron capture detector [98a,1253,2025]. For such... 

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. 

With a few exceptions, most of the detectors used in GC were invented specifically for this technique. The major exceptions are the thermal conductivity detector (TCD, or katharometer) that was preexisting as a gas analyzer when GC began, and the mass spectrometer (or mass selective detector, MSD) that was adapted to accept the large volumes and the fast scan rates needed for GC peaks. Most recently, other spectroscopic techniques like IR and atomic plasma emission have been coupled to the effluent from gas chromatographs, serving as GC detectors. 

Thermal conductivity detector (TCD, katharometer) 1 X 10" g propane (in helium carrier gas) 1 X 10 Universal response, concentration detector Ultimate sensitivity depends on analyte thermal conductivity difference with carrier gas Since thermal conductivity is temperature dependent, response depends on cell temperature Wire selection depends on chemical nature of analyte Helium is recommended as carrier and make-up gas. When analyzing mixtures containing hydrogen, one can use a mixture of 8.596 (mass/mass) hydrogen in helium... 

Several forms of gas sensor based upon thermal conductivity are available. The most common type of detector (the katharometer) consists of a number of hot-wire sensors arranged in a Wheatstone Bridge circuit (Fig. 6.54)(8). A small current i is supplied to heat each arm of the bridge. The heat transfer coefficient h for... 

Two types of detector are in common use the katharometer, which measures changes in thermal conductivity, and the flame-ionization detector, which turns the volatile components into ions and registers the change in electrical conductivity. 

The Katharometer (Thermal Conductivity and Hot Wire) Detector Data Acquisition and Processing Synopsis References Chapter 5... 

The detector is a transducer which may be either a katharometer (thermal conductivity transducer) or a flame ionization detector (electric conductance transducer). The difference between the values of a physical constant for any sample component and gas carrier is transformed, by the detector, into an electric signal proportional to the concentration of that component in the gas phase the components are thus sensed . 

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