Katharometer detector

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

Figure I. Chromatogram of reaction mixture after separating the solid phase. Conditions column, 2 meters 2% silicone oil on Chromosorb W, 40-60 mesh hydrogen feed rate, 30 mL/min. temperature, 180°C. detector, katharometer...

This type of analysis requires several chromatographic columns and detectors. Hydrocarbons are measured with the aid of a flame ionization detector FID, while the other gases are analyzed using a katharometer. A large number of combinations of columns is possible considering the commutations between columns and, potentially, backflushing of the carrier gas. As an example, the hydrocarbons can be separated by a column packed with silicone or alumina while O2, N2 and CO will require a molecular sieve column. H2S is a special case because this gas is fixed irreversibly on a number of chromatographic supports. Its separation can be achieved on certain kinds of supports such as Porapak which are styrene-divinylbenzene copolymers. This type of phase is also used to analyze CO2 and water. 

Most sensor volumes, whether in LC (e.g., a UV absorption cell) or in GC (e.g., a katharometer cell), are cylindrical in shape, are relatively short in length and have a small length-to-diameter ratio. The small length-to-diameter ratio is in conflict with the premises adopted in the development of the Golay equation for dispersion in an open tube and, consequently, its conclusions are not pertinent to detector sensors. Atwood and Golay [12] extended the theory of dispersion in open tubes to tubes of small length-to-diameter ratio. The theory developed is not pertinent here as it will be seen that, with correctly designed cells, that dispersion from viscous sources can be... 

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. 

Figure 3.14 A katharometer. A constant voltage is applied to the detector filaments and the resulting current produces a heating effect. In the reference column (carrier gas only), because the heat losses from the filament are constant, its resistance will also be constant. The heat loss from the test filament will, however, vary with the gas composition and the resulting changes in resistance can be monitored.

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... 

The instrument used to obtain these chromatograms was fitted with a katharometer detector and a 6-foot 34-inch i.d. column. When a flame ionization detector was used, about 25 additional components, eluted after the peaks shown in Figures 1 and 2, were detected. 

With detectors, it is also necessary to pay attention to their protection against corrosion and various deposites which can affect quantitative results. The katharometer is usually equipped with resistant filaments, usually nickel- or gold-plated. Silyl derivatives decompose in the flame of the FID into silicon oxide, which deposits on the electrodes and reduces the response of the detector significantly. Hence, when analysing silyl derivatives, the electrodes must be cleaned more often than usual. 

The noise levels of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g., the katharometer) are sometimes measured under static conditions (i.e., no flow of carrier gas). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held respon-... 

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. 

The sensitivity of the katharometer is only about 10 g/mL (probably the least sensitive of all GC detectors) and has a linear dynamic range of about 500 (the response index lying between 0.98 and 1.02). It is, however, a general detector and will sense all permanent... 

Of the many detectors that are discussed in the literature, we shall consider three which are the most important for gas-phase kinetic studies. These are the katharometer, the hydrogen flame ionisation detector (fid), and the gas density balance. 

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... 

Linear and cyclic polydimethylsiloxanes were determined on a column of silicone elastomer (Lukopren M) on Rysorb BLK at various temperatures according to the components to be determined. The relative error of measurement of a single component was less than + 7.5%424,425. The determination of methylphenylpolysiloxanes of boiling point up to 500 °C has been reported426 on silanised crushed firebrick containing various amounts of polymethylsiloxane liquid. A katharometer detector was used. With a mixture of ten methylphenylpolysiloxanes (b.p. 100-500 °C) satisfactory separation without decomposition was obtained. 

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