TCD cell

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. 

Thermal Conductivity Detector (TCD). The TCD cell is a metal block in which cavities have been drilled to accommodate the transducers, which can be either thermistors or resistance wires (so-called hot wires, Figure 8.8). Thermistors are most sensitive at low temperatures and find limited... 

FIGURE 27-9 Schematic of (a) a TCD cell, and (b) an arrangement of two sample detector cells and two reference detector cells. (From J. Hinshaw. LC-GC. 1990, 8. 298. With permission.)... 

As noted earlier in this chapter, a small cell volume is desirable for faithful reproduction of peak shapes and greater sensitivity. Typically, TCD cells have volumes around 140 piL which are very good for packed columns or wide bore capillaries. Their use with narrow capillaries has not become routine, but cells are available with volumes down to 20 and several studies have shown that good chromatograms can be obtained in some cases [18,19]. Make-up gas is usually required when capillary columns are used with TCDs. An extremely small cell has been made by etching a nL volume on a silica chip for a micro-GC instrument. Another manufacturer uses a small volume (5 pL) single-cell TCD in its operation the two gas streams (sample and reference) are passed alternately through the cell at a frequency of 10 times per second [5]. 

FIGURE 6.8 Geometry of a typical TCD cell. The supports for the axial filament are insulated from the stainless-steel body. Dimensions are in centimeters. (Reprinted with permission from previous edition. Copju ight 1995 John WUey and Sons, Inc.)... 

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. 

As each component exits the chromatographic column, it is channeled into an infrared (IR) gas cell and the component s IR spectrum obtained. A thermal conductivity detector (TCD) (see Chapter 13) can be used to determine when a component is emerging from the column. The TCD detector does not destroy the sample and none of the common gases used in GC have IR spectra, and thus do not interfere with the spectrum of eluting components. Half-peak height is a common time to obtain the spectrum of that component and the setup for detection and obtaining the spectrum can thus be automated [6,13],... 

TCD = inhibitory activity on TNF-a-induced cell death in mouse hepatocytes [192] ... 

Two important parts of the electronics for a TCD are not showr in Figure 5.8. In practice the four cells of a detector rarely match. A control is usually provided so that the output voltage can be nulled before a chromatogram is run. This is usually done with variable resistors connected in the bridge. Often there are two controls on the front panel "Fine" and "Coarse" Balance. Another necessary control, at least when an integrator is not used, is an attenuator switch. This reduces the response on the recorder by various amounts so that the larger peaks can be kept... 

This is why TC detectors are made of cells mounted closely together, embedded in metal, with the assembly meticulously insulated. In spite of this effort, the cells are not exactly matched in heat transfer to the metal they are embedded in. This means that it is important to control the temperature of the detector body accurately. In most modern instruments, the TCD tempera-control circuit produces much better thermal stability (though not necessarily accuracy) than even the chromatographic oven control. 

One of the best features of thermal conductivity detectors with helium carrier gas is the ease of quantitative analysis. It has been shown experimentally that relative response factors, where sample weight is used, are independent of (a) type of detector (filament or thermistor), (b) cell and sensor temperature, (c) concentration of sample, (d) helium flowrate, and (e) detector current. In addition, relative response factors change only slightly within a series of homologous compounds. The first systematic study of TCD responses in helium was done by Rosie and Grob and are summarized in reference (6). 

Solution Let zone 1 represent one tube and zone 2 represent the effective plane 2, that is, the unit cell for the tube bank. Thus Aj = tcD and A2 = C are the corresponding zone areas, respectively (per unit vertical dimension). This notation is consistent with Example 3. Also put i = 0.8 with 2 = 1.0 and define R = C/D = 12/5 = 2.4. The gray plane effective emissivity is then calculated as the total view factor for the effective plane to tubes, that is, J2-1s % For R = 2.4, Fig. 5-15, curve 5, yields the refractory augmented view factor F2ji = 0.81. Then is... 

The performance of the sensors tested here can be snmmarized as follows Sensor A (TCD) was not sensitive enongh to detect H2 anywhere, even up to 7000 pL/L in the calibration cell. It was however sensitive to condensing water vapor, reading the eqnivalent of 3000 pL/L H2 at 25 °C and 100 % relative humidity. 

In this study, we used the modified Wicke-Kallenbach cell which is tubular membrane cell type. Permeation measurements were performed in the 293K-373K, Oatm-Satm range for H2, N2, CO2 and CH4. Feed gas and retentate gas were controlled by MFC(Mass Flow Controller, Tylan Co.) and BPR(Back Pressure Regulator). Permeate gas flux was measured by soap bubble flow meter, MFM (Mass Flow Meter, Teledyne Co.) and wet gas meter. Especially, MFM was used to measure kinetics of membrane permeation. Separated and retentate gas composition was analyzed by on-line GC(HP 5890 II, TCD type). Helium was used as carrier gas and sweeping gas. Temperature was detected by RTD(Hanyoung. Co.) at inlet, inner cell and skin of cell. Pressures were detected by pressure transducers(Deco Co.) at inlet and permeate part. 

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. 

Detectors can also be grouped into concentration-dependent detectors and mass-flow-dependent detectors. Detectors whose responses are related to the concentration of solute in the detector cell, and do not destroy the sample, are called concentration-dependent detectors, whereas detectors whose response is related to the rate at which solute molecules enter the detector are called mass-flow-dependent detectors. Typical concentration-dependent detectors are TCD and GC-FTIR. Important mass-flow-dependent detectors are the FID, thermoionic detector for N and P (N-, P-FID), flame photometric detector for S and P (FPD), ECD, and selected ion monitoring MS detector. 

Figure 1. A comparison of the band shape obtained with a conventional "micro" TCD and a systematically designed small volume cell. Note the improvement both in width and the reduction in "tailing". (Methane peak, digitized data) Reproduced, with permission, from Ref. 1. Copyright 1977, Journal of Chromatographic Science,...

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