Thermal conductivity detector sensitivity

For measuring the inert species, some of which are present in the majority of gases, the thermal-conductivity detector (TCD) is often the detector of choice for gas analyses. Since the TCD is a concentration detector and its sensitivity is lower than that of mass-flow detectors such as the flame-ionization detector (FID), relatively high concentrations of compounds in the carrier gas are needed. This means that packed columns, with their high loadability, are still quite popular for such analyses. 

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. 

Thermal Conductivity Detector In the thermal conductivity detector (TCD), the temperature of a hot filament changes when the analyte dilutes the carrier gas. With a constant flow of helium carrier gas, the filament temperature will remain constant, but as compounds with different thermal conductivities elute, the different gas compositions cause heat to be conducted away from the filament at different rates, which in turn causes a change in the filament temperature and electrical resistance. The TCD is truly a universal detector and can detect water, air, hydrogen, carbon monoxide, nitrogen, sulfur dioxide, and many other compounds. For most organic molecules, the sensitivity of the TCD detector is low compared to that of the FID, but for the compounds for which the FID produces little or no signal, the TCD detector is a good alternative. 

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. 

Detectors range from the universal, but less sensitive, to the very sensitive but limited to a particular class of compounds. The thermal conductivity detector (TCD) is the least sensitive but responds to all classes of compounds. Another common detector is the flame ionization detector (FID), which is very sensitive but can only detect organic compounds. Another common and very sensitive detector is called electron capture. This detector is particularly sensitive to halogenated compounds, which can be particularly important when analyzing pollutants such as dichlorodiphenyltrichloroethane (DDT) and polychlorobiphenyl (PCB) compounds. Chapter 13 provides more specific information about chromatographic methods applied to soil analysis. 

The thermal conductivity detector (TCD) is a universal detector that is nondestructive, which is a major advantage for preparative work (Dybowski and Kaiser, 2002). However, it is not sensitive enough for many of the analyses discussed later. This detector operates on the principle that a hot body loses heat at a rate dependent on the composition of the material surrounding it (Burtis et ah, 1987). In a TCD, two filaments are heated, one in carrier gas, and the other in the column effluent. The voltages required to maintain the filament at a constant temperature are measured and compared. When compounds elute from the column the voltage of the sample filament is different from that of the filament in carrier gas and is recorded as a peak (Burtis et al., 1987). 

The onset of thermal diffusion depends on the gas concentrations, the sample surface area, the rate at which the sample cools to bath temperature, and the packing efficiency of the powder. In many instances, using a conventional sample cell, surface areas less than 0.1 m can be accurately measured on well-packed samples that exhibit small interparticle void volume. The use of the micro cell (Fig. 15.10b) is predicated on the latter of these observations. Presumably, by decreasing the available volume into which the lighter gas can settle, the effects of thermal diffusion can be minimized. Although small sample quantities are used with a micro cell, thermal conductivity detectors are sufficiently sensitive to give ample signal. 

The thermal conductivity detector used in the continuous flow method can sense signals corresponding to less than 0.001 cm of adsorption with 1 % accuracy, causing it to be considerably more sensitive to small amounts of adsorption than the volumetric or gravimetric methods. 

In the past, thermal conductivity detectors were most common in gas chromatography because they are simple and universal They respond to all analytes. Unfortunately, thermal conductivity is not sensitive enough to detect minute quantities of analyte eluted from open tubular columns smaller than 0.53 mm in diameter. Thermal conductivity detectors are still used for 0.53-mm columns and for packed columns. 

The sensitivity of a thermal conductivity detector (but not that of the flame ionization detector, described next) is inversely proportional to flow rate It is more sensitive at a lower flow rate. Sensitivity also increases with increasing temperature differences between the filament and the surrounding block in Figure 24-17. The block should therefore be maintained at the lowest temperature that allows all solutes to remain gaseous. 

Response to organic compounds is proportional to solute mass over seven orders of magnitude. The detection limit is 100 times smaller than that of the thermal conductivity detector (Table 24-5) and is reduced by 50% when N2 carrier gas is used instead of He. For open tubular columns, N2 makeup gas is added to the H2 or He eluate before it enters the detector. The flame ionization detector is sensitive enough for narrow-bore columns. It responds to most hydrocarbons and is insensitive to nonhydrocarbons such as H2, He, N2, 02, CO, C02, H2Q, NH NO, H2S, and SiF4. 

The most general purpose detector for open tubular chromatography is a mass spectrometer. Flame ionization is probably the most popular detector, but it mainly responds to hydrocarbons and Table 24-5 shows that it is not as sensitive as electron capture, nitrogen-phosphorus, or chemiluminescence detectors. The flame ionization detector requires the sample to contain SlO ppm of each analyte for split injection. The thermal conductivity detector responds to all classes of compounds, but it is not sensitive enough for high-resolution, narrow-bore, open tubular columns. 

If qualitative information is required to identify eluates, then mass spectral or infrared detectors are good choices. The infrared detector, like the thermal conductivity detector, is not sensitive enough for high-resolution, narrow-bore, open tubular columns. 

Sensitivity. This property of the gas chromatographic system largely accounts for its extensive use. The simplest thermal conductivity detector cells can detect 100 ppm or less. Utilizing a flame ionization detector one can detect a few parts per million with an electron capture detector or phosphorous detector parts per billion or picograms of solute can easily be measured. This level of sensitivity is more impressive when one considers that the sample size used is of the order of a microliter or less. 

Thermistors, which are metal oxide beads used as temperature-sensitive resistors, have been used in thermal conductivity detectors since the mid-1950s. They offer several advantages. 

Direction of the gas chromatographic effluent into a vessel containing activated carbon attached to an automatic recording electromicrobalance is the basis for the device known as the Brunei mass detector (47). This is an absolute analytical method and requires no calibration, and in fact, can be used to calibrate other detectors which have unpredictable responses. The sensitivity of the detector is in the same range as the thermal conductivity detector. 

Thermal conductivity detectors used in gas chromatographs do not respond equally to all FAMlis. To correct for varying detector sensitivity, peak area for each FAME should be multiplied by the proper response correction factor (RCF) (Table E6.2). If extra time is available, you may want to calculate your own response correction factors for the fatty acid methyl esters. The factors are experimentally determined on a gas chromatograph by comparing the area under a GC peak due to a known amount of compound to the area under a GC peak represented by a reference compound. 

The most common detectors for GC are the non-selective flame ionisation detector and thermal conductivity detector. For element speciation, selectivity is definitely advantageous, allowing less sample preparation and less demanding separation. Of the conventional GC detectors, the electron capture detector is very sensitive for electrophilic compounds and therefore has some selectivity for polar compounds containing halogens and metal ions. It has been used widely... 

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... 

Procedure (See Chromatography, Appendix IIA.) Use a suitable gas chromatograph equipped with a thermal conductivity detector (F and M Model 810, or equivalent), containing a 0.61-m x 6.35-mm (od) stainless steel column (Perkin Elmer Instruments, or equivalent) packed with 20% Silicone SE-30, by weight, and 80% Diatoport S (60- to 80-mesh), or equivalent materials. Program the column temperature from 100° to 270°, heated at a rate of 15°/min. Set the injection port temperature to 300° the bridge current at 140 mA and the sensitivity, lx for the integrator (Infotronics CRS-100, or equivalent) and 2x for the recorder. Use helium as the carrier gas, with a flow rate of 100 mL/min. 

This analysis has limitations. The thermal conductivity detector has a low sensitivity for H2 yet high concentrations cannot be allowed because response is not linear. Consequently, the flow of helium through the electrolysis cell must be adjusted to Keep the H2 concentration in the sensitive range. 

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