Thermal conductivities detector

Fig. 21.2. Gas-solid chromatogram of vapor sample from bulging drum (column 5-ft X 1/8 in o.d. aluminum packed with molecular sieve 5A 60-80 mesh column temperature ambient 25°C to 28°C detector thermal conductivity at 80°C carrier gas Ar at 15cm3/min. (Reprinted/redrawn with permission from Analyt. Chem., 56, 603A (1984). Copyright 1984 American Chemical Society.)...

There are some detectors (thermal conductivity, for example) which respond to the concentration in the detector, rather than the mass flowrate. Consider such a detector used with a quarter-inch column, where the typical carrier flowrate, Fc, is 1 cm3/sec. Suppose the MDL is measured to be 16 ng/sec. The MDL can be made a factor of two better (8 ng/sec) by going to an eighth inch column (where the flow is typically 0.5 cm3/sec). This is because, when the same mass flowrate of compound is going through the detector as before, there is less carrier gas diluting the sample, so the detector performs with high sensitivity. 

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

Special identification detectors Thermal conductivity detector Thermal energy analyzer Temperature-programmed reduction Thermoparticulaie analysis Thermal volatilization analysis Thin-layer chromatography Titrimetry Volume changes... 

Detector Thermal conductivity thermistor at 15 mA Thermal conductivity thermistor at 15 mA Thermal conductivity thermistor at 15 mA... 

In Fig. 12.20 are shown schematically the chief hybridizations possible between chromatography and various spectrometric techniques. The simplest configuration (Fig. 12.20a) involves the linkage between the column and the spectrometer detection zone via a suitable interface —the information is obtained from the spectrometer only. This configuration is one of the commonest in GC-MS [42-44], HPLC-MS [45], HPLC-plasma emission (ICP, MIP) [46], SFC-MS [47] and HPLC-NMR [48] hybridizations. In the configuration In Fig. 12.20b, the typical non-destructive detector (thermal conductivity In GC and UV-visible In HPLC) of the chromatograph provides an ordinary chromatogram ... 

There are three types of detectors Thermal Conductivity Detector (TCD), Flame Ionization Detector (FID) and Electron Capture Detector (ECD). In these detector systems, EID exhibits the highest sensitivity for the detection of hydrocarbon materials and TCD has been used commonly for the detection of inorganic components and organic materials except for hydrocarbons. The ECD is used specifically for the detection of materials with large electronegativity, such as alkyl halides. Recently, GC has been equipped with a mass spectrometry system (GC-MS). 

The connection to an appropriate detector flame ionization detector, thermal conductivity detector, flame photometric detector, nitrogen phosphorous detector (FID, TCD, FPD, NPD, etc.) is achieved via the four- or six-port valve being operated manually or electronically. The carrier gas (helium, nitrogen, argon, artificial air, etc.) flows via the valve only through the sampling column, at low flow rates (e.g., 25 cm /min) held constant during the experiments. 

Another important instrument required in modem LC is a sensitive or selective detector for continuous monitoring of the column effluent. In GC. the differences in physical properties of the mobile phase (carrier gas) and the sample are great enough for universal detectors with good sensitivity to be used (e.g., flame ionization detector, thermal conductivity detector, - Gas Chromatography). The problem in LC is that the physical properties of the mobile phase and the sample are often very similar, which makes the use of a universal detector impossible. Nevertheless, presently available LC detectors are very sensitive, are generally selective, and have a relatively wide range of applications (see Table 1). 

Among the detectors, thermal conductivity is still used where high sensitivity is not required. A flame ionization (or -ionization) detector is most common at the present time. An electron capture detector is especially sensitive for halogenated substances. 

Column length, 5 feet column temperature, 98 stationary phase, 30 per cent of Carbowax 1500 on 36- to 85-mesh Chromosorb nitrogen flow rate, 20 ml per minute flash-heater temperature, 145° detector, thermal conductivity cell detector current, 175 mA sample size, 30 recorder, Honeywell-Brown, 1 mV full-scale deflection, chart speed 12 inches per hour. 

Detectors—Thermal Conductivity—If a methanation reactor is used, a flame ionization detector is also required. To determine carbon monoxide with a flame ionization detector, a methanation reactor must be inserted between the column and the detector and hydrogen added as a reduction gas. Details on the preparation and use of the reactor are given in Appendix XI. 

Detector thermal conductivity thermal conductivity thermal conductivity hydrogen flame Ionization... 

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