Thermal conductivity detector mechanism

In temperature-programmed reduction (TPR), a flow of inert gas (N2 or Ar) containing approximately 5 vol% H2 is passed through the catalyst bed of a flow reactor containing a reducible solid catalyst (66). By monitoring continuously the H2 concentration in the gas flow and its eventual consumption with a thermal conductivity detector when heating the sample with a linear temperature ramp of ca. 10 K/min, the rates of reduction are obtained as a function of time (or temperature). The total amount of H2 consumed determines the reduction equivalents present in the catalyst, and detailed analysis of the experiment permits the kinetic parameters of the reduction process to be determined and provides information on the reduction mechanisms. The characteristic numbers, which depend on the experimental parameters (amount of reducible species present, H2 concentration, flow rate, and temperature ramp), have been defined (66,67). These numbers must be kept in certain ranges for optimal performance of the experiment. 

Besides the universal detector systems, for example electron capture, flame ionisation and thermal conductivity usually coupled with gas chromatographic columns, various other detectors are now being used to provide specific information. For example, the gas chromatograph/mass spectrometer couple has been used for structure elucidation of the separated fractions. The mechanics of this hybrid technique have been described by Message (1984). Other techniques used to detect the metal and/or metalloid constituents include inductively coupled plasma spectrometry and atomic absorption spectrometry. Ebdon et al. (1986) have reviewed this mode of application. The type and mode of combination of the detectors depend on the ingenuity of the investigator. Krull and Driscoll (1984) have reviewed the use of multiple detectors in gas chromatography. 

The main types of instruments measuring humidity in gases are various types of hygrometers (gravimetric, mechanical, condensation, infrared absorbance detector, electric sensor, thermal conductivity, Al203/sili-con, P2O5) and psychrometers. 

The electrical requirements for the TCD are much simpler than those for other gas chromatographic detectors. The mechanical requirements, however, are usually demanding, particularly thermal control. It is extremely important to control the temperature of the detector very well. To accomplish this, the cells of the TCD are mounted closely together, embedded in a metal block, with the entire assembly meticulously insulated. Often, the temperature control of the circuit provides better thermal stability than the chromatographic oven. Insulation of the detector prevents heat transfer by thermal conduction from the chromatographic oven. If heat is transferred through the flowing carrier gas, variations in the gas flow will likely be the source of noise and drift. 

Boron-doped diamond presents another attractive material with low and stable background current and noise over a wide potential range, corrosion resistance, high thermal conductivity, and high current densities. Usually no mechanical or electrochemical pretreatment of BDD film electrode is needed. Therefore, BDD film electrodes find use also in the area of environmental analysis for organic explosive determinations. BDD-based electrochemical detector allowed, e.g., amperometric detection of 2,4,6-trinitrotoluene, 1,3-dinitrobenzene, and 2,4-dinitrotoluene over the 200-1,400 ppb range, with detection limits at the 100 ppb level. ... 

Unlike optical detection, integrated electrical detectors (impedimetric, electrochemical, most mechanical and thermal, certain magnetic) require electrode interconnections and on-chip electrical traces (conductive pathways) to connect to external electronics. Although this is an added complication, it is typically easier to implement than full monolithic integratiOTi of an optical excitation-and-detection system. 

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