Thermal/heat detectors

Thermal/heat detectors Reliable Low cost Slow Affected by the wind Indocn use... 

By far the most used detector is the thermal conductivity detector (TCD). Detectors like the TCD are called bulk-property detectors, in that the response is to a property of the overall material flowing through the detector, in this case the thermal conductivity of the stream, which includes the carrier gas (mobile phase) and any material that may be traveling with it. The principle behind a TCD is that a hot body loses heat at a rate that depends on the... 

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. 

Determination of oxygen. The sample is weighed into a silver container which has been solvent-washed, dried at 400 °C and kept in a closed container to avoid oxidation. It is dropped into a reactor heated at 1060 °C, quantitative conversion of oxygen to carbon monoxide being achieved by a layer of nickel-coated carbon (see Note). The pyrolysis gases then flow into the chromatographic column (1 m long) of molecular sieves (5 x 10-8 cm) heated at 100 °C the CO is separated from N2, CH4, and H2, and is measured by a thermal conductivity detector. 

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. 

The catalytic reforming of CH4 by CO2 was carried out in a conventional fixed bed reactor system. Flow rates of reactants were controlled by mass flow controllers [Bronkhorst HI-TEC Co.]. The reactor, with an inner diameter of 0.007 m, was heated in an electric furnace. The reaction temperatoe was controlled by a PID temperature controller and was monitored by a separated thermocouple placed in the catalyst bed. The effluent gases were analyzed by an online GC [Hewlett Packard Co., HP-6890 Series II] equipped with a thermal conductivity detector (TCD) and carbosphere column (0.0032 m O.D. and 2.5 m length, 80/100 meshes), and identified by a GC/MS [Hewlett Packard Co., 5890/5971] equipped with an HP-1 capillary column (0.0002 m O.D. and 50 m length). 

The instrumentation for temperature-programmed investigations is relatively simple. The reactor, charged with catalyst, is controlled by a processor, which heats the reactor at a linear rate of typically 0.1 to 20 °C min . A thermal conductivity detector or, preferably, a mass spectrometer measures the composition of the outlet gas. 

Carbon dioxide chemisorptions were carried out on a pulse-flow microreactor system with on-line gas chromatography using a thermal conductivity detector. The catalyst (0.4 g) was heated in flowing helium (40 cm3min ) to 723 K at 10 Kmin"1. The samples were held at this temperature for 2 hours before being cooled to room temperature and maintained in a helium flow. Pulses of gas (—1.53 x 10"5 moles) were introduced to the carrier gas from the sample loop. After passage through the catalyst bed the total contents of the pulse were analysed by GC and mass spectroscopy (ESS MS). 

All reactions involving lactic acids were performed in 300 mL Parr Autoclave batch reactor. All reagents, including the resin catalyst, were charged into the reactor and heated up to the desired reaction temperature. Stirring was commenced once the desired temperature was reached this was noted as zero reaction time. Reaction sample were withdrawn periodically over the course of reaction and analysed for ester, water and alcohol using a Varian 3700 gas chromatograph with a thermal conductivity detector (TCD) and a stainless steel... 

Temperature-programmed reduction (TPR) gives information on the reduction behavior of the Co catalysts. The spectra were recorded by the instrument ChemBET 3000 (Quantachrome Instruments) equipped with a thermal conductivity detector. Before analysis the samples were dried overnight (at least 12 h) at 373 K. The reduction was carried out in a hydrogen mixture of 10% H2 in Ar with a heating rate of 10 K/min. 

Thermal or heat detectors respond to the energy emission from a fire in the form or heat. The normal means by which the detector is activated is by convention currents of heated air or combustion products or by radiation effects. Because this means of activation takes some time to achieve thermal detectors are slower to respond to a fire when compared to some other detection devices. 

Usually there ll be at least two thermal conductivity detectors in the instrument, in a bridge circuit. Both detectors are set in the gas stream, but only one gets to see the samples. The electric current running through them heats them up, and they lose heat to the carrier gas at the same rate. 

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). 

Elemental composition Ba 69.58%, C 6.09%, O 24.32%. The compound is digested with nitric acid under heating and the solution is analyzed for barium by atomic absorption or emission spectrometry (see Barium). Carbon dioxide may be determined by treating a small amount of the solid with dilute HCl and analyzing the evolved gas by GC using a thermal conductivity detector or a mass spectrometer. The characteristic mass of CO2 is 44. 

The number of acid sites on pillared clays was determined by means of temperature programmed desorption (TPD) of ammonia. In each TPD experiment, a sample weighing about 0.5 g was treated in vacuo for 1 h at a given temperature in the range 400 - 600°C. Ammonia was adsorbed at a desired temperature (100-300°C) for 30 min and evacuated for 30 min. This sample was heated to 700°C at a rate of 10°C/min and desorbed ammonia was monitored by thermal conductivity detector. As water was desorbed simultaneously with ammonia, the ammonia TPD spectrum was obtained by point-by-point subtraction of the water desorption spectrum obtained with the sample which had not adsorbed ammonia. 

Figure 13.3 Flow outgassing with thermal conductivity detector, (a) a, Purge gas source b, needle valve c, cold trap Dj and D2, thermal conductivity detectors e, flow selector valve f, cell with powder g, flow meter, (b) Heated filaments 1 and 3 are D2,2 and 4 are. ...

Any gas may be used as the carrier as long as it does not react with the sample and/or stationary phase. However, other properties must be considered depending upon the type detector employed. With a thermal conductivity detector one uses a gas with high heat conductivity because thermal conductivity of a gas is inversely proportional to the square root of the molecular weight Thus, very low molecular weight gases are optimum. Helium is... 

Gas chromatography was used to determine n-paraffin distribution in the oil and wax samples. An F and M Instrument Company Model 500 chromatograph was used with an uncompensated single column, a helium carrier gas flow rate of 25 ml/min., and a thermal conductivity detector. The column was 4.8 mm in diameter and 3.3 m in length, and was packed with 3% Dexil 300 on Chromo-sorb P. The block and injection port temperatures were maintained at 673 K. The column was temperature-programmed from 348 K to 673 K at a rate of 5.7 K per minute. Peak identification was aided by the use of internal standards of decane, dodecane, and hexadecane. The baseline was determined by heating without sample injection. Response values were not available for the various areas on the traces, so the analyses were reported as % by area. 

A 8] The separation layer consists of platinum coated on a silicon substrate. Owing to the low heat capacity, quick temperature cycles can be executed and a resolution down to 7 ppm can be achieved with the integrated thermal conductivity detector. The standard length of the capillary is 860 mm with a channel width of 60 pm. A measurement cycle takes less than 60 s. 

The instrumentation for TP investigations is relatively simple the set-ups for TP reduction (TPR) and TP oxidation (TPO) studies of catalysts are shown in Figure 2.1. The reactor, charged with catalyst, is controlled by a processor, which heats the reactor at a rate of typically 0.1 to 20 °C min-1. A thermal conductivity detector measures the hydrogen or oxygen content of the gas mixture before and after reaction. For TPR, one uses a mixture of typically 5% H2 in Ar, or for TPO 5% 02 in He, to optimize the thermal conductivity difference between reactant and carrier gas. With this type of apparatus, a TPR (TPO) spectrum is a plot of the hydrogen (oxygen) consumption of a catalyst as a function of temperature. 

Ammonia TPD Measurement. The acidic properties of the catalysts were characterized using temperature programmed desorption (TPD) of ammonia. The experiments were carried out on a flow-type apparatus equipped with a fixed-bed and a thermal conductivity detector. The samples were activated in a helium flow of 5 L/h at 773 K for 1 hour. 300 mg of the H+-form of each dehydrated sample were used to perform the ammonia TPD. Pure ammonia, with a flow rate of 3 L/h, was then passed through the sample at 423 K for 30 min. The sample was subsequently purged with helium at the same temperature for 1.5 hours in order to remove the physisorbed ammonia. The TPD was performed under a helium flow of 6 L/h from 423 K to 873 K with a heating rate of 10 K/min and subsequently at the final temperature for 30 min. 

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