Thermal conductivity detector operation

Figure 9. The hydrogen transfer system. Hydrogen diffuses out of the main carrier gas stream through the heated palladium tube and is swept to a second detector by a stream of nitrogen and determined there, while the remaining components are determined at the first detector. In the commercial instrument the two sides of a single thermal conductivity detector operate independently as if they were two separate detectors.

The general operation of the pilot scale reactor has be previously described by Pareek et. al. [3]. However, modifications were required to allow the injection of the gas and liquid tracers, and their subsequent detection at the outlets. The liquid tracer, 5mL Methyl blue solution (lOgL" ), was injected via a syringe inserted into the liquid feed line. Outlet samples were measured with a Shimadzu 1601 UV-Vis Spectrophotometer at a wavelength of 635nm. A pulse (20mL) of helium gas tracer was introduced using an automated control system, with the outlet concentration monitored in real-time with a thermal conductivity detector. Runs were carried out based on a two-level... 

Fig. 14.6. Schematic diagram of a capillary thermal conductivity detector showing both the reference and analytical flows. Reprinted with permission from the 6890 Gas Chromatograph Operating Manual. Copyright (2004), Agilent Technologies.

The thermal conductivity detector (TCD) operates on the principle that gases eluting from the column have thermal conductivities different from that of the carrier gas, which is usually helium. Present in the flow channel at the end of the column is a hot filament, hot because it has an electrical current passing through it. This filament is cooled to an equilibrium temperature by the flowing helium, but it is cooled differently by the mixture components as they elute, since their thermal conductivities are different from... 

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). 

Figure 2.10—Thermal conductivity detector. To the left is a schematic showing the path of the carrier gas. To the right is a schematic of the TCD and its operating principle, based on an electrical Wheatstone bridge (equilibrium exists when R /R2 = Ri/Ra)-...

The purpose of the carrier is to transport the sample through the column to the detector. The selection of the proper carrier gas is very important because it affects both column and detector performance. Unfortunately, the carrier gas that gives the optimum column performance is not always ideal for the particular detector. The detector that is employed usually dictates the carrier to be used. For instance, an electron capture detector operating in the pulsed mode requires an argon-methane mixture a thermal conductivity detector works best with hydrogen or helium. The most common carrier gases are listed in Table 6.1. 

DA, MEK, methyl vinyl ketone (MVK), propionaldehyde (PrH), and acetaldehyde (AcH) were analysed by on-line gas chromatography using a Varian 3400 GC equipped with a thermal conductivity detector and a 2m column containing 25% w/w B.B -oxydipropionitrile on Chromosorb W (80-100 mesh) operated at 60°C He was used as the carrier gas. Acetic acid (AcOH) was collected in 2ml of water from the effluent stream over a period of 1 hour and later analysed on a Porapak QS column at 150°C. CO2 was tested by removal of 2ml samples from the exit of the reactor with a gas syringe and injecting them onto a Porapak QS column operated at 60°C. 

Camphor has been determined by Baines and Proctor (183) in a number of essential oils and pharmaceutical preparations using gas chromatographic technique. The apparatus employed has a thermal conductivity detector with 1+ in. platinum wire 0.001 in. diameter of nominal resistance 250 Ohms. The wires in the channels being matched to within 0.1 Ohm. The bridge current was 200 mA and the output recorded on a 2.5 mV recorder. 2% ethylbenzene was added to the standards and samples as an internal standard. The operating conditions were as... 

RECOMMENDED OPERATING RANGES FOR HOT WIRE THERMAL CONDUCTIVITY DETECTORS... 

The following table provides guidance in the operation of hot wire thermal conductivity detectors. The operating trances are provided in mA dc for detector cells operated between 25 and 200PC.1 The current ranges and the cold resistances provided are for typical wire lengths and configurations. 

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. 

Detectors Use a thermal conductivity detector or a flame ionization detector or a mass spectrometer, operating all as recommended by the manufacturer. 

In the normal operation of the reactor the flow rate and composition of the HiS feed stream both fluctuate. In the past, each time either variable changed the required SO2 feed rate had to be reset by adjusting a valve in the feed line. A control system has been installed to automate this process. The H2S feed stream passes through an electronic flowmeter that transmits a signal Rt directly proportional to the molar flow rate of the stream, hi. When hf = 100 kmol/h. the transmitted signal R = 15 mV. The mole fraction of H2S in this stream is measured with a thermal conductivity detector, which transmits a signal R. Analyzer calibration data are is follows ... 

Thermal Conductivity Detector. The TCD is based on the principle that addition of a compound to a gas alters the thermal conductance of the gas. It is used often with capil-laiy GC. The operating principle of the ECD is based on the reaction between electronegative compounds, such as fluorine, chlorine, bromine, and iodine, and thermal electrons. Because not all compounds contain these functional groups, derivatization with reagents containing polychlorinated or polyfluorinated moieties is a common practice used with an ECD. 

Isopropanol dehydration over AI2O3 calcined at different temperatures was studied in a pyrex glass steady state system. The fixed bed (50 mg) tubular reactor was operated at differential regime (% conversion<10), in the 423temperature range and atmospheric pressure. The feed was composed of a N2 (Praxair) stream saturated with isopropanol at room temperature. The analysis of effluents from the reactor was carried out by gas chromatography with a Gow-Mac Series 750 apparatus equipped with a thermal conductivity detector and a Porapak Q packed column. 

The small volume thermal conductivity detector used In this study was a Y-type [ 10,11,12] gas flow pattern 30 Cow-Mac Model 10-955 cell (Gow-Mac Instrument Company, Madison, New Jersey). This cell was compared with the standard Varlan TCD (Gow-Mac Model 10-952) In a Varlan series 3700 gas chromatograph. The standard electronics were modified for operation with the 30 pi TCD. All measurements were made In the constant mean temperature mode. The carrier gas was He. The flow rates were regulated by two 0-60 ml/mln mass flow controllers (model 1000, Porter Instrument Company, Hatfield, Fa.) or by a pressure regulator (Model 8601, Brooks Instrument Division, Emerson Electric Company, Hatfield, Pa.). The capillary column was attached to a modified 1/16 to 1/16 Swagelok union which In turn was connected to the appropriate TCD. All make-up flows were regulated so that the total flow through both the reference and the sample sides were matched. 

The standard thermal conductivity detector employed by Varian is a Y-geometry 140 yi cell consisting of four matched filaments arranged in a Wheatstone bridge configuration and operated in the constant mean temperture mode. The cell was replaced by a 30ul TCD and the electronics were modified to compensate for change in filament resistance. 

Figure 2.14 Thermal conductivity detector. Left, schematic showing the carrier gas passage. Right, the cross-sectional scheme of the metal block with its operating principle, based on an electrical Wheatstone bridge assembly (equilibrated when Rj/Rj = R3/R4).

Detectors vary in their response to the compounds being sensed, with respect both to the amount and to the class of compound. Since the thermal conductivity of a mixture of two gases (the carrier gas and the sample) is not necessarily a linear function of composition, absolute quantitative measurements with thermal-conductivity detectors can only be obtained after calibration with standard compounds. However, response to structurally similar compounds is quite uniform, and reliable, relative, quantitative data can be obtained with this detector. Ionization detectors give a signal that is related directly to the mass or the concentration of the components, and the response is uniform over a broad range of operating conditions these detectors are, therefore, ideally suited for quantitative work. 

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