Hot wire detector

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

There are several types of detectors, devices that can tell when a sample is passing by them. They detect the presence of a sample and convert it to an electrical signal that s turned into a GC peak (Fig. 109) on the chart recorder. The most common type is the thermal conductivity detector. Sometimes called hot-wire detectors, these devices are very similar to the filaments you... 

There are four main types of detectors used in GC thermal conductivity detector (TCD), also called a hot wire detector, flame ionization detector (FID), electron capture detector (ECD), and quadruple mass spectrometer (MS)... 

A commercial single point instrument contains a linearization network which corrects for the hot-wire detector nonlinearity. This procedure allows a built-in digital integrator to linearly integrate the signals so that the surface area is given directly on a digital display. Analysis time on this apparatus is extremely short, usually under 10 min. 

Fig. 13.1 Schematic diagram of apparatus IS, ion source O, atomic beam oven F, Faraday cup EM, electron multiplier HW, hot-wire detector. Long and short dashed line, ion beam solid line, Na beam dashed lines, laser beams (from ref. 1).

There are other major problems with peak assignment on the basis of the areas. These problems relate to the reproducibility of peak area measurements under widely varying conditions. Ideally, the area of a peak remains constant even if its capacity factor varies. However, varying the conditions may affect the peak areas. If the column temperature is changed in GC, then the flow rate may be affected. Peak areas will change (by a constant factor) if concentration-sensitive detectors such as the hot wire detector (H WD katharo-meter) are used, but not with mass flow sensitive detectors (such as the flame ionization detector, FID). 

Ideally, the degrees of sensitivity of the two detectors should also be comparable. Because of this last reason, it is not attractive to combine a hot wire detector with a flame ionization detector in GC (a combination that also conflicts with the third limitation above) or a differential refractometer with a UV spectrometer in LC. 

Procedure Use a gas chromatograph equipped with a hotwire detector and a suitable sample-injection system or on-column injection. Under typical conditions, the instrument contains a 1/4-in. (od) x 6- to 8-ft column, or equivalent, maintained isothermally at 70° to 80°. The flow rate of dry carrier gas is 50 to 80 mL/min, and the sample size is 15 to 20 pL (for the hot-wire detector). The column selected for use in the chromatograph depends on the components to be analyzed and, to a certain extent, on the preference of the analyst. The columns 1, 2, 3, and 4, as described under Toluene, may be used as follows (1) This column separates acetone and methanol from their aqueous solution. It may be used for... 

The catalytic tests were carried out in flow mode with the effluent gas analyzed by an on-line gas chromatograph which was provided with an Alltech CTR-1 column and a hot wire detector. The catalyst (ca.l20 mg) underwent the standard pretreatment in flowing 10% O2 in He at 773 K overnight. The reactant blend (0.46% NO + 0.46% CsHg + 5% O2 + He balance) was passed through the catalyst at the total flow of 100 mL (NTP)/min and total pressure of 1 atm in the range of temperature 423-873 K with a 0.5 h isothermal step at any selected temperature. GHSV = 25.000 h. ... 

Fig. 4-6. Detector circuits for vapor-phase chromatography, (a) Thermistor detector Di, D=, Victory Eng. Corp. 32A12 thermistors Ri, Ri, 1,0000 wire wound resistors Rz, 1,0000 Helipot Rt, 10,0000 1% carbon film resistor Rs, 5,0000 1 % carbon film resistor Rt, Rj, 2,5000 1 % carbon film resistor Sw, single-pole four-position switch, (b) Hot-wire detector Rz, filament current control, to adjust filament current between 150—300 ma ( 20 ohm 5w) Ri, R4, reference detectors Rs, R, sample detectors Re, zero control 20 Re, 600 1 % carbon film resistor R, 300 1 /, carbon film resistor R, 150 1% carbon film resistor Rse, 7.5Q 1 % carbon film resistor Rs, 7.SCI 1 % carbon film resistor M, 300 ma meter,- S, single-pole six-position sv/itch.

The katharometer detector [sometimes spelled cath-erometer and often referred to as the thermal conductivity detector (TCD) or the hot-wire detector (HWD)] is the oldest commercially available gas chromatographic (GC) detector still in common use. Compared with other GC detectors, it is a relatively insensitive detector and has survived largely as a result of its almost universal response. In particular, it is sensitive to the permanent gases to which few other detectors have a significant response. Despite its relatively low sensitivity, the frequent need for permanent gas analysis in many industries probably accounts for it still being the fourth most commonly used GC detector. It is simple in design and requires minimal electronic support and, as a consequence, is also relatively inexpensive compared with other detectors. 

Instead of measuring the attenuation of a beam, one may also count the ions produced with very high efficiency by the use of channelplates or a hot-wire detector [387], an approach which has mainly been applied in laser spectroscopy, where high sensitivity can be achieved by space charge amplification. The principle of the thermionic diode is that the atomic vapour under study is formed within the detector, and a current limited by the space charge is obtained by appropriately biasing a diode, consisting of an external anode (often the outer wall of the vacuum system, formed by a metal tube) and a heated cathode made of a suitable material to emit many electrons (thoriated W is suitable in many cases). A sketch of... 

A mixture of pentane, hexane, and heptane is separated by gas chromatography. A number of different types of colutrms can be used, and a simple thermal conductivity or hot wire detector is satisfactory. The instrument response for each compound is calibrated by running a standard mixture of the compounds. The order of separation is determined by miming the individual compounds. 

There are a large number of GC detectors available but the majority of GC separations are monitored by the flame ionization detector (FID), the nitrogen phosphorus detector (NPD), the electron capture detector (BCD) or the katherometer detector (or Hot Wire Detector). Tlie latter is almost exclusively used in gas analysis and rarely used in chiral chromatography and so will only be briefly described here. Furthermore, the FID is used in probably 90% of all chiral analyses. However, before describing the construction and function of each detector the subject of detector specifications needs to be discussed. 

The Katharometer (Thermal Conductivity and Hot Wire) Detector Data Acquisition and Processing Synopsis References Chapter 5... 

Chromosorb. The coated support was packed in stainless steel columns (0.5 cm id, about 0.8 m long) and served as the stationary phase in IGC. The stationary phases were contacted with selected vapours at very high dilution and described procedures were followed [6, 11] to measure the net retention volume, V , of the vapours probing the deposited solids. A Perkin-Elmer Sigma-2 chromatograph with hot wire detector was used. All determinations were in the range 30 - 60 °C, and values of V were measured in at least triplicate, with a reproducibility better than 4%. 

One widely used detector is the nondestructive, thermal conductivity detector, sometimes called a hot-wire detector. A heated wire in the gas stream changes its electrical resistance when a substance dilutes the carrier gas and thus... 

The thermal conductivity detector (TCD), also called the hot-wire detector or katliarometer, measures the change in the thermal conductivity of the mobile-phase stream. Helium is the recommended carrier gas because of its high thermal conductivity. When an analyte is present in the carrier gas, the thermal conductivity will drop and less heat is lost to the cavity wall. A filament in the detector cell operated under constant voltage will heat up and its resistance will increase. This resistance is measured and recorded. 

The catalyst was crushed and a sieve fraction (0.4-0.63 mm) was taken. A 10 vol % Ha/Ar flow (50 Vmin )was passed through a sample of 1(X) mg. The temperature was raised wdth 5 /min- The hydrogen consumption was measured with a hot-wire detector. 

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