Types of detectors

In spite of the large number of detectors, they can be classified into just two groups  

Detector Spectral range (/rm) Time constant D (cmHz /2 w ) 

In intrinsic photoelectric detectors, electrons are excited from the valence band to the conduction band by photon absorption. The conductivity increases due to the increment in the carrier densities in both the conduction and the valence bands. The excitation process is possible provided that the photon energy of the incident radiation is greater than the energy gap of the semiconductor. 

On the other hand, in extrinsic detectors, electrons or holes are created by incident radiation with photons of energy mnch lower than the energy gap. As can be observed from Fignre 3.11(b), the inclnsion of impnrities leads to donor and/or acceptor energy levels within the semicondnctor gap. Thns, the energy separation between these impnrity levels and the valence/condnction bands is lower than the energy 

The main limitation of photoelectric detectors is the noise cansed by thermal excitation of the carriers from the valence band or from the impnrity levels. If there is a large dark current (a cnrrent generated by the detector in the absence of incident hght), the sensitivity of the photoelectric detector becomes poor (only very intense beams will indnce an appreciable change in the detector condnctivity). In order to rednce the dark cnrrent, photoelectric detectors are nsnally cooled dnring operation. 

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In TOF-SARS [9], a low-keV, monoenergetic, mass-selected, pulsed noble gas ion beam is focused onto a sample surface. The velocity distributions of scattered and recoiled particles are measured by standard TOF methods. A chaimel electron multiplier is used to detect fast (>800 eV) neutrals and ions. This type of detector has a small acceptance solid angle. A fixed angle is used between the pulsed ion beam and detector directions with respect to the sample as shown in figure Bl.23.4. The sample has to be rotated to measure ion scattering... 

Alternatively, the ions in a mass spectrometer can also arrive at a multipoint collector as a temporally dispersed beam. Therefore, at any point in time, all ions of the same m/z value arrive simultaneously, and different m/z values arrive at other times. Ail elements of this collector detect the arrival of ions of one m/z value at any one instant of time. This type of detector, which is also an array, is called a microchannel plate collector of ions. 

A more recent, and superior, type of detector, which also benefits from the multiplex advantage, is the charge-coupled device (CCD). The CCD, as used for spectroscopy, has been developed from the CCD detector used in a camcorder. 

An alternative type of spectrometer is the energy dispersive spectrometer which dispenses with a crystal dispersion element. Instead, a type of detector is used which receives the undispersed X-ray fluorescence and outputs a series of pulses of different voltages that correspond to the different wavelengths (energies) that it has received. These energies are then separated with a multichannel analyser. 

Detectors. Two general types of detectors are used in x-ray medical imaging scintillation and gas ionisation. Scintillation detectors are used for both conventional projection and computerized tomographic imaging. Ionization detectors have been used only in CT appHcations. All detectors used in detection of x-ray radiation must be linear and have a maximum efficiency at the wavelength of the x-ray photon to be detected. 

Jin X-ray detector. Two different types of detectors are commonly used. 

Various types of detector tubes have been devised. The NIOSH standard number S-311 employs a tube filled with 420—840 p.m (20/40 mesh) activated charcoal. A known volume of air is passed through the tube by either a handheld or vacuum pump. Carbon disulfide is used as the desorbing solvent and the solution is then analyzed by gc using a flame-ionization detector (88). Other adsorbents such as siUca gel and desorbents such as acetone have been employed. Passive (diffuse samplers) have also been developed. Passive samplers are useful for determining the time-weighted average (TWA) concentration of benzene vapor (89). Passive dosimeters allow permeation or diffusion-controlled mass transport across a membrane or adsorbent bed, ie, activated charcoal. The activated charcoal is removed, extracted with solvent, and analyzed by gc. Passive dosimeters with instant readout capabiUty have also been devised (85). 

The alkah flame-ionisation detector (AFID), sometimes called a thermionic (TID) or nitrogen—phosphoms detector (NPD), has as its basis the fact that a phosphoms- or nitrogen-containing organic material, when placed ia contact with an alkaU salt above a flame, forms ions ia excess of thermal ionic formation, which can then be detected as a current. Such a detector at the end of a column then reports on the elution of these compounds. The mechanism of the process is not clearly understood, but the enhanced current makes this type of detector popular for trace analysis of materials such as phosphoms-containing pesticides. 

It is clear that for this type of detector to be effective and produce the true Gaussian form of the eluted peak, then (Ca) must, at all times, be unity and, consequently, the... 

Such effects principally cannot be observed in multi band detectors such as a UV diode array detector or a Fourier transform infrared (FTIR) detector because all wavelengths are measured under the same geometry. For all other types of detectors, in principle, it is not possible to totally remove these effects of the laminar flow. Experiments and theoretical calculations show (8) that these disturbances can only be diminished by lowering the concentration gradient per volume unit in the effluent, which means that larger column diameters are essential for multiple detection or that narrow-bore columns are unsuitable for detector combinations. Disregarding these limitations can lead to serious misinterpretations of GPC results of multiple detector measurements. Such effects are a justification for thick columns of 8-10 mm diameter. 

Various types of detectors that recognize heat and/or smoke utilizing fused bimetallic strips, ionization chambers and the interruption of a light beam by smoke or other combustion products. It is important to select the most appropriate form of detector for the environment. Insurers give a modest discount from premiums if the alarm installation complies with the insurance rules. 

Detectors. Although electrical conductance has been widely used for detecting ions in ion chromatography, the scope of the technique has been considerably extended by the use of other types of detector. It is convenient broadly to classify detectors into two series. 

A summary of these characteristics for different types of detectors is given in Table 8.2. 

A detailed description of the various detectors available for use in HPLC is beyond the scope of the present text and the reader is recommended to consult the monograph by Scott.55 A brief account of the principal types of detectors is given below. 

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. 

Element-selective detectors. Many samples, e.g. those originating from environmental studies, contain so many constituent compounds that the gas chromatogram obtained is a complex array of peaks. For the analytical chemist, who may be interested in only a few of the compounds present, the replacement of the essentially non-selective type of detector (i.e. thermal conductivity, flame ionisation, etc.) by a system which responds selectively to some property of certain of the eluted species may overcome this problem. 

An electrical output for a polarographic detector is produced according the above reactions. Again, various electrolytes can be used for this type of detector, but they are usually based on KC1 or AgCl with additional high molecular mass compounds, which are added to prevent the loss of electrolyte during sterilisation. 

A later General Electric x-ray photometer26 is noteworthy because it uses current ionization chambers (2.6) as detectors. Improved means of external amplification made it possible to use this type of detector in a satisfactory photometer with the simple circuit shown in Figure 3-9. 

Element Analyt- ical Line Wavelength, A Crystal Slit Width, in. Path Type of Detector Detector Path Length, cm Detector Window"... 

There are three different types of detector noise, short term noise, long term noise and drift. These sources of noise combine together to give the composite noise of the detector. The different types of noise are depicted in figure 3. 

The aromatic nucleus adsorbs in the UV and thus, the derivative can be detected by a UV detector. This is the most common type of chemical derivatization but the derivative may be chosen to be appropriate for different types of detector. For example, the solute can be reacted with a fluorescing reagent, producing a fluorescent derivative and thus be detectable by the fluorescence detector. Alternatively, a derivative can be made that is easily oxidized and, consequently, would be detectable by an electrochemical detector. 

Figure 3. Optical and infrared detector zoology . The wavelength region is stated on the first row, with corresponding wavelength (in /rm) shown on the second row. The type of detector material and associated manufacturers are shown in the boxes below, which also depict the wavelength coverage possible with each kind of detector material.

The advantages of this type of detector are that even a noisy photodiode can easily be turned into a quantum limited detector and that the spectrum of the source can be analyzed precisely. 

Secondly, the intensity of response for a certain compound from one type of detector is not necessarily the same as that obtained from the other detector. This should not be unexpected, since the two detectors are measuring quite different properties of the analyte, in this case UV absorption at a particular wavelength and how readily it is ionized and fragmented under the conditions employed. These properties are urn-elated. 

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