Choice of Detectors

Gaseous samples are preferably measured in ionization chambers (a radiation), proportional counters or Geiger-Muller gas counters. The samples are introduced into the counters or passed through with a gas stream (flow counters). 

Solid samples containing high-energy p emitters can be measured with end-window Geiger-Muller counters, whereas proportional counters are suitable for the measurement of a and p emitters. Self-absorption of a and p rays in solid samples may play an important role and requires special attention in the case of a and low-energy p radiation. Very thin and homogeneous layers can be obtained by electrolytic or vapour deposition of the radionuclides to be measured or by solvent extraction and subsequent evaporation of the solvent. 

Surface-barrier detectors are very useful for detection and measurement of a particles. The internal counting efficiency for a particles is = 1.0. If the geometry G of the arrangement of a source and detector is well-defined and self-absorption S in the sample can be neglected, absolute activities of a emitters can be determined. The performance of surface-barrier detectors is conventionally tested by recording the spectrum of a calibration source, e.g. a Am source. 

For the measurement of y emitters in solids Nal(Tl) scintillation detectors or Ge detectors are most suitable, depending upon whether high counting efficiency or high energy resolution is required. For comparison, the spectra of Co taken with a Nal(Tl) scintillation detector and with a Ge(Li) detector are plotted in Fig. 7.16. 

Kind of radiation Ionization chambers Proportional counters Geiger-Muller counters Scintillation detectors Semiconductor detectors 

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In many cases, it is necessary to complement physical security by the installation of an intruder alarm system in order to achieve the standard of security commensurate to the risk exposure. The scope of protection to be afforded by the alarm system depends on the security risk, but it may embrace fences, windows, doors, roofs, walls, internal areas, yards and external open areas, and vehicles inside and outside buildings. There is a comprehensive range of detection devices, but the choice of detector is critical to ensure that it provides the desired level of protection and is stable in the particular environment. 

The detector. The function of the detector, which is situated at the exit of the separation column, is to sense and measure the small amounts of the separated components present in the carrier gas stream leaving the column. The output from the detector is fed to a recorder which produces a pen-trace called a chromatogram (Fig. 9.1fr). The choice of detector will depend on factors such as the concentration level to be measured and the nature of the separated components. The detectors most widely used in gas chromatography are the thermal conductivity, flame-ionisation and electron-capture detectors, and a brief description of these will be given. For more detailed descriptions of these and other detectors more specialised texts should be consulted.67 69... 

The choice of detector is often crucial to the success of a particular HPLC method. A number are in routine use, including the UV, fluorescence, electrochemical, conductivity and refractive index detectors, and each has particular advantages and disadvantages, details of which can be found elsewhere [2-4],... 

Detection in 2DLC is the same as encountered in one-dimensional HPLC. A variety of detectors are presented in Table 5.2. The choice of detector is dependent on the molecule being detected, the problem being solved, and the separation mode used for the second dimension. If MS detection is utilized, then volatile buffers are typically used in the second-dimension separation. Ultraviolet detection is used for peptides, proteins, and any molecules that contain an appropriate chromophore. Evaporative light scattering detection has become popular for the analysis of polymers and surfactants that do not contain UV chromophores. Refractive index (RI) detection is generally used with size exclusion chromatography for the analysis of polymers. 

Another difficulty in the gas chromatographic separation of amino acids is the choice of detector and it may be necessary to split the gas stream and use two different detectors. The flame ionization detector, which is commonly used, is non-specific and will detect any non-amino acid components of the sample unless purification has been performed prior to derivatization. In addition the relative molar response of the flame ionization detector varies for each amino acid, necessitating the production of separate standard curves. As a consequence, although gas chromatography offers theoretical advantages, its practical application is mainly reserved for special circumstances when a nitrogen detector may be useful to increase the specificity. 

The laser light travels through the epifluorescence or side port of the microscope. A dichroic mirror reflects the laser light and passes the green fluorescence to either of the detectors. Detectors are positioned on the bottom port of the inverted microscope or the top port of the upright microscope. The choice of detector is discussed in more detail below. Broadband and band-pass filters placed in the detection path prevent residual IR from reaching either of the detectors. 

There are a variety of FPA detectors available that are sensitive in the NIR spectral region. The optimal choice of detectors depends on several factors desired wavelength range, whether the application will be laboratory based or part of a process environment, the sensitivity needed to adequately differentiate sample spectra and price. The figure of merit most often used to describe detector performance is specific detectivity or D, which is the inverse of noise equivalent power (NEP), normalized for detector area and unit bandwidth. NEP is defined as the radiant power that produces a signal-to-dark-current noise ratio of unity. 

The pump, injector, column, and detector are connected with tubing of narrow inner diameter. The inner diameter of the tubing that is used between the injector and column and also between the column and the detector must be as narrow as possible (0.010 inch or less for analytical work) to minimize band broadening. The choice of detector is based on intrinsic properties of the solute. Often more than one detector can be used to maximize sample information and confirm peak identities. For example, an absorbance detector could be placed in series with a conductivity detector for the visualization of a charged, chromophoric solute. 

The choice of detectors in LC is often a trade-off between wide scope and high sensitivity. For instance, the refractometer is readily available and easy to operate it can detect most compounds (wide scope), but it often has a lack of sensitivity for many compounds. A variable-wavelength UV detector offers a good choice for solutes that have some UV absorbance capability. Absorption at a specific wavelength results in a more selective and sensitive detection mode than a refractometer, but only for UV-absorbing compounds. In other specific cases the fluorescence or electrochemical detector can be used. These have a high sensitivity for individual compounds but are also limited in the number and type of compounds they can detect (narrow scope). 

The first and last terms correspond to oscillating polarizations that emit light at twice the frequency of E and 2 respectively, and the other two terms correspond to sum- and difference-firequency mixing of the two frequencies. In general, experiments can be conducted in such a way as to strongly favour one non-linear process above the others. However, if this is not possible, steps can be taken to detect the ouqmt of a single process through the use of appropriate filters, spatial separation of the emitted beams and choice of detector. 

The molecular weight distribution of SAN copolymers can be determined by gel permeation chromatography [10]. SAN copolymers are soluble in common solvents such as tetrahydrofuran. The solubility characteristics of SAN copolymers, combined with the commercial availability of columns and data analysis software, make gel permeation chromatographic analysis a rapid and routine procedure. The choice of detectors can allow for the determination of absolute molecular weight [11]. Selection of multiple detectors enables characterization of compositional heterogeneity as a function of molecular weight [12], as illustrated in Figure 13.3. 

One of the most important decisions that is left to the analyst when operating a liquid chromatograph is the choice of detector sensitivity. In some instruments the output from the sensor is monitored continuously over its entire dynamic range and so sensitivity is not an optional experimental parameter. Nevertheless, in this case, the sample size determines the concentration range over which the eluted solutes are monitored and thus an optimum sample size must be chosen. The detector should never be operated at its maximum sensitivity unless such conditions are enjoined by limited sample size or column geometry. Provided that there is adequate sample available, and the sample concentration when eluted is within the linear dynamic range of the detector, the maximum sample size that the column can tolerate should be used. This ensures that the detector noise is always minimal... 

The choice of detector can be quite critical. Uv detectors are very sensitive but are of little use if molecules without chromophores are being separated. A refractive index detector is universally applicable but has the drawback that gradient elution is virtually impossible. 

The flow diagram in Figure 10.4 is intended as a guide and is the way the author would normally approach a new HPLC analysis. Reversed-phase chromatography is assumed and this will mean evaporation of solvent and dissolution in mobile phase if using the hquid-liquid extraction path. No mention has been made of direct aqueous injection as the times that this technique can be employed in environmental analysis are few indeed. It can be seen that the author s choice of detector is fluorescence then electrochemical then UV. 

With the major constituents in foods the choice of LC detector is often the most important issue. Compounds such as vitamins, carbohydrates etc. may not have a strong ultraviolet (UV) chromophore. Therefore refractive index (RI) detection and, increasingly, electrochemical detection are often used. As discussed later, the choice of detector is even more important when determining the concentration of components in the foodstuff rather than the bulk constituent. 

An analytical separation technique requires a detection method responding to some or all of the components eluting from the separation system. The choice of detector is determined by the demands of the sample and analysis. For Field-Flow Fractionation (FFF) techniques many of the detection systems have evolved from those used in liquid chromatography (LC) techniques. 

There exist several different detectors suitable for detecting the analytes after the chromatographic separation. Some commercial detectors used in LC are ultraviolet (UV) detectors, fluorescence detektors, electrochemical detectors, refractive index (RI) detectors and mass spectrometry (MS) detectors. The choice of detector depends on the sample and the purpose of the analysis. 

Although many methods for the quantitation of uric acid are described in the literature, the most popular methods today employ the uricase-mediated reaction however, the specificity of this reaction may be compromised by the choice of detector reaction, owing to either an interfering enzyme or a molecule that competes in the final color generation step. Today reactions that generate a visible end product are preferred because of the higher color yield however, care should be taken that interference caused by ascorbate, bilirubin, and unspecified interferants in plasma from patients with kidney failure is minimized. 

The choice of detector is critical to the specific types of chemicals to be analyzed. Some units will detect everything and the output will resemble noise some will detect only certain types of chemicals. The latter type is preferred, and in the case of nitrosamines, the thermal energy analyzer/detector is the method of choice. Because smoke condensate is filthy, the cheaper, packed column is invariably used. Generally it gives adequate separation of the nitrosamines. 

CE has several advantages over 1C. In terms of theoretical plates, CE has at least ten times the separation of a typical IC system. Separations by CE are fast and it is relatively easy to adjust experimental conditions to obtain an adequate separation of sample ions. CE is a truly micro method that permits the use of very small samples. However, CE has a number of drawbacks that include a limited choice of detectors, a mediocre detection sensitivity, occasional problems with reproducibility, and a perception that CE is a somewhat exotic technique that is more complicated than IC. 

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