Classification of detectors

Historically, the first and the oldest detector of x-rays, which was in use for many decades, is the photographic film. Just as the visible light, the x-ray photons excite fine particles of silver halide when the film is exposed to x-rays. During the development, the exposed halide particles are converted into black metallic silver grains. Only the activated silver halide particles, i.e. those that absorbed several x-ray photons (usually at least 3 to 5 photons), turn into metallic silver. 

This type of detector is simple but is no longer in common use due to its low proportionality range, and limited spatial and energy resolution. Furthermore, the film development process introduces certain inconveniences and is time consuming. Finally, the information stored on the developed photographic film is difficult to digitize. 

the photographic film vaguely resembles a ratemeter because the intensity is extracted from the degree of darkening of the spots found on the film - the darker the spot, the higher the corresponding intensity because the larger number of photons have been absorbed by the spot on the film surface. The three most commonly utilized types of x-ray detectors today are gas proportional, scintillation, and solid-state detectors, all of which are true counters. 

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Another classification of detector is the bulk-property detector, one that measures a change in some overall property of the system of mobile phase plus sample. The most commonly used bulk-property detector is the refractive-index (RI) detector. The RI detector, the closest thing to a universal detector in lc, monitors the difference between the refractive index of the effluent from the column and pure solvent. These detectors are not very good for detection of materials at low concentrations. Moreover, they are sensitive to fluctuations in temperature. 

Detectors are primarily classified with respect to the technique with which they are used, viz. GC detectors, LC detectors and TLC detectors, and this type of classification is generally accepted. There is more debate regarding the secondary classification of detectors and, unfortunately, there are three methods in common use. Each of these methods of classification will be considered in detail. 

The detector can be considered as the "soul" of a HPLC system. Connected to the outlet end of the column, its role is to monitor the column effluent in real time. Detectors can be the most sophisticated and expensive component of the system. Classification of detectors is of two sorts, selective detectors which give different responses depending on the molecular structure of the sample under analysis, or universal detectors, for whom the response is similar for most compounds. Absorbance and fluorescence detectors are termed selective detectors, while the refractive index (RI) is a "universal detector". The Ultraviolet-Visible (UV-Vis) detector is more selective and sensitive, being able to detect amounts as low as lO g/mL, while the RI detector s sensitivity is in the range of lO g/mL. Therefore selective detectors can be used to minimise interference from unwanted components. As for fluorescence detectors, their sensitivity is in the range of lO i g/mL for... 

There is one further classification of detectors. Detectors can analyze the molecules of the mixture with and without destroying of the molecule of the component. These two classes of detectors are called destructive and non-destructive detectors, respectively. The latter type of detector is very sensitive to the concentrations of the different compounds in the mixture. Also, the volume of the measurement cell is important for this type of detector. 

The second alternative classification is to define detectors as specific and non-specific detectors. In this sense a specific detector would be exemplified by the fluorescence detector as it detects only those substances that fluoresce. An example of a non-specific detector would be the refractive index detector that detects all substances that have a refractive index different from that of the mobile phase. The classification of detectors as specific and non-specific is acceptable, but in this book detectors will be classified as bulk property detectors and solute property detectors as it more closely associates the detector with its basic method of measurement. 

The slow development of LC from the time of Tswett, to the late 1950 s was entirely due to the lack of high sensitivity on-line detectors. Since, the inception of effective LC detectors there has been a continuous synergistic interaction between column development and detector development which has resulted in the present highly sophisticated LC systems of today. There are a number of ways of classifying LC detectors, specific and non-specific detectors, mass and concentration sensitive detectors and finally bulk property and solute property detectors. The classification of detectors as bulk property and solute property detectors is recommended. Bulk property detectors respond to a change in some overall property of the eluent such as refractive index or dielectric constant whereas solute property detectors respond to some property that is unique to the solute alone. In practice solute property detectors are rarely ideal and many respond, at least weakly, to the same property of the mobile phase as well as the solute. 

Figure 4.11 Schematic classification of different types of HPLC detectors...

There are two types of detectors under UV/IR classifications. Both of the types respond to frequencies in the UV wavelength and IR in the CO2 wavelength. In both types simultaneous presence of the UV and IR signals must be available for alarm conditions. In the simple voting device an alarm is generated once both conditions are met. In the ratio device, satisfaction of the ratio between the level of UV signal received and IR signal received must also be achieved before an alarm condition is confirmed. 

Figure 1.14 — Classification of flow-through sensors according to external shape. SMZ sensing microzone D detector W waste. For details, see text.

One other, very descriptive classification of flow-through sensors is based on the location of the active microzone and its relationship to the detector. Thus, the microzone can be connected (Figs 2.6. A and 2.6.B) or integrated (Fig. 2.6.C) with the measuring instrument. Sensors of the former type use optical or electric connections and are in fact probe sensors incorporated into flow-cells of continuous analytical systems they can be of two types depending on whether the active microzone is located at the probe end (e.g. see [17]) or is built into the flow-cell (e.g. see [18]) — in this latter case. 

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