Detector Principles

Electrochemical detection depends on a catalytic oxidation or reduction at the surface of an electrode located in the column effluent stream. This reaction produces a net charge transfer at the electrode surface. The resulting current is measured and provides a basis for quantirication of sample species. 

Ions that can be analyzed by electrochemical detection include cyanide, sulfide, hypochlorite, ascorbate, hydrazine, arsenite, phenols, aromatic amines, bromide, iodide, and thiosulfate [53], nitrite and nitrate [54.55], cobalt and iron [46], and others. The list may be extended through the technique of post-column derivatization to include many more ions such as carboxylic acids, halide ions, alkaline earth ions, and some transition metal ions [57,58). An example of an electrochemical reaction to detect ions is shown by Eq. 4.8. 

Working electrode Range Basic solutionl (volts) Acidic solution (volts)  

The potential limits for glassy carbon do not cut off sharply and must be determined experimentally. Complexing anions such as halides or cyanide will limit the positive range by several tenths of a volt. 

Within this working range, the presence of a reactive sample will give rise to a cui-rent/potential (// ) curve (Fig. 4.3). This curve is unique for a particular sample/elu-ent/electrode combination. It is characterized by a half-wave potential and by a diffusion current plateau. The half-wave potential (the potential half way up to the diffusion current plateau) is defined as the potential needed to induce electrolysis of the electroactive species. As the potential is increased, the electrolysis current also increases because more ions migrate to the electrode and become oxidized (or reduced). The electrolysis current eventually forms a plateau in the HE curve because, ultimately, the amount of current is limited by the rate of diffusion of ions to the electrode surface. In normal operation, the electrochemical detector potential is set at the smallest potential possible that is still on the diffusion current plateau. The detector should be on the plateau for consistent performance, but at the lowest potential to lessen the chance of side reactions. 

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Detector Principle of Operation Selectivity Sensitivity Linear Range MDQ1 Stability... 

Type of Detector Principle of Operation Selectivity Umit of Detection Comments... 

The electron capture detector principle is based on the phenomenon that electronegative specifies (CX) can react with thermal electrons to form... 

The flame-based GC selective detectors derive their response from a specific flame emission (flame photometric detectors), or certain secondary ionization processes subsequent to the combustion in a flame (thermionic or alkali-flame detectors). Recent advances in the detector principles and their applications, as pertinent to biochemical uses, will now briefly be reviewed. 

A variety of chromatographic detectors have been described in the literature. Because of limited space, only the most common commercially available detector principles are described in detail here. 

Nearly all LC detector principles produce an output signal which is related to the concentration of the solute in the eluent and are therefore concentration sensitive. 

Almost continuous position information can be obtained from an MCP-PMT with a resistive anode or with a micromachined wedge-and-strip geometry [247, 248, 262, 312] (see also Sect. 6, page 213). The detector principles are shown in Fig. 3.11. 

Fig. 4.11 Kinetic Inductance Detector principle of operation. An absorbed photon breaks apeirt a Cooper pair resulting in an excess quasi-particle population (a). The complex impedance of the film is altered by increasing the kinetic inductance and a microwave resonance circuit is used to sense...

EVAPORATIVE LIGHT-SCATTERING DETECTOR Principle of Operation... 

The new instrument introduced for inspection of multi-layer structures from polymeric and composite metals and materials in air-space industry and this is acoustic flaw detector AD-64M. The principle of its operation based on impedance and free vibration methods with further spectral processing of the obtained signal. 

The methods and means for ecological diagnostics make rapid strides among all the NDT and TD developing areas. To provide the atmosphere monitoring recently the good results were achieved in the development of surface-acoustics wave sensors (SAW), laser measuring systems, infrared detectors and systems based on other physical principles. 

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

In contrast to IR and NMR spectroscopy, the principle of mass spectrometry (MS) is based on decomposition and reactions of organic molecules on theii way from the ion source to the detector. Consequently, structure-MS correlation is basically a matter of relating reactions to the signals in a mass spectrum. The chemical structure information contained in mass spectra is difficult to extract because of the complicated relationships between MS data and chemical structures. The aim of spectra evaluation can be either the identification of a compound or the interpretation of spectral data in order to elucidate the chemical structure [78-80],... 

The principle of headspace sampling is introduced in this experiment using a mixture of methanol, chloroform, 1,2-dichloroethane, 1,1,1-trichloroethane, benzene, toluene, and p-xylene. Directions are given for evaluating the distribution coefficient for the partitioning of a volatile species between the liquid and vapor phase and for its quantitative analysis in the liquid phase. Both packed (OV-101) and capillary (5% phenyl silicone) columns were used. The GG is equipped with a flame ionization detector. 

A scintillator, sometimes known as the Daly detector, is an ion collector that is especially useful for studies on metastable ions. The principle of operation is illustrated in Figure 28.4. As with the first dynode of an electron multiplier, the arrival of a fast ion causes electrons to be emitted, and they are accelerated toward a second dynode. In this case, the dynode consists of a substance (a scintillator) that emits photons (light). The emitted light is detected by a commercial photon... 

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