Other optical techniques

Many of the more specialised techniques for the optical examination of specimens have not been covered above, for reasons of space. These include conoscopy [35], infrared microscopy [46], fluorescence microscopy [47], and, of particular interest in connection with polymers, Raman microscopy [2]. 

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SHG, more so than other optical techniques, can give deceptive results due to propagation phenomena. Disregarding for the moment optical absorption effects, non-phase-matched SHG displays the behavior pictured in Figure 2 (18). The harmonic intensity I w emerging from a sample depends on the sample thickness / in a... 

Unlike the dependence of Aeff on film thickness alone (dNc /dd) that is sometimes used as a figure of merit for guided mode molecular sensors, 5m0d captures both the index and thickness dependence of the sensor response in a single parameter. While Dopt does not uniquely determine the film response for other optical techniques such as ellipsometry and reflectance difference, once d and n of the film are known, the optical thickness can be evaluated and comparisons are made between guided mode sensors and other techniques. 

Luminescence, in particular photoluminescence, constitutes a well-established discipline in analytical science where the cited hallmarks include remarkable sensitivity, wide dynamic range and low detection limits (-10under suitable conditions). These collective merits are often umivaled by other optical techniques, and hence its wide adoption in the life sciences for determining trace constituents in biological and environmental samples. Moreover, its fast response, high spatial resolution and remote sensing capabilities make it attractive for real-time analytical problems such as process manufacturing (process analysis or PAT) and field applications. ... 

Luminescence is often much more sensitive to molecular dynamics than other optical techniques where temperature, viscosity, pH and solvent effects can have a significant influence on the emission response. Analyte degradation for light sensitive fluors and photobleaching for static measurements also influence the emission signal. Because of the wide variety of potential matrix effects, a thorough investigation should be conducted or the sample matrix well understood in terms of its potential impact on emission response. A complete discussion on the fate of the excited states and other measurement risk considerations can be found elsewhere. ... 

Apart from optical microscopy, there are some other optical techniques which are truly surface sensitive and have found widespread use. Examples are ellipsometry (see Section 9.4.1), total internal reflection fluorescence (TIRF) [316], and surface plasmon resonance techniques [348],... 

Several other optical techniques that rely on various mechanisms by which light interacts with matter, including absorption, reflection, elastic scattering and autofluorescence, are also being developed for cancer diagnostics. This section will discuss these optical biopsy techniques. 

Other optical techniques have also been used to monitor analytical ultracentrifugation experiments. These included refractive index (Schlieren optics) as well as interference patterns produced using Rayleigh- or Lebedev-type optics.11 In both cases, data are acquired using photographic or video recording equipment. Examples of data obtained with these different optical systems are shown in Figure 13.11. 

Several other optical techniques now are widely used in automated systems. Among these tecliniques are the following ... 

Table 9 presents the potential application of turbidimetry and other optical techniques such as absorptiometry for high concentration. This type of measurement can be proposed for the estimation of TSS, according to the nature of the sample. In any case, the calibration step must be carefully established and regularly checked. 

Spectroscopies such as UV-visible absorption and phosphorescence and fluorescence detection are routinely used to probe electronic transitions in bulk materials, but they are seldom used to look at the properties of surfaces [72]. As with other optical techniques, one of the main problems here is the lack of surface discrimination, a problem that has sometime been b q)assed by either using thin films of the materials of interest [73, 74], or by using a reflection detection scheme. Modulation of a parameter, such as electric or magnetic fields, stress, or temperature, which affects the optical properties of the sample and detection of the AC component of the signal induced by such periodic changes, can also be used to achieve good surface sensitivity [75]. This latter approach is the basis for techniques such as surface reflectance spectroscopy, reflectance difference spectroscopy/reflectance anisotropy spectroscopy, surface photoadsorption... 

Optical detectors are by far the most common in FIA, even though only a few dedicated cells have been designed as most of the cells, particularly photometric and fluorimetric, manufactured for other flow methods can be readily adapted for use in FIA. On the other hand, other optical techniques such as chemiluminescence [42] and refractometry [43], of more limited use in this methodology, have favoured the development of new types of cell suited to specific needs. At this point it is worth noting the simplicity of the joint... 

These criteria limit the applicahility of fluorescent dye-based sensors to only a handful of analytes. If a fluorescence-based method does not exist or is Incompatible with an optical sensor, an absorption sensor is the only alternative. There exist a large number of different absorbing dyes that are sensitive, selective and absorb in convenient regions of the spectrum. Several absorbance-based fiber-optic sensors have been described (8,25.26). Unfortunately, a major drawback with absorption spectroscopy is its inherent insensitivity. We decided to explore the possibility of using other optical techniques that can be used with indirect methods. A successful example has been presented which combines the sensitivity of the fluorescence measurement with the specificity of an absorbing dye and is based on an energy transfer mechanism (15). 

Results from Yang et al. s study are in good agreement with other work gas velocity in the core region increases with solids circulation rate it decreases near the wall and, gas wall velocities are zero [101]. Other optical techniques, used to measure solids velocities, also may be applied to infer gas velocities. For example, in the core region suspension densities are low and particles may be reasonably well-dispersed therefore, the slip velocity is approximately equal to the particle terminal velocity. The measured particle velocity in a dispersed suspension of solids then can be readily translated into a local gas velocity. 

The combination of molecular depth profiting and molecular imaging opens up the opportunity for 3D imaging. The possibility of carrying out 3D molecular analysis on biological systems, for example, is really exciting because in principle, mass spectrometry does not require active markers (as is the case for other optical techniques) and spatially resolved chemistry may be accessible without altering the system. 

Due to all the complications which split and shift the peaks. It Is a di Bcult job to interpret n.m.r. spectra. Usualfy services of an expert are required. Sensitivity of the old generation n.m.r. Instruments was much less as compared to other optical techniques and siich other techniques as gas chromatography, and ma spectrometry. The modem day n.m.r. instmments. called the Fourier transform n.m.r. are built around a small, high speed digital computer. These instruments have revolutionized the practice of njnj. in organic chemlstty. 

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