Optical techniques limitations

Although some optical techniques, such as soft X-ray absorption and optical reflectance measurements, provide comparative information about solids with higher energy resolution, EELS enjoys several unique advantages over optical spectroscopies. First of all, unlike optical reflectance measurements which are sensitive to the surface condition of the sample, the transmitted EELS represents the bulk properties of the material. Secondly, EELS spectra can be measured with q along specific controllable directions and thus, can be used to study the dispersion of plasmons, excitons, and other excitations [8.1-8.5]. Such experiments offer both dynamics as well as symmetry information about the electronic excitations in solids. In addition, the capability to probe the electronic structure at finite momentum-transfer also allows one to investigate the excited monopole or quadrupole transitions, which cannot be directly observed by conventional optical techniques limited by the dipole selection rule. 

Molecular ion mass interferences are not as prevalent for the simpler matrices, as is clear from the mass spectrum obtained for the Pechiney 11630 A1 standard sample by electron-gas SNMSd (Figure 4). For metals like high-purity Al, the use of the quadrupole mass spectrometer can be quite satisfiictory. The dopant elements are present in this standard at the level of several tens of ppm and are quite evident in the mass spectrum. While the detection limit on the order of one ppm is comparable to that obtained from optical techniques, the elemental coverage by SNMS is much more comprehensive. 

Only a few reviews have appeared in which application of the limiting-current method is discussed from a chemical engineering viewpoint. In the review of Tobias et al (T3) mentioned earlier, the authors examined the knowledge available on electrochemical mass transport during the early stages of its application in 1952. Ibl (II) reviewed early work on free convection, to which he and his co-workers contributed notably by development of optical methods for study of the diffusion layer. A discussion of the application of optical techniques for the study of phase boundaries has been given by Muller (M14). 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

For samples thicker than the depth of field, the images are blurred by out-of-focus fluorescence. Corrections using a computer are possible, but other techniques are generally preferred such as confocal microscopy and two-photon excitation microscopy. It is possible to overcome the optical diffraction limit in near-field scanning optical microscopy (NSOM). 

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. ... 

The application of NIRS does not resolve all concerns associated with accurate blend uniformity determination. Key questions arise, such as (1) is the NIR beam sampling within a unit dose and (2) are an effective number of unit doses being sampled throughout a blend These questions are not limited to NIRS, but are applicable to any diffuse reflectance optical technique. 

Motivating the research is the need for systematic, quantitative information about how different surfaces and solvents affect the structure, orientation, and reactivity of adsorbed solutes. In particular, the question of how the anisotropy imposed by surfaces alters solvent-solute interactions from their bulk solution limit will be explored. Answers to this question promise to affect our understanding of broad classes of interfacial phenomena including electron transfer, molecular recognition, and macromolecular self assembly. By combining surface sensitive, nonlinear optical techniques with methods developed for bulk solution studies, experiments will examine how the interfacial environment experienced by a solute changes as a function of solvent properties and surface composition. 

A variety of experimental techniques have been used to obtain information about the third-order optical nonlinearities and optical power limiting behavior of materials. This section includes descriptions of those techniques that have been used or have potential use with organometallics. For an excellent source of information about other techniques, the interested reader is directed to Ref. 6. 

In the mechanical mixture, the dispersion of the rubbery phase is coarse in the graft copolymer, by contrast, it is so fine that it is difficult to detect the elastomer by this optical technique of limited resolving power. However, measurements of the temperature dependence of the mechanical loss show that the elastomer is present as a distinct phase. 

By taking advantage of the deep penetration depth offered by CARS microscopy with near-infrared laser pulses, CARS tissue imaging has been demonstrated to complement other label-free optical techniques, such as auto-fluorescence that is limited to a few chemical species and optical tomography, second harmonic generation (SHG), and third harmonic generation (THG)... 

A limiting factor in noninvasive optical technology is variations in the optical properties of samples under investigation that result in spectral distortions44 8 and sampling volume (effective optical path length) variability 49-54 These variations will impact a noninvasive optical technique not only in interpretation of spectral features, but also in the construction and application of a multivariate calibration model if such variations are not accounted for. As a result, correction methods need to be developed and applied before further quantitative analysis. For Raman spectroscopy, relatively few correction methods appear in the literature, and most of them are not readily applicable to biological tissue.55-59... 

At high concentration, when molecules are no longer isolated in space, a conventional optical microscope is unable to resolve them within the diffraction limit. Efforts have been made to circumvent the diffraction limit by engineering the point spread function using nonlinear optical techniques. Spatial resolution of 20 nm in a cell has been demonstrated without using a proximal probe.67... 

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