Other reflection techniques

External reflection. This is not as well developed a technique as internal reflection the physics of reflection of light from surfaces is less accommodating to the infrared spectroscopist. Smooth or shiny surfaces are particular problems. Specular reflection from the surface itself is governed by Fresnel s equations—the reflectance depends on a complicated combination of refractive index, sample absorbance and polarisation. Consequently, samples where the reflectance is mainly from the surface give rise to spectra which bear little relation to conventional transmission spectra. A transformation known as the Kramers-Kronig transformation does exist which attempts to convert a specular reflectance spectrum into a conventional-looking one. It is not 100% successful, and also very computer-intensive. For these reasons, specular reflectance is not commonly used by the analyst. 

The uses for DRIFT often overlap with KBr discs or mulls. However, much less sample preparation is required and interference by the matrix (KBr or the mulling agent) is avoided. This may be essential if the sample 

INTRODUCTION TO SURFACTANT ANALYSIS Output mirror Input mirror 

The reflectance spectra of many materials are very often too strong if the sample is used neat. Mixing with excess KBr may be required to reduce intensity. 

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Quite early in the history of infrared spectroscopy, reflections in form of reststrahlen bands were observed indicating strong absorption. Nowadays infrared spectroscopy is widely applied to organic compounds which are comparably weak absorbers. For their studies such reflection methods are favoured, whose results are interpretable in almost the same way as transmittance spectra are. However, there is an increasing interest in applying other reflection techniques also to such samples. Advantageous aspects are ... 

Attenuated Total Reflection, Multiple Internal Reflection and Other Reflection Techniques... 

Basic Techniques - Liquid, Solution, Dispersion Dispersive Techniques Films, Solvent Cast, Hot Press. Microtome Attenuated Total Reflection, Multiple Internal Reflection and Other Reflection Techniques Pyrolysis. Microscope, etc. 

Diffuse reflectance (DR) is successfully applied to obtain IR spectra of rough (scattering) or dull surfaces, i.e. of media intractable by other reflection techniques. The interpretation of DR spectra, however, is sometimes handicapped by the fact that they may be a mixture of AB and SR spectra. DR spectroscopy is a sensitive tool, especially when used with an IR Fourier transform (FT) spectrometer (DRIFT). 

An additional advantage to neutron reflectivity is that high-vacuum conditions are not required. Thus, while studies on solid films can easily be pursued by several techniques, studies involving solvents or other volatile fluids are amenable only to reflectivity techniques. Neutrons penetrate deeply into a medium without substantial losses due to absorption. For example, a hydrocarbon film with a density of Ig cm havii a thickness of 2 mm attenuates the neutron beam by only 50%. Consequently, films several pm in thickness can be studied by neutron reflecdvity. Thus, one has the ability to probe concentration gradients at interfaces that are buried deep within a specimen while maintaining the high spatial resolution. Materials like quartz, sapphire, or aluminum are transparent to neutrons. Thus, concentration profiles at solid interfaces can be studied with neutrons, which simply is not possible with other techniques. 

The single most severe drawback to reflectivity techniques in general is that the concentration profile in a specimen is not measured directly. Reflectivity is the optical transform of the concentration profile in the specimen. Since the reflectivity measured is an intensity of reflected neutrons, phase information is lost and one encounters the e-old inverse problem. However, the use of reflectivity with other techniques that place constraints on the concentration profiles circumvents this problem. 

With the exception of single-crystal transmission work, most solids are too opaque to permit the conventional use of ultraviolet/visible (UV/VIS) electronic spectroscopy. As a result, such work must be performed through the use of diffuse reflection techniques [8-10]. Important work has been conducted in which UV/VIS spectroscopy has been used to study the reaction pathways of various solid state reactions. Other applications have been made in the fields of color measurement and color matching, areas which can be of considerable importance when applied to the coloring agents used in formulations. 

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. 

Vukjovic et al.199 recently proposed a simple, fast, sensitive, and low-cost procedure based on solid phase spectrophotometric (SPS) and multicomponent analysis by multiple linear regression (MA) to determine traces of heavy metals in pharmaceuticals. Other spectroscopic techniques employed for high-throughput pharmaceutical analysis include laser-induced breakdown spectroscopy (LIBS),200 201 fluorescence spectroscopy,202 204 diffusive reflectance spectroscopy,205 laser-based nephelometry,206 automated polarized light microscopy,207 and laser diffraction and image analysis.208... 

In solid state physics, the sensitivity of the EELS spectrum to the density of unoccupied states, reflected in the near-edge fine structure, makes it possible to study bonding, local coordination and local electronic properties of materials. One recent trend in ATEM is to compare ELNES data quantitatively with the results of band structure calculations. Furthermore, the ELNES data can directly be compared to X-ray absorption near edge structures (XANES) or to data obtained with other spectroscopic techniques. However, TEM offers by far the highest spatial resolution in the study of the densities of states (DOS). 

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],... 

IRES Versus Other Reflection Vibrational Spectroscopies. In order to achieve a sensitivity sufficient to detect absorption due to molecules at submonolayer coverages, some sort of modulation technique is highly desirable. Two candidates for modulation are the wavelength and the polarization state of the incident light. The former has been successfully applied to single crystal studies by Pritchard and co-workers (5j, while the latter is the basis of the Toronto ellipsometric spectrometer and of the technique employed by Bradshaw and coworkers (6) and by Overend and co-workers (7). The two different techniques achieve comparable sensitivities, which for the C-0 stretching mode of adsorbed carbon monoxide amounts to detection of less than 0.01 monolayer. Sensitivity, of course, is very much a function of resolution, scan rate, and surface cleanliness. 

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. 

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