Optical match

The infra-red measurements were of two types, normal-film measurements with the sample sandwiched between KBr plates, and tilted-film experiments with the sample sandwiched between 45° prisms of KBr, in each case with layers of Nujol to provide optical matching. Whereas the 1616 cm 1 Raman line occurs in a region well clear of other lines so that it was satisfactory to measure peak intensities, the infra-red spectrum of PET shows many overlapping bands. Accurate assessment of absorption intensities therefore requires the computer separation of the spectrum into a set of overlapping peaks (shown to be Lorentzian in profile) and a linear background. The procedures adopted and the band assignments are discussed in detail by Hutchinson et al. 6). 

The first glass polyalkenoate cement had a C, of 0-76, which was far too high, but improved modem materials are more acceptable and a value as low as 0-52 has been reported for one of these (Crisp, Abel Wilson, 1979). Knibbs, Plant Pearson (1986b) have found that most glass polyalkenoate cements have a good optical match with tooth enamel. 

Ideally, the solutions used in the experiment and calibration should be optically matched (i.e., their transmittances should differ by less than 2%). 

As a separate issue, the ring-opening reaction of the spirooxazine and the spiropy-ran can be sensitized by triplet energy donors[73,113-115]. A typical absorption rise for a benzophenone-sensitized ring-opening reaction is compared to that of the unsensitized fast absorption rise for NOSI3 in Fig. 13 [73,113]. In this case, both solutions were optically matched at the excitation wavelength. 

In many cases, the profile a spectroscopist sees is just the instrumental profile, but not the profile emitted by the source. In the simplest case (geometric optics, matched slits), this is a triangular slit function, but diffraction effects by beam limiting apertures, lens (or mirror) aberrations, poor alignment of the spectroscopic apparatus, etc., do often significantly modify the triangular function, especially if high resolution is employed. 

Under appropriate reaction conditions, regular hexagonal platelets (ca. 50 nm thick and 20 pm in diameter) can be obtained. The high refractive index (n — 2.0), high aspect ratio (> 200), and the extremely even surface of basic lead carbonate make it an optical match to natural pearl essence. 

For accurate results, it is advisable to use optically matched pairs or sets of cuvettes (i.e., cuvettes manufactured to a high specification to ensure they have the same optical parameters). 

Experimental evidence was presented, moreover, that a clear distinction is possible between micellar and (W/O) microemulsion regions according to the amount of solubilized water. Also, an optical matching phenomenon was observed which is of considerable significance regarding an understanding of the molecular properties of microemulsions in general. 

Disc rotation can be automatically triggered from the GPC upon injection or at some preset delay so sample collection is virtually unattended. The speed of rotation of the disc is adjusted to match the time of the evolution of the sample from the chromatograph. At a speed of 10 deg/min, 36 min of sample collection can take place on each disc. The Ge discs are easily cleaned and can be reused. After collection, the disc is removed and placed in a 3X beam condenser within the PTIK optics cabinet. The beam condenser is designed to provide an optical match between the FTIR beam size, which is mm, and the size of the deposited sample, 3 mm. Once seated on the platform in the beam condenser, the disc is rotated beneath the IR beam and spectra of individual samples collected. If the FTIR system has the capability of continually collecting spectra, then the spectra of the polymer deposit can be displayed continuously, thus generating an IR chromatogram. If this software is not available, the spectra may be individually collected and displayed by scanning the disc from point to point. 

Richards, F. M. The matching of physical models to three-dimensional electron-density maps a simple optical device. J. Molec. Biol. 37, 225-230 (1968). Richards, F. M. Optical matching of physical models and electron density maps early developments. Methods in Enzymology 115, 145-154 (1985). 

When it was observed that the DNQ/novolac resists in use at the time became more transparent on exposure (i.e., they bleached). Dill used this change in optical absorbance as a way of monitoring the kinetics of exposure. He developed a first-order model of exposure consisting of three parameters A, B, and C, now called the Dill model, which accurately fit the experimental measurement results of the transmittance of a resist coating on an optically matched glass wafer as a function of exposure dose. The Dill parameters A, B, and C, describe bleachable absorbance, nonbleachable absorbance, and exposure rate constant, respectively." ... 

If 1,3-pentadiene, either the cis or the trans isomer, dissolved in a river water sample, is irradiated, isomerization occurs (p. 213) producing a photostationary state (Fig. 6.13). The same steady-state (60% trans) was achieved starting with either isomer and reflects a balance between the cis trans and the trans cis processes. The composition of the photostationary state observed with different water samples and humic acids are summarized in Table 6.11. Solutions were adjusted to be optically matched each showing an absorbance of 0.20 at 366 nm. A small effect of wavelength is observed with smaller proportions of the trans isomer observed at 313 nm compared to 366 nm. 

The absorbance for just the second cell is A = —log 0.50 = 0.30. The two cells are identical in their absorbance of light. Identical or optically-matched cells are required for accurate quantitative analysis using spectroscopy in many cases. 

When double-beam instrumentation is used, two cells are needed one for the reference and one for the sample. It is normal for absorption by these cells to differ slightly. This causes a small error in the measurement of the sample absorption and can lead to analytical error. For most accurate quantitative work, optically matched cells are used. These are cells in which the absorption of each one is equal to or very nearly equal to the absorption of the other. Large numbers of cells are manufactured at one time and their respective absorptivities measured. Those with very similar absorp-tivities are designated as optically matched cells. Matched cells are usually etched near the top with an identification mark and must be kept together. It is important for the analyst to understand that even closely matched cells will show small differences in absorption due to differences in raw material characteristics. (The transmission of matched cells will also change due to normal use, so a new cell of the same match code will not necessarily match an older, used cell.) Less commonly used cells (other than the 1 cm type) can be supplied in matched sets of two or four cells. The proper use of matched cells is to fill both the sample and the reference cells with the solvent and run a baseline spectrum, which is stored by the instrument computer system. The sample cell is then cleaned and sample solution put into it, while the reference cell and its solvent are left in place. After measuring the sample spectrum, the baseline is subtracted from the sample spectrum by the computer. This approach will correct for small differences in the cells. It is also important that the sample cell be... 

The increasing residual scattering at higher temperatures (see Fig. 9) is straightforward explained by the temperature dependent fluctuations of the interfacial surfactant layer covering the dispersed droplets (26). The fluctuations can be observed since the static contributions of the refractive index increments of the dispersed particles and the solvent (oil) are optically matched. Hence, except for the fluctuations of the surfactant molecules in the monolayer the microemulsions discussed in the present paper show a remarkable monodispersity. 

For an extraction of precise energy values from measured absorption edges additional information is required one may, for example, take the energy differences between the distinct bound and continuum levels from optical spectroscopy data of the next element (optical match, Z + 1 rule). With this method inner-shell binding energies have been determined with absolute accuracies of about The experimental binding energies... 

The first optical matching step is the source to the flow cell. A typical analytical flow cell with a 1 mm diameter is larger than the 0.5-nim-diameter... 

The second, more challenging, optical matching step is the flow cell to the spectrograph. For the conventional type of flow cell described here, the numerical aperture (NA) of the flow-cell (defined in Fig. 3) and that of the spectrograph are drastically different, as is the shape of the slit compared with the circular cross section of the cell. If these mismatches are not addressed in the design, optical efficiency is poor. 

However, experimentally, room temperature fluorescence quantum yields (p) can be determined by comparison with standards of known quantum yield (t p ). The emission quantum yields of these reference compounds should be independent of the excitation wavelength and the absorption and emission range of the sample (cp) and reference (ref) compound should match as much as possible. In practice, the quantum yield is determined by comparison of the integrated area under the emission spectra of optically matched solutions of the samples (f I(A) dA) and that of the suitable reference compound (f I(Ay dA). The absorbance values should be kept as low as possible to avoid inner filter effects. In these conditions, using the same excitation wavelength, the unknown fluorescence quantum yield (0p ) is calculated using Eq. 15.5 [4],... 

Phosphorescence quantum yields (c ph) are obtained by collecting the phosphorescence emission spectra from optically matched solutions (at the excitation wavelength) of the samples and the reference compound and by applying the following equation. 

In a spectrofluorimeter, the sensitised phosphorescence emission spectra of singlet oxygen from optically matched solutions of the samples and that of the reference compound should be obtained in identical experimental conditions (see Fig. 15.3). The singlet oxygen formation quantum yield is then determined by comparing the integrated area under the emission spectra of the samples solutions (J I(Ay dA) and that of the reference solution (f /(Ay dA) and applying Eq. 15.10,... 

Care must be taken in order to have diluted solutions of the compounds and the reference optically matched at the laser excitation wavelength. Typically, we should have standards to obtain the value for the three available wavelengths of a Nd YAG laser 266, 355 and 532 nm. Optical parametric amplifiers can be used to tune other wavelengths, but these are not always available and always reduce the laser intensity reaching the sample. Therefore, the i values are generally determined using as standards naphthalene in ethanol (sj = 24,500 mol dm cm at 415 nm, ( t = 0.8) when the laser excitation is with the fourth harmonic (2ex = 266 nm) of a Nd YAG, benzophenone in benzene (ex — 7,220 mol dm cm at 530 nm, = 1) with Aex — 355 nm and tetra-phenyl-porphyrin in toluene (ej = 6,000 mol dm cm at 790 nm, (pj — 0.82) for 2ex = 532 nm [2, 15]. 

Rieka, J., Borkovec, M. and Hofmeier, U., Coated droplet model of microemulsions optical matching and polydis-persity, J. Chem. Phys., 94, 8503-8509 (1991). 

For many tissues in vivo measurements are possible only in the geometry of the backscattering. The corresponding relation can be written on the basis of diffusion approximation. For the semi-infinite medium and the source and detector probes, separated by a distance normal and optically matched to the sample surface, the reflecting flux is given by [119]... 

The sapphire surface was highly polished to make the surface roughness as small as possible (<30 A rms), in order to minimize the possibility of stray light excitation of the bulk solution. Toluene was chosen not only because it is a good solvent for PMMA but also, and more importantly, because its refractive index is very close to that of PMMA (1.494 compared to 1.492). Under such conditions of optical matching, it is reasonable to assume that the refractive index of the solution is independent of the polymer concentration. This simplifies considerably the data analysis. 

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