Scanning extinction measurements

In these density profiles the latex particles, added before starting the experiment, migrate to that position in the cell where their density coincides with the density of the surrounding medium. The position of the particles can be recorded by schlieren optics or, if there is a particle density distribution, more precisely by scanning extinction measurements normally used for the characterization of proteins. Thus the density and extinction profile in the ultracentrifugation cell yield a criterion for the density distribution and hence, because of the correlation between chemical composition and particle density, a criterion for the composition distribution or heterogeneity of the latex particles. 

The local position of the particles in the cell can be recorded in all cases by scanning extinction measurements. Only in the special case of chemically uniform particles can the schlieren optical method also be used. 

Although measurements with diffractometer interfaced with EDC cameras have been performed at 80-100 kv, however, this old-type system has a lot of limitations linked to the extremely long time (several hours) to scan ED patterns and the beam size (from microns to mm) of the electron diffraction cameras. Again, the problem of correcting intensities from dynamical contribution has not been addressed satisfactory, as primary extinction (dynamical) corrections have been proposed for known stmctures using the Blackman formula . ... 

Measured extinction spectra for aqueous suspensions of polystyrene spheres—the light scatterer s old friend—are shown in Fig. 11.19. Water is transparent only between about 0.2 and 1.3 jam, which limits measurements to this interval. These curves were obtained with a Cary 14R spectrophotometer, a commonly available double-beam instrument which automatically adjusts for changing light intensity during a wavelength scan and plots a continuous, high-resolution curve of optical density. To reproduce the fine structure faithfully, the curves were traced exactly as they were plotted by the instru-... 

In this protocol, commercially purchased carotenoid standards are dissolved in a suitable solvent and the absorbance measured at its maximum wavelength (A.max). Using published extinction coefficients and taking into consideration the dilution factor, the concentration of the standard carotenoid is calculated. The spectrum is also scanned in order to evaluate the fine structure (see Spectral Fine Structure in Background Information). The carotenoid solution should ideally be assayed by HPLC as described in unit F2.3 to establish chromatographic purity and thus correct the calculated concentration. 

Knollenberg and Veal [80] discuss operation, design and performance of optical counters in general and a review of extinction optical particle counters has been presented by Sommer [81] cit. 82). A comprehensive review of laser-based techniques for particle size measurement, covering both stream scanning and field scanning methods, contains 167 references [83]. 

Table I, provided yet another instrumental approach for balloon experiments intended to measure the extinction of solar radiation by stratospheric aerosol. Spectrometer 4, Table I, based on the use of a pyroelectric vidicon image device, was developed to measure the strong absorption bands of non-isonuclear molecules ( 2-5 ym range). Recently we have developed spectrometer 5, Table I, based on the use of a self-scanned solid state pyroelectric array sensor. The main advantages of this sensor, over the pyroelectric vidicon, are its improved sensitivity and reduced channel-to-channel cross-talk.

The absorbance change dAfdt will then follow the current, provided that species R has a measurable absorbance (depending, of course, on the concentration and the extinction coefficient). Instrumentally, it is also possible to plot dAjdE, which is related to dAjdt by the scan rate, v ... 

The quantitative evaluation of the amount of dyes in a spot can be carried by the traditional method of scrapping the spot from the plate, dissolving the dyes in a solvent and measure the extinction of the solution. This method is time-consuming and the reproducibility is fairly poor. To date, quantitative evaluation methods such as videodensitometry and scanning densitometry have also found application in the TLC analysis of... 

The spot can be measured as a whole or in segments when a suitably shaped light beam is used. Usually the spot area is scanned with a narrow beam and the diminution in light intensity summed for the spot area. One might expect that, under the conditions mentioned on p. 136, a linear relation would be found between the areas under the extinction curves thus obtained and the amounts of substance applied. As Klaus [351] has shown, the absorption integrals are, however, independent of the spot shape only when the transmission is relatively high, the spot area not too small and the spot fairly symmetrical. Standard and sample must be chromatographed on the same layer in order to compensate for individual infiuences of the adsorption layer. 

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