Stray coefficients

The quantitative description of diffuse reflectance measurements is known from thin-layer chromatography. Taking a powdery sample with such a large thickness (d = oo) that no reflection at the support material can be observed, the reflectivity of the sample is connected to the stray coefficients as well as with the absorption coefficient k. Kubelka and Munk [97]... 

The electronic transitions which produce spectra in the visible and ultraviolet are accompanied by vibrational and rotational transitions. In the condensed state, however, rotation is hindered by solvent molecules, and stray electrical fields affect the vibrational frequencies. For these reasons, electronic bands are very broad. An electronic band is characterised by the wave length and moleculai extinction coefficient at the position of maximum intensity (Xma,. and emai.). 

In most spectrophotometric analytical procedures, the absorbance of an unknown is compared directly with that of a calibrator or series of calibrators. Under these circumstances, minor errors in wavelength calibration, variation in spectral bandwidths, or presence of stray light are compensated for and do not usually contribute serious errors. Use of a series of calibrators covering a wide range of concentrations also provides a measure of Hnearity (i.e., agreement with Beer s law for a given procedure and instrument). When calculations are based on published or previously determined values for molar absorptivities or absorption coefficients, however, the spectrophotometer must be checked more rigorously. Performance verification of spectrophotometers on a periodic basis also improves reliability of routine comparative analyses. 

The best imbedding liquid was found to be castor oil for measurements in water. Castor oil has an acoustic impedance close to that of water (so that the percentage of incident sound energy reflected at the interface is small) and a rather high absorbtion coefficient. The probe is placed in such way that the direction of propagation of the sound wave is perpendicular to the thermocouple wires. The response of the probe follows the pattern described above (see Figure 7). According to the authors, this type of probe is extremely useful because (i) it is small in size, (ii) it has a low input electrical impedance, and (iii) it is not sensitive to stray radio frequency fields. 

The model molecule consists on n + 1 light-scattering beads each with identical isotropic polarizabilites a. The beads are connected by springs (or segments in the previous description) which provide a restoring force linear in the displacement if some beads stray from their equilibrium separations. Each bead interacts with the surrounding medium through identical frictional coefficients f and, in addition, Brownian forces are exerted on the beads by solvent molecules, (see Fig. 8.8.1). 

The curves in Fig. 1 do not include the short wave band observed by Smith (1929) at 940 A. in acid. The intensity of this band is uncertain owing to the experimental difficulty of excluding stray radiation in the spectrophotometer. Smith gave the molar extinction coefficient of this band as 3.5 X 10. ... 

Q The visual pigment chroniophore retinal (vitamin A aldehyde) has a molar extinction coefficient (in ethanol) of 43,000 at 375 nm. (a) What should be the absorbance at 375 nm of a 45.0 pmol dm solution of retinal in a I cm cuvette (b) If, due to stray light... 

The simplest and most inexpensive temperature monitors are thermocouples, such as copper-constantin (type T). Although the temperature coefficient of the thermoelectric effect is low (39 /nV/K at room temperature for type T), there exist excellent tables for common thermocouples which make them extremely reproducible (see, e.g., the helpful handbook on temperature measurements from Omega Engineering, Stamford, CT). Unfortunately, below about 30 K the thermoelectric coefficient decreases by at least a factor of 10 and thermocouples become not useful (unless one uses exotic and expensive couples like gold/iron). Further, because the voltages measured are so low, artifacts such as ground loops and stray return currents from the typical 1-5 A current being applied to the heater can lead to serious errors. 

The stray light contribution is calculated as the product of the blackbody emission from the Sun multiplied by the scattering coefficient (user defined), transmitted... 

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