Wavelength absorption, effect

Their physical properties closely resemble those of pterin, which has a basic pKt, of 2.20 and an acidic one of 7.86 associated with N-1 protonation and a hypsochromic shift of the long-wavelength absorption band in the UV spectrum, and N-3 deprotonation effecting a bathochromic shift respectively (Table 4). The xanthopterin (4) and isoxanthopterin types... 

Fig. 7—2. Spectral data to illustrate absorption and enhancement effects for three transition elements. (To avoid crowding, only part of the cobalt absorption curve is shown.) See Table 7-1. Case B. Substitution of A1 for Fe decreases absorption of incident beam and has little effect on analytical line. Net positive absorption effect. Case C. Substitution of Pb for Fe decreases absorption of primary beam but greatly increases absorption of analytical line. Net negative absorption effect. Case D. Note wavelength relationship indicated in figure. Enhancement impossible. Case E. Note wavelength relationship in figure. Enhancement occurs.

The change in the absorption effect with chemical composition can be calculated in simple cases by properly adjusting a (Equation 7-5) to allow for the change in mass absorption coefficient. The mass absorption coefficient of the sample for a given wavelength is given by... 

The earlier discussion (7.3) leads one to expect absorption and enhancement effects in these high-temperature alloys. For example, the results for nickel will be very sensitive to the iron content. Nickel Ka has a wavelength about 0.1 A shorter than that of the iron K edge (Figure 7-2), and a slight increase in the iron content consequently means an increased absorption of nickel Ka, which tends to reduce the apparent nickel content (negative absorption effect). Furthermore, this increased absorption of nickel Ka will enhance iron Ka, whence an... 

It is sometimes convenient to use a properly chosen scattered line in the background for comparison to avoid having to add an internal standard to the sample.37 The experience of the Applied Research Laboratories shows that the effects of variations or fluctuations in the equipment and of particle size in powdered samples can be eliminated satisfactorily in this way. In some cases, absorption and enhancement effects are also adequately compensated. We have already mentioned (7.8) that scattered reference line and the analytical line will be subject to considerably different absorption effects if the two lines differ appreciably in wavelength. Everything depends upon the selection of a satisfactory scattered reference line, and this is done empirically. 

The addition of a secondary solute or wavelength shifter can serve to offset much if not all of the action of tagged nitrocompds in reducing counting efficiency. For expl nitrocompds, a shift of the emission spectrum considerably into the visible region where absorption effects are not so pronounced is clearly indicated. The secondary solute POPOP has been found to be most efficient for this purpose (Ref 2). This enhanced effect on the scintillation process is illustrated in Fig 2 for p-Nitrotoluene... 

It is interesting to note that the longest wavelength absorption band of these homocycles more and more shifts to lower energies as the molecular size increases. This effect can most clearly be seen in the spectrum of S. ... 

Stability of anthocyanins can be attained by self-association, that is, when two or more anthocyanin molecules are associated. This effect was verihed by increasing the concentration of the cyanidin 3,5-diglucoside solution from 10 M to 10 M with a consequent bathochromic shift in maximum wavelength absorption in the visible region. "... 

The fluorescence excitation spectrum of a Q-CdS sample, with several maxima in the absorption spectrum, also has a number of peaks. However, the maxima in the two spectra do not always occur at the same wavelengths This effect is not surprising, as excitation at different wavelengths leads to the excitation of particles of different sizes which do not have the me fluorescence intensity at the wavelength where the fluorescence is recorded. 

Stewart, J. E., Spectral-bandwidth effects of variable-wavelength absorption detectors in liquid chromatography, ]. Chromatogr., 174, 283, 1979. 

We have mentioned above that if reaction occurs at defects in the structure this may lead to nontopochemical products. Such an effect may be important under certain circumstances thus, the monomer molecule and its arrangement in the crystal must be such that the excitation energy will be transferred from the ordered crystal to defects at a high rate. Inter alia this requires that the long-wavelength absorption band of the monomer be an intense one, so that this effect could be significant in the anthracenes, for example. Further, the defect population of most crystals is extremely small thus, for a finite amount of the defect dimer to be formed, additional defects must be created, for example as a result of strain-induced slip, as the reaction proceeds. 

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