Christiansen distortion effect

When measuring spectra of very hard crystals, sometimes distorted bands, almost like in derivative spectra, can be seen with solid KBr or mulling techniques. This is caused by the Christiansen effect(8), which appears if too large crystals are left in the... 

When a continuous film was obtained, no Christiansen effect was observed. However, sometimes good spectra were obtained from discontinuous films. The photomicrographs and IR spectra shown in Figure 5 were taken using an IR microscope. For the polyethylene deposit consisting of droplets 10-30 pm in diameter, no spectral distortion was observed. In another deposit from the same SEC run when droplets ranged from 2 to 10 pm in diameter, a distorted spectrum was obtained. 

The Effect of Polymer Type on Film Morphology. At relatively high polymer concentrations, all types of polymers used, and their blends formed continuous films and provided excellent spectra. Blend composition and the presence or absence of crystallization did not cause Christiansen efiect distortions. However, at concentrations usual for SEC analysis, all polymers showed evidence of not wetting the KBr surface, yielded discontinuous films, and often had poor spectra. These results indicate, with respect to spectral distortions, that the main importance of polymer type is its infiuence on continuous-film formation. 

For the selection of optical windows, besides such parameters as useful spectral range, mechanical resistance and solubility, the refractive index also has to be taken into account The refractive index of the windows should match that of the liquid sample in order to minimize reflection losses, stray light and distortions of band shapes (Christiansen effect). NaCl and KBr are very suitable for organic analytes. Inorganic analytes may have much higher refractive indices. The higher the refractive index, the higher the reflection losses for the incident IR radiation. 

Crystalline samples sometimes produce spectra with distorted band shapes, an effect known as the Christiansen effect [see Potts (1963) and Table 1.4]. Also, polymorphic forms of the same substance frequently show differences in infrared spectra. An example is N-benzoyl-2,3,4,6-tetra-0-benzoyl- -D-glucosylamine, a compound that exists in a form with melting point 113-115°C which, when heated to 117-120°C and allowed to crystallize from the melt, gives a form with melting point 184°C having a somewhat different spectrum in Nujol (Tipson, 1968). Also, different crystal habits (same melting point) of a compound may display partially differing spectra, especially if examined as mulls, in which little pressure is applied. Shifts of up to 20 cm" for certain bands have been observed (Barker et al., 1956) for crystalline and amorphous forms of some carbohydrates. In all such instances, however, spectra of samples of each of the forms, recorded after dissolution in the same solvent, or as a molten substance, are identical. 

In crystalline solids the effects of randomness of environment and of quantized rotation are absent, and very sharp vibration bands can be observed (Rgure 8.7(c)), especially in the Raman Effect. Surface reflection effects vary a lot in the vicinity of an IR absorption band (because the refractive index of the sample varies - a phenomenon known as anomalous dispersion) and distort its apparent profile (known as the Christiansen effect). Samples must be prepared very carefidly by fine grinding, to ensme that high quality spectra can be obtained. 

Here k is the molar extinction coefficient and 5 is a scattering coefficient which varies with particle size and packing. While the Christiansen effect seen in transmission spectra of powders appears to be absent, spectral distortion can occur in diffuse reflection spectra if the particle size is not uniformly fine. Strongly scattering, or black samples such as coal can be handled by this technique. 

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