Christiansen effect

Christiansen effect analy chem Monochromatic transparency effect when finely powdered substances, such as glass or quartz, are immersed in a liquid having the same refractive index. kris-chan-san i fekt ... 

Gow-Mac has recently marketed a refractometer which is based on the Christiansen effect . The system consists of a sample flow cell which is packed with a solid material having the same refractive index as the mobile phase. Visible light is therefore transmitted directly through the cell. When a solute enters the cell, the refractive index of the mobile phase alters, causing a change in the quantity of transmitted light which reaches the detector. A variety of solid materials is available for use with a large number of solvent systems. 

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... 

Figure 1. Spectra of CR using different sample preparation methods. The spectra have been measured (a) as a solid KBr disk with deficient grinding of CR (Christiansen effect visible), (b) as a solid KBr disk with CR well reground, (c) dissolving CR into acetone and deposition on a KBr disk, and (d) by cryodeposition GC/FTIR (Source M. Soderstrom, unpubhshed results)...

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. 

A simple method to prepare a solid sample is called the potassium bromide pressing technique. The prerequisite for this method is that the particles of the solid sample are smaller than the wavelength, which must be measured. Larger particles lead to the unwanted Christiansen effect. 

P.D. Delmas, N.H. Bjarnason, B.H. Mitlak, A.C. Ravoux, A.S. Shah, W.J. Huster, M. Draper, C. Christiansen, Effects of raloxifene on bone mineral density, serum cholesterol concentrations, and uterine endometrium in postmenopausal women [see comment], N. Engl.J. Med. 1997, 337, 1641-1647. 

Differential refractometers detect the changes in refractive index of eluate when compared with the refractive index of the mobile phase. Four principles are exploited in the construction of differential refractometers deflection of light passing the boundary surface between the substances with different optical density deflexion refractometers), and subsequent light scattering in case of multiple refraction refractometers based on Christiansen effect), the Fresnel law of light reflection reflection refractometers) and, finally, the interference of light interferometers). 

The Christiansen effect, the strong dependence of the transmission of light through a heterogenous system on the difference in the RI values of the phases, can be used for RI measurement in LC. An instrument capable of 10 RI noise level... 

C = Christiansen effect D = deflection type F = Fresnel type. 

Linearity, however, is probably a problem vn th the Christiansen effect system... 

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. 

Water resistant, front-surface aluminum mirrors can be used as deposition substrates, followed by spectral acquisition in the R-A mode. The smooth and hard surface properties of mirrors complicate efficient analyte deposition (spreading of the analytes) when the eluent is not completely evaporated. The spectral data recorded from these substrates should be in close resemblance to the spectra obtained from transmission measurements, because the band intensities are controlled by a double-pass transmittance mechanism. However, still spectral differences between R-A and KBr disk spectra can be observed, including absorption-band shifts and asymmetries. Furthermore, the effect of light scattering (Christiansen effect) may become apparent when the spot thickness exceeds a certain level and anomalous relative band intensities may be observed in R-A spectra of certain analytes deposited on flat substrates when compared to transmission spectra acquired from KBr disks. In order to minimize these... 

Christiansen effect n. When finely powdered substances, such as glass or quarts, are immersed in a liquid of the same index of refraction complete transparency can only be obtained for monochromatic light. If white light is employed the transmitted color corresponds to the particular wavelength for which the two substances, solid and liquid have exactly the same index of refraction. Due to differences in dispersion the indices of refraction will match for only a narrow band of the spectrum. 

ES Pett gohn, EB Christiansen. Effect of particle shape on free-settling rates of isometric particles. Chem Eng Prog 44 157-172, 1948. 

Different cells packed with appropriate materials were necessary to cover the refractive index range of 1.31 to 1.60. A diagram of a Christiansen effect detector is shown in Figure 6. 

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. 

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