Light diffraction methods

Frake et al. compared various chemometric approaches to the determination of the median particle size in lactose monohydrate with calibration models constrncted by MLR, PLS, PCR or ANNs. Overall, the ensuing models allowed mean particle sizes over the range 20-110/tm to be determined with an error less than 5 pm, which is comparable to that of the laser light diffraction method nsed as reference. Predictive ability was similar for models based on absorbance and second-derivative spectra this confirms that spectral treatments do not suppress the scattering component arising from differences in particle size. 

Prior to the development of the light diffraction methods and photon correlation spectroscopy to be described in chapters 6 and 8, sedimentation methods were the most widely used methods for characterizing the size distribution of a powder. In this chapter we will discuss the basic concepts involved in sedimentation techniques and review two or three methods still in relatively wide use. We will also describe briefly some of the methods which have been used extensively in past studies so that the reader may be able to assess the value of published data from the archives of powder technology. The reader interested in a detailed discussion of classical sedimentation size characterization techniques should refer to textbooks covering these earlier methods of size distribution in detail [1, 2]. 

Experimental equipment for X-ray diffraction methods has improved enormously in recent years. CCD detectors and focusing devices (Goepel mirror) have drastically reduced the data acquisition time. Cryogenic systems have been developed which allow structural studies to be extended down to the liquid helium temperature range. These developments have had important implications for SCO research. For example, fibre optics have been mounted in the cryostats for exploring structural changes effected by light-induced spin state conversion (LIESST effect). Chaps. 15 and 16 treat such studies. 

The vesicle size is an important parameter not only for in-process control but particularly in quality assurance, because the physical stability of the vesicle dispersion depends on particle size and particle size distribution. An appropriate and particularly quick method is laser light scattering or diffraction. Laser light diffraction can be applied to particles > 1 pm and refers to the proportionality between the intensity of diffraction and the square of the particle diameter according to the diffraction theory of Fraunhofer. 

Phase transitions Differential thermal analysis (DTA) or differential scanning calorimetry (DSC), light scattering and various spectroscopic techniques, diffraction methods (especially X-ray diffraction), measurement of thermal expansion and any other property changing with the transition... 

The quality of the relief thus obtained can be determined directly in the course of etching because the light reflected from the etched grating produces a diffraction pattern that characterizes the depth and shape of the grating profile at any given moment. To this end, one has just to measure the intensity of the reflected light. The method possesses a high sensitivity the occurrence of the relief can be detected when its depth is only of the order of 0.01 /an. 

Conclusion. This book is concerned with optical and X-ray methods but there are of course other methods of studying the structures of crystals and molecules. The diffraction of electrons and of neutrons depends on the same general principles as that of X-rays or visible light but there are important theoretical differences, and the expert mental arrangements are very different. It is not proposed to deal with either of these subjects here, but a few remarks will be made on their relation to the X-ray diffraction method. 

Another optical technique, called the back-light scattering (Kossel-diffraction) method can also be used to investigate structure in food emulsions and foams. In this method, the emulsion (or foam) in a transparent vessel is illuminated by a collimated laser beam (See... 

The characterization of emulsions by particle size distribution analysis has been facilitated in recent years by a range of new instruments. Most of these instruments employ laser light diffraction principles, and have replaced older spectrophotometric methods. 

In this book, particles larger than 1 pm are of primary interest, and thus, only the Fraunhofer diffraction method, which can account for particles larger than 2-3 pm, is discussed here. The Fraunhofer diffraction theory is derived from fundamental optical principles that are not concerned with scattering. To obtain the Fraunhofer diffraction, two basic requirements must be satisfied. First, the area of the particle or aperture must be much smaller than the product of the wavelength of light and the distance from the light source to the particle or aperture. Second, this area must also be smaller than the product... 

Intermediate Alkali Feldspars. The alkali feldspars with intermediate composition vary in their TL signal when seen in detail. All of them differ significantly as a group from the end members. However, several interesting common features throw light on their overall behaviour. These features suggest a new means for identifying structure, which differs from the traditional optical and X-ray diffraction methods. 

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