Laser Diffraction Technique

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

A first study refers to liquid water [77]. The signals AS q,x) and A5[r, x] were measured using time-resolved X-ray diffraction techniques with 100 ps resolution. Laser pulses at 266 nm and 400 nm were employed. Only short times X were considered where thermal expansion was assumed to be negligible, and... 

The apphcation of the laser diffraction technique is sometimes questioned becanse it measures geometric instead of aerod5mamic particle diameters. However, the aerod5mamic diameter can be calcnlated when the dynamic shape factor and density are known. Moreover, the dynamic shape factor (x) of micronized particles will often be only shghtly higher than 1 and so is the tme particle density (1.0 < pp < 1.4 g cm ). As a conseqnence, the aerodynamic diameter differs only slightly from the eqnivalent volume diameter (see Eq. 3.1). 

A first study refers to liquid water [77]. The signals AS q,x) and A5[r,r] were measured using time-resolved X-ray diffraction techniques with 100 ps resolution. Laser pulses at 266 and 400 nm were employed. Only short times x were considered, where thermal expansion was assumed to be negligible and the density p to be independent of x. To prove this assumption, the authors compared their values of AS q, x) to the values of AS q) obtained from isochoric (i.e., p = const) temperature differential data [78-80]. Their argument is based on the fact that liquid H2O shows a density maximum at 4 °C. Pairs of temperatures Ti, T2 thus exist for which the density p is the same constant density conditions can thus be created in this unusual way. The experiment confirmed the existence of the acoustic horizon (Fig. 8). 

These practical issues of particle shape and dispersion are not intended to cast aspersions on the laser diffraction technique rather, these factors have been discussed to bring awareness around the analytical results that are obtained when these factors are present. Laser diffraction has proven itself to be a reliable, robust technique for particle size analysis. When the assumption of nonaggregated spherical particles is violated, there are clear manifestations in the calculated particle size distribution. When analyzing drug substances that are used in low-dose solid oral formulations, the impact of these manifestations can be particularly impactful as there is often a limited number of API lots to be used for method development. Therefore, the analyst must be aware of these issues prior to the commencement of method development to avoid these pitfalls. In addition to the information contained in ISO 13320, Snorek et al. have written a summary around the general practices of laser diffraction measurements in the pharmaceutical industry.19... 

Since image analysis techniques are not as widely used as laser diffraction techniques, the common tendency is to compare all size data back to laser diffraction... 

The authors thank R. J. Collins and J. A. Corruthers of the Department of Electrical Engineering, University of Minnesota, for their many helpful suggestions and discussions on the application of laser diffraction techniques. They also thank W. T. Peria of the same department for loaning some laser equipment. The various services provided by the Department of Aerospace Engineering and Mechanics of the University of Minnesota are gratefully acknowledged. 

Laser diffraction technique is able to generate the envelope diameter distribution of the aggregates in the aerosol cloud, as well as the surface and mass fractal dimensions as explained in Light Scattering under both Boundary and Surface Fractal Dimensions and Mass Fractal Dimension. However, validation of Eq. (25) by comparison studies between the impaction and laser diffraction techniques is required. 

Laser-scattering techniques have been used in a variety of operational modes to size pharmaceutical aerosols. These may be listed as forward scattering, Fraunhofer diffraction, and laser Doppler velocimetry, in conjunction with either accelerating nozzles or acoustic excitation. 

Particle size analysis should be carried out on all batches of salt candidates to establish suitable recrystallization procedures. A rapid assessment of both the particle size and crystal habit can be carried out by Scanning Electron Microscopy (SEM). Laser diffraction techniques can provide a rapid assessment of the particle size distribution using less than lOmg of material. It is also useful to retain a photomicrograph for each batch. 

To assess emulsion formation, the normal approach is to measures the droplet size distribution using, for example laser diffraction techniques. A useful average diameter d is,... 

Mass spectrometry can be improved by using the MS/MS tandem system that assures a high resolution for structure determination. The structural analysis cannot exist without MS, FT-IR, and NMR techniques accompanied by X-ray diffraction, neutron diffraction, and laser Raman techniques. 

A significant role in future research will be played by molecular dynamics simulation as well as other new and very promising computational techniques, made possible by the advent of supercomputers. Also of importance are new and more sophisticated experimental techniques, such as NMR, laser correlation spectroscopy, neutron diffraction, and laser fluorescence bleaching recovery. These new techniques will surely yield new insights into the mechanisms of transport of small molecules in both rubbery and glassy polymers. 

Laser diffraction technique The technique of evaluating the size of grains in a sediment mixture by evaluating the amount of diffraction of light by the particles. 

Field scan techniques Laser diffraction, sound attenuation, the photon... 

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