Particle size analysis laboratory techniques

Frequency domain photon migration (FDPM) has been investigated as an optical technique with potential application to particle size analysis,67 albeit in a laboratory environment. The approach could be readily implemented in situ, with appropriate... 

We have included a new chapter on pariiclc size determination (Chapter 34). The physical and chemical properties of many research materials and consumer and industrial produces arc intimately related to their particle size distributions. As a result, particle size analysis has become ati iniporlant technique in many research and industrial laboratories. 

Sampling is by far the most important part of particle size analysis by microscopy (and probably all particle size techniques). A kilogram of drug substance will contain many millions of particles. Since, at most, the particle size analysis samples a few thousand particles, the measured particles must be selected with care. Allen [20] presents an extensive discussion of bulk sampling issues relevant to all particle size analysis, irrespective of the particular technique. Our interest, though, is primarily directed toward sampling as it relates to the specimen used for particle size analysis by microscopy. We will assume that the 50 mg or so of sample dehvered to the laboratory is truly representative of the bulk powder. 

It is often said that particle size analysis can be only as good as the sampling technique used for collecting the sample. As all laboratory techniques (and also some on-line techniques) use a small sample taken from a production stream in which particle stratification or segregation often takes place, the correct sampling technique is critical for the accuracy of the eventual particle size analysis data. 

Chu 1991 Schmitz 1990). For example, the dynamic version of the diffusing wave spectroscopy described in Vignette V is a form of DLS, although in diffusing wave spectroscopy the method of analysis is different in view of multiple scattering. Most of the advanced developments are beyond the scope of this book. However, DLS is currently a routine laboratory technique for measuring diffusion coefficients, particle size, and particle size distributions in colloidal dispersions, and our objective in this section is to present the most essential ideas behind the method and show how they are used for particle size and size distribution measurements. 

Analytical characterization includes measurement of absolute sizes and concentrations of species present in the catalyst. For the purpose of clarity, these techniques have been organized, starting with the bulk macroscopic properties, down to the component, microscopic features. The underlying goal of analytical characterization is to provide information about the sample which will allow research personnel to relate the properties measured to some aspect of a catalyst s performance, either in the field or in the evaluation laboratory. Macroscopic characterization includes both chemical compositions and physical properties such as particle size, density and total surface area. Chemical analysis techniques are well... 

Fairs [79] used a projection microscope for training purposes. This technique can also be used for size analysis [80] but is not recommended for particles smaller than 2 pm. Hamilton et. al. [81 c/7.82] demonstrated the need to train operators and showed that gross count differences on the same samples at different laboratories were much reduced after interlaboratory checks. 

Because there are many ways to define particle size, it follows that there are many different methods for analysis of size involving an equally diverse array of instruments and techniques. Up-to-date summaries of these methods and instruments, and the theory behind the size determination, can be found in Knapp etal. (1996), Washington (1992), Syvitski (1991) and McManus (1988). The more traditional techniques (i.e., non-automated and generally non-electronic) have been discussed in Krumbein Pettijohn (1938), A.S.T.M. (1959), Irani Callis (1963), Muller (1967) and Jelfnek (1971), and are also summarized in most standard sedimentological textbooks and laboratory manuals (e.g., Lewis McConchie, 1994a, b Boggs, 1995 Lewis, 1984 Friedman Johnson, 1982 Blatt et al., 1980). 

A particle size measurement does not have a meaning unless the objective of the measurement is also specified. Thus, the techniques which should be used depend entirely on the accuracy which is required and the circumstances of place and time in which the measurements must be made. There is no such thing as the "best particle size technique unless the circumstances are also specified. A particle analysis laboratory cannot operate on the same basis as some normal analytical laboratories, samples come in and numbers go out. In a particle analysis, the question must always first be addressed. "What do we need to know " Another way of saying this is that, eventually, particle analysis is an engineering tool, not a basic science. Engineers must use all the known laws of science to solve the particular problem with which they are confronted in the most elegant manner. 

KBr Pellets, In many laboratories nujol mulls have been replaced to a large extent by the KBr-pellet technique. In this technique the sample is again ground to a fine, uniform particle size (preferably under 2 pi), but is mixed with an alkali halide -usually KBr) instead of being dispersed in nujol. This sample-KBr mixture is pressed into a pellet. In this technique both the sample and the KBr are weighed, thus making the technique useful for quantitative analysis for many types of samples. Normally pellets are made to have a total weight of 300 mg of this total, 0.5-1 % is sample. 

Hatch extended his method of analysis to size-distribution curves ranging from coarse-screen analysis through fine particles measured microscopically. While excellent results were obtained by using this technique on laboratory samples, the method cannot be generalized to cover all types of distributions encountered in practice. As already explained in Chapter 3, size-frequency distributions may assume a variety of shapes. The Hatch development applies only to distributions which follow the normal or log-probability law. When size-distributions are hyperbolic in the lower extremes and follow normal log-probability laws in the upper extremes, the Hatch analysis must necessarily fail. Nevertheless, the relationships developed by Hatch have a far-reaching practical importance... 

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