Particle size analysis instrumental method

Data from a number of different particle size analysis instrumental methods including light scattering, field flow fractionation, hydrodynamic chromatography and microscopy were obtained for a series of polymethylmethacrylate latexes and were compared to DCP results (2). These and other comparative results have demonstrated the accuracy of the instrument and method. The reproducibility and precision of the instrument also were studied and are reported elsewhere ( 1 ). 

This is the classical method of carrying out particle-size analysis. Coulter supply two instruments—the Model ZM (video display optical) and the top-of-the-range multisizer—the latter having built-in video display of results. 

As already indicated in Chapter 7, the introduction of laser technology has already had a major impact on light-scattering methods. These have found particular application in the development of new methods of particle sizing, and several instruments are now available commercially which arc designed for the automatic determination of particle size distributions. These methods are being developed steadily, especially in terms of the associated computer software needed for the rapid analysis of experimental data. In particular, while the measurement of the particle size in monodisperse systems is well established, the mathematical analysis for polydisperse systems and for non-spherical particles presents problems which are not yet fully solved. 

This section of the chapter is divided into two parts. The first part discusses the instruments and methods used to evaluate particle morphology. It pays particular attention to nomenclature since the words used in this field are often ambiguous. The second part deals with the details of particle size analysis by microscopy. It pays particular attention to sampling issues and to the use of image analysis. 

Section 6 consists of four chapters devoted to miscellaneous instrumental methods with emphasis on thermal, radiochemical, and automated methods. A chapter on particle size analysis is also included in this final section. 

There are two important reasons for comparing the performance of different particle sizing techniques/instruments for a particular apphcation. First, it is necessary to establish their reliability or, in the case of certain instruments, to verify that they are capable of yielding substantially the same particle size or PSD. The degree of success depends not only on the techniques in question but also on the nature of the system to which they are applied. Second, it may become clear that the use of two (or more) different measurement techniques to characterize a given kind of material may yield additional information that might not be obtainable from a single analysis method and which therefore would otherwise be overlooked. 

Information on particle size may be obtained from the sedimentation of particles in dilute suspensions. The use of pipette techniques can be rather tedious and care is required to ensure that measurements are sufficiently precise. Instruments such as X-ray or photo-sedimentometers serve to automate this method in a non-intrusive manner. The attenuation of a narrow collimated beam of radiation passing horizontally through a sample of suspension is related to the mass of solid material in the path of the beam. This attenuation can be monitored at a fixed height in the suspension, or can be monitored as the beam is raised at a known rate. This latter procedure serves to reduce the time required to obtain sufficient data from which the particle size distribution may be calculated. This technique is limited to the analysis of particles whose settling behaviour follows Stokes law, as discussed in Section 3.3.4, and to conditions where any diffusive motion of particles is negligible. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

Many diffuse-reflectance instruments are available. Some employ several interference filters to provide narrow bands of radiation. Others are equipped with grating monochromators. Ordinarily, calibration is often a stringent requirement as samples must be acquired of the material for analysis that contain the range of analyte concentrations likely to be encountered. It may be useful to grind solid samples to a reproducible particle size. Equations are developed and used for the analysis. Once method development has been completed and validated, solid samples can be analyzed in a few minutes. Accuracy and precision are reported to be of 1 to 2% relative. 

Particle size is one of the principal determinants of powder behavior such as packing and consolidation, flow ability, compaction, etc., and it is therefore one of the most common and important areas of powder characterization. Typically, one refers to particle size or diameter as the largest dimension of its individual particles. Because a given powder consists of particles of many sizes, it is preferable to measure and describe the entire distribution. While many methods of size determination exist, no one method is perfect (5) two very common methods are sieve analysis and laser diffraction. Sieving is a very simple and inexpensive method, but it provides data at relatively few points within a distribution and is often very operator dependent. Laser diffraction is a very rapid technique and provides a detailed description of the distribution. However, its instrumentation is relatively expensive, the analytical results are subject to the unique and proprietary algorithms of the equipment manufacturer, and they often assume particle sphericity. The particle size distribution shown in Figure 1 was obtained by laser diffraction, where the curves represent frequency and cumulative distributions. 

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