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A Guide to Choosing a Particle Sizer By Bruce B. Weiner PhD, Co-founder and Consultant to Brookhaven Instruments Corporation
The choice of a particle size analyser has never been more difficult. There are several techniques from which to choose and variations within each technique. Sales literature claims of specification and performance have become highly inflated, confusing the first-time buyer; the result has been to hinder and not help the decision making process. Many particle sizing instruments were originally designed to address specific problems. Although some have found additional uses, there is still some truth to the notion that certain techniques are best suited for particular tasks. The idea that one instrument will suit every particle sizing need, and hence solve all problems, is not supported in practice.
Limited in scope
This guide does not specifically address imaging problems, shape analysis, single particle counting, nor sizing of airborne particles. Examples are drawn from particle sizing in liquids where the amount of material is not of primary concern; the ‘dirty water’ or microcontamination problem is excluded.
This document is a brief summary based on many years of experience with the modern methods of particle size analysis; it is not definitive. New techniques and new applications of old techniques appear at an ever-increasing rate. Yet, the concepts presented here are general enough to be of value for several years to come. The author would welcome any comments you may have and is always available to answer any specific questions.
Classifications Particle sizing techniques can be classified in several ways.
Size Range: Many interesting applications in particle size analysis centre around 1 micron. Figure 1 shows several commercially available techniques for particle sizing with a purposefully ‘fuzzy’ demarcation around 1 micron.
Why is the region around 1 micron so important? There are several reasons.
First, this region is roughly the dividing line between sedimentation and centrifugation. For particles that are dense and/or larger, sedimentation works well. For particles that are not dense and/or smaller, centrifugation works well. Since both density and size play a role, the choice of technique depends on both of these properties.
Second, this region is roughly the dividing line between Fraunhoffer Diffraction (FD) and light scattering. For particles that are larger, the classical FD technique is independent of the refractive index of the particle. For particles that are smaller, the scattering pattern depends significantly on the refractive index of the particle.
Third, measurements become increasingly difficult with zone counting (ZC: electro- and photozone) techniques below this region. Electrozone techniques suffer from signal-to-noise problems, and photozone techniques suffer from diffraction effects as do optical scanners. In addition ZC techniques suffer from increasing coincidence errors at these smaller sizes. Fourth, the ability to resolve images with an optical microscope becomes increasingly difficult below about a micron.
All of the statements above are generalisations. Yet they provide good, first-order, estimates of the practical working limits of any one technique. In special cases these limits may be exceeded. But be wary of size range claims without qualification.
Imaging vs. Nonimaging: Instruments based on imaging are, potentially, capable of measuring shape, structure, and texture in addition to concentration and size. They can, ideally, distinguish between different compositions. Imaging techniques include optical and electron microscopy, video, holography, and photography. Image analysers are often, but mistakenly, thought of as the primary method of particle size analysis.
Yet image analysis has many disadvantages and difficulties. Typically, too few particles are measured to give reliable statistical results. Manual image analysis is subjective, slow, and labour intensive. Like other single particle counters, image analysers may suffer from coincidence effects. When automated and computerised, the cost mounts, and coincidence effects may be more difficult to recognise.
Non-imaging techniques yield equivalent spherical diameters (ESD). This is the diameter of a sphere that would give the same result as the actual particle. Thus, different techniques may yield different equivalent spherical diameters for the same particle. These differences are valuable: They reveal information on the shape, structure, or texture of the particle. Nevertheless, if definitive information of this type is required, then an image analyser is necessary.
Degree of Separation: Another major classification is the degree to which particles are separated prior to measurement. There are three categories here: single particle counting; fractionation, both partial and high resolution; and ensemble averaging.
Single particle counters (SPCs) include image analysers, electro- and photozone counters, and particle scanners. Like image analysers, SPCs suffer from coincidence counting effects. The zone counters are also subject to clogging of the zone. Additionally, electrozone counters normally require high salt concentrations to work properly, and this may cause aggregation. Yet SPCs are the preferred choice when particles must be counted as well as sized.
Figure 1. Commercially available particle sizing techniques (mostly liquid suspensions)
Fractionation techniques include sieving, sedimentation, centrifugation, and various forms of particle chromatography. Depending on how the measurement is carried
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