Particle Size Measurements Using Optical Microscopy

5 Particle Size Measurements Using Optical Microscopy 

The globe-and-circle graticules are used to compare the projected particle area with a series of circles in the ocular graticule. The size of spherical particles can be expressed by the diameter, but for irregularly shaped particles various statistical 

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Particle size measurement is one of the essential requirements in almost all uses of colloids. However, our discussion in Section 1.5 makes it clear that this is no easy task, especially since even the definition of particle size is difficult in many cases. A number of techniques have been developed for measuring particle size and are well documented in specialized monographs (e.g., Allen 1990). Optical and electron microscopy described in the previous section can be used when ex situ measurements are possible or can be acceptable, but we also touch on a few nonintrusive methods such as static and dynamic light scattering (Chapter 5) and field-flow fractionation (see Vignette II Chapter 2) in other chapters. 

Voltammetric Sizing The average size of inert microparticles deposited on an electrode surface has been determined using cyclic voltammetric measurements [45]. This method was proposed to be used for smaller particles (i.e., if optical microscopy is not possible) as an inexpensive alternative solution to scanning electron microscopy (SEM). 

Washington (1992) has discussed the concepts and techniques of particle size analysis and its role in pharmaceutical sciences and other industries. There are many different methods available for particle size analysis. The techniques most readily available include sieving, optical microscopy in conjunction with image analysis, electron microscopy, the Coulter Counter and laser diffractometers. Size characterization is simple for spherical particles, but not for irregular particles where the assigned size will depend on the method of characterization used. Table 6.2 lists particle size measurement methods commonly used and the corresponding approximate useful size range (Mullin 1993). 

Particle size measurement with microscopy is a direct observation method, where individual particles are observed directly, so that their shape and the degree of agglomeration can be identified at the same time. It provides straightforward information on a ceramic powder. There are three types of microscopies optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Optical microscopes can be used for particle sizes larger than... 

Dodecyl acrylate microspheres, crosslinked with hexanediol diacrylate, were prepared by emulsion polymerisation and characterised by scanning electron microscopy, particle size measurement, and differential scanning calorimetry. The microspheres were doped with 9-(diethylamino)-5-(octadecanoylimino)-5H-benzo(alpha)phenoxazine (ETH 5294), a chromoionophore whose fluorescence emission spectrum is sensitive to pH. These microspheres adsorb anions selectively according to their lipophilicity, and therefore can be used as anion-selective optical sensors. The responses of the microspheres to chloride, nitrate and perchlorate ions were reported. 27 refs. 

Microscopy is often referred to as an absolute method for the determination of size and size distribution of small particles because it allows direct visualization and measurements of individual particles. Three commonly used types are optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). 

Third, in selecting a particle sizing technique, one must consider the size of the material itself. Most techniques have effective upper and lower limits of detection. Laser diffraction, for example, can measure particles from approximately 0.01 pm to several millimeters. With optical microscopy and image analysis, it becomes very difficult to resolve features that are smaller than a 0.3 pm because of the wavelength of light used in conventional optical microscopes.1... 

Microscopy is a widely used particle sizing technique in which individual particles are observed and measured. Optical microscopy is used for examination of particles from about 150—0.8 microns. For smaller particles an electron microscope is needed. A single particle has an infinite number of linear dimensions and it is only when they are averaged that a meaningful value is yielded. When a linear dimension is measured parallel to some fixed direction, the size distribution of these measurements reflects the size distribution of the projected areas of the particles [3]. ... 

Some light scattering measurements require knowledge of a particle s optical properties. The light scattering analysis uses these optical properties in a model for the calculation of particle size, and for accurate results you need to choose a model that best represents these optical properties. Picking an improper model will yield inaccurate results. Microscopy only requires a small amount of material and can be used to quickly measure these optical properties. 

The particles can be visualized using optical or electron microscopy, and specific dimensions can be measured with less subjectivity. The advent of computers also allows particles to be sized by image analysis techniques based on optical or electron photomicrographs. 

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