Size methods microscope

Before choosing a particle sizing technique, examination of the samples under a microscope is usually wise becau.se the range of sizes and shapes present can then he estimated. Most particle size methods are sensitive to particle shape and all are limited with respect to the particle size range. Particles with approximately spherical shapes are measured most accurately. Needles and other shapes that differ significantly from spherical are often analyzed by microscopy. 

Standard latexes, and other reference materials, are available from various organizations [18—24]. One of the calibrations standards available to fineparticle scientists are latex spheres which were made on board the space shuttle in 1985. Because these spheres were formed in the absence of gravity they are perfectly spherical. The National Bureau of Standard makes available standard reference material in the form of ten micron microspheres mounted on glass slides. In the first type of slide a few thousand microspheres are deposited as a regular array on a glass microscope slide. In the other type, the fineparticles are randomly distributed [18]. A series of standard non-spheri-cal fineparticles have been prepared by the Community Bureau of Reference Commission of the European Community for use in comparing the performance of size methods. These reference powders are known as BCR standards and several publications are available describing the use of such reference materials [19]. 

Microscopic observation of aerosol particles permits direct measurement of particle size. This is in contrast to indirect methods such as sedimentation, impaction, mobility analysis, and light scattering, wherein the particle size is estimated from the measurement of a property related to size. Microscopy also provides the opportunity to observe particle shapes, and it requires only an extremely small amount of sample. Linear measurements made with a microscope can be very accurate and, often serve as a primary measurement for the calibration of other aerosol-sizing methods. However, microscopic methods for determining particle size distributions are, in general, tedious and require consistency, skill, and careftd preparation. 

Light microscopy allows, in comparison to other microscopic methods, quick, contact-free and non-destmctive access to the stmctures of materials, their surfaces and to dimensions and details of objects in the lateral size range down to about 0.2 pm. A variety of microscopes with different imaging and illumination systems has been constmcted and is conunercially available in order to satisfy special requirements. These include stereo, darkfield, polarization, phase contrast and fluorescence microscopes. 

One of the most important uses of specific surface determination is for the estimation of the particles size of finely divided solids the inverse relationship between these two properties has already been dealt with at some length. The adsorption method is particularly relevant to powders having particle sizes below about 1 pm, where methods based on the optical microscope are inapplicable. If, as is usually the case, the powder has a raiige of particle sizes, the specific surface will lead to a mean particle size directly, whereas in any microscopic method, whether optical or electron-optical, a large number of particles, constituting a representative sample, would have to be examined and the mean size then calculated. 

Physical testing appHcations and methods for fibrous materials are reviewed in the Hterature (101—103) and are generally appHcable to polyester fibers. Microscopic analyses by optical or scanning electron microscopy are useful for evaluating fiber parameters including size, shape, uniformity, and surface characteristics. Computerized image analysis is often used to quantify and evaluate these parameters for quaUty control. 

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

Microscopic identification models ate similar to the CMB methods except that additional information is used to distinguish the source of the aerosol. Such chemical or morphological data include particle size and individual particle composition and are often obtained by electron or optical microscopy. 

Microscope Methods In microscope methods of size analysis, direct measurements are made on enlarged images of the particles. In the simplest technique, linear measurements of particles are made by using a cahbrated scale on top of the particle image. Alternatively, the projected areas of the particles can be compared to areas of circles. 

Membrane Cliaraeterization MF membranes are rated bvtliix and pore size. Microfiltration membranes are imiqiielv testable bv direct examination, but since the number of pores that rnav be obsen ed directlv bv microscope is so small, microscopic pore size determination is rnainlv useful for membrane research and verification of other pore-size-determining methods. Furthermore, the most critical dimension rnav not be obseiA able from the surface. Few MF membranes have neat, cvlindrical pores. Indirect means of measurement are generallv superior. Accurate characterization of MF membranes is a continuing research topic for which interested parties should consult the current literature. 

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