Particle microscopic methods

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. 

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. 

Comparison of Microscopic Method of Particle Size Analysis — Tungsten M-10. ... 

J.W. Lavitt, A Microscopic Method for the Determination of the Particle (Crystal) Size Distribution of 2 Micron RDX , PATR 1909 (1953) 20) J.W. Lavitt, An Improved Micro-... 

Kaye, An Electron Microscope Method for the Determination of the Particle Size Distribution and Particle Shape of Colloidal and Ball-Milled Lead Azide , PATR 2133 (1955) 25a) A.T. 

The advantage of the optical microscope method is that it provides direct and absolute information on the particles under characterization. Its chief disadvantage is that it can only provide data on the particles on the slide, and it can therefore be biased by the method used to prepare the slide. 

Probably the most extensive use of particle morphology and microscopy has been in the area of chemical microscopy. With this approach, derivatives of the analyte species are prepared, crystallized, and identified through the morphological characteristics of these derivatives [21]. Most of these applications have been superseded by modem methods of analysis, but the microscopic method can still be used by skilled practitioners for the study of trace quantities of analyte. The literature developed during the heyday of chemical microscopy is too large to be reviewed here, but advances in the field are still chronicled in the Annual Reviews issue of Analytical Chemistry [22]. A substantial review of the optical characteristics of organic compounds is available [23]. 

In all microscopic methods, sample preparation is key. Powder particles are normally dispersed in a mounting medium on a glass slide. Allen [7] has recommended that the particles not be mixed using glass rods or metal spatulas, as this may lead to fracturing a small camel-hair brush is preferable. A variety of mounting fluids with different viscosities and refractive indices are available a more viscous fluid may be preferred to minimize Brownian motion of the particles. Care must be taken, however, that the refractive indices of sample and fluid do not coincide, as this will make the particles invisible. Selection of the appropriate mounting medium will also depend on the solubility of the analyte [9]. After the sample is well dispersed in the fluid, a cover slip is placed on top... 

Therefore, no detailed discussion on the interaction of any liposomes with any particular cell type should be stated here. We refer to several recent publications for a study of interaction of DOPE CHEMS liposomes and COS-7 and HUVEC (109) and for a study on size-dependent uptake of particles into B16-F10 (72). The combination of flow cytometry and a microscopic method (e.g., spectral bio-imaging) turned out to be highly useful both to study the initial mode of internalization and to follow the intracellular fate of liposomes and other particulate carrier systems. 

We presented fully self-consistent separable random-phase-approximation (SRPA) method for description of linear dynamics of different finite Fermi-systems. The method is very general, physically transparent, convenient for the analysis and treatment of the results. SRPA drastically simplifies the calculations. It allows to get a high numerical accuracy with a minimal computational effort. The method is especially effective for systems with a number of particles 10 — 10, where quantum-shell effects in the spectra and responses are significant. In such systems, the familiar macroscopic methods are too rough while the full-scale microscopic methods are too expensive. SRPA seems to be here the best compromise between quality of the results and the computational effort. As the most involved methods, SRPA describes the Landau damping, one of the most important characteristics of the collective motion. SRPA results can be obtained in terms of both separate RPA states and the strength function (linear response to external fields). 

The method selected depends upon the kind of material to be measured. If particles are confined to narrow limits of size, screens or microscopic methods of direct measurement may be used. When particles are distributed over a wide range of sizes we must choose indirect methods such as sedimentation or centrifuging. There is no simple method of measurement in either case, and the results are not always susceptible of interpretation unless the composition of the material is known. This will be even more evident when we consider sedimentation methods applied to particles varying widely not only in size but also in density. 

Microscopic methods have certain advantages provided that a representative distribution of particles can be prepared for examination. Using refined techniques, sizes as small as 0.5 /x are readily measured with ordinary microscopes. Electron microscopes permit resolution to sizes thousands of times smaller, and indeed, this method is at present the only one which can be used on discrete particles of extremely fine size. The two-dimensional aspects of microscopic measurements often render this technique unsatisfactory. Furthermore, it is not always possible to obtain necessary shape factors to yield accurate volume and surface computations. 

An attractive interaction arises due to the van der Waals forces between molecules of colloidal particles. Depending on the nature of dispersed particles, the Keesom forces (or the dipole-dipole interaction), the Debye forces (or dipole-induced dipole interaction), and the London forces (or induced dipole-induced dipole interaction) may contribute to the van der Waals interaction. First, the van der Waals interaction was theoretically computed using a method of the pairwise summation of interactions between different pairs of molecules of the two macroscopic particles. This method called the microscopic approximation neglects collective effects, and, as a consequence, misrepresents the Hamaker constant. For many problems of a practical use, however, specific features of the total interaction are determined by a repulsive part, and such an effective, gross description of the van der Waals interaction may often be accepted [3]. The collective effects in the van der Waals interaction have been taken into account in the calculations of Lifshitz et al. [4], and their method is known in the literature as the macroscopic approach. 

The preservation of particle character and size throughout polymerization itself is very hard to determine. The size of the final polymer particles is easily determined by light scattering or microscopic methods since the dispersions can be diluted without changing the particle size. Measurements of the emulsion droplets in concentrated media on the other hand are a very difficult task and have already been discussed above. 

In the most simplistic means of defining particle shape, measurements may be classified as either macroscopic or microscopic methods. Macroscopic methods typically determine particle shape using shape coefficients or shape factors, which are often calculated from characteristic properties of the particle such as volume, surface area, and mean particle diameter. Microscopic methods define particle texture using fractals or Fourier transforms. Additionally electron microscopy and X-ray diffraction analysis have proved useful for shape analysis of fine particles. 

Initiation of polymerization and individual phases of polymer growth on Si02-supported catalyst particles can be followed by a combination of kinetic and microscopic methods. A few minutes after exposure to propylene the polymerization rate reaches an initial maximum, which is followed by a period of low activity (Figure 23). In a third phase the polymerization rate rises again and in a final, fourth phase, a broad maximum of activity is reached. This... 

Light Microscopic Method. Phase contrast microscopy (PCM) accurately assesses fiber exposure levels for fibers 5 pm in length and >0.25 pm in diameter. Furthermore, PCM cannot differentiate between asbestos and nonasbestos fibers. Currently, the standard method for the determination of airborne asbestos particles in the workplace is NIOSH Method 7400, Asbestos by Phase Contrast Microscopy (NIOSH 1994a). OSHA considers that sampling and analytical procedures contained in OSHA Method ID-160 and NIOSH Method 7400 are essential for obtaining adequate employee exposure monitoring. Therefore, all employers who are required to conduct monitoring are required to use these or equivalent methods to collect and analyze samples (OSHA 1994). In NIOSH Method 7400, asbestos is collected on a 25 mm cellulose ester filter (cassette-equipped with a 50 mm electrically-conductive cowl). The filter is treated to make it... 

The results of measurements by the microscopic method show that the electrophoretic mobility of the particles varies with the distance from the wall of the cell particles close to the wall move in a direction opposite to that in which those in the center migrate. In any event, the results show an increase in velocity from the walls to the center of the cell. The explanation of this fact lies in the electro-osmotic movement of the liquid a double layer is set up between the liquid and the walls of the cell and under the influence of the applied field the former exhibits electro-osmotic flow. For the purpose of obtaining the true electrophoretic velocity of the suspended particles it is neceasary to observe particles at about one-fifth the distance from one wall to the other. A more accurate procedure is to make a series of measurements at different distances from the side of the cell and to apply a correction for the electro-osmotic flow. The algebraic difference of the corrected electrophoretic velocity and the speed of the particles near the walls gives the electro-osmotic mobility of the liquid in the particular cell. If the solution contains a protein which is adsorbed on the surface of the walls of the vessel and on the particles, it is possible to compare the electrophoretic and electro-osmotic mobilities in one experiment reference to the significance of such a comparison was made on page 532. 

Fig. 3.2 The British Standard Graticule (Graticules Ltd) [11], Diagram from BS 3406 (1963), Confirmed April 1993 Methods for the Determination of the Particle Size of Powders-. Part 4, Optical Microscope Method. (Reproduced by permission from British Standards Institution, 2 Park Street, London Wl, from whom copies of the complete standard may be obtained.)...

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