Direct measurement of particle size

In fact, pore size and surface area are established by an underlying parameter the particle size of the (secondary) particles. Direct measurement of particle sizes yields information on surface area and porosity and vice versa. 

HRTEM allows the direct measurement of particle sizes larger than 5-10 A. Depending on contrast, thickness of the sample and location of the Pt particle on the support, particles smaller then 5-10 A are invisible. As these particles contain only a few atoms, they consist exclusively of surface atoms. Therefore, these small particles are extremely important in catalysis. 

The major advantage of microscopic methods is their direct measurement of particle size. In many of the alternative methods, at least one automated data-interpretation or calculation step is inserted between the instrumental analysis and establishing the estimate of particle size. This reduces the subjectivity of the measurement while increasing the likelihood of interpretive errors. 

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. 

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

Insitec now forms part of Malvern Instruments but is still based in California for process and laboratory R D. The EPCS are laser-based instruments for in-line particle measurements that provide information on particle volume concentration and size distribution. EPCS instruments are part of the larger group of electro-optical instruments (MALLS) whose principle of operation is based on light scattering from a group (or ensemble) of particles. Unlike other instruments operating on this principle, the EPCS can perform direct measurements of particle laden flow stream provided the concentration is within operating limits. 

One of the major uses of transmission electron microscopy in the area of catalysts is the measurement of particle size distributions for supported metals. Chemical techniques can only effectively be used to obtain a global value for the dispersion. By observing the particles directly in transmission electron microscopy, it is possible to check the heterogeneity, and detect the existence of bimodal size distributions. Figure 9.8 shows an example of metallic particles distributed in a zeolite. 

The measurement of particle size is a key issue in the formulation of many pharmaceutical products. Particle size distribution is known to directly influence physical properties of powders, such as dissolution rate, powder flow, bulk density, and compressibility. Conventional methods of particle size measurement include sieve analysis and laser diffractometry. ... 

If we knew the primary particle size and coefficient of penetration (both of which can be determined from TEM images), we would then obtain the number of primary particles as a direct measure of the size of the aggregate. 

Electron microscopy allows one to analyze the average particle size, the number of particles per agglomerate, and the projected area from which a calculation of the void volume of each aggregate can be done. Centrifugal sedimentation allows direct measurement of the size distribution of aggregates... 

As indicated above, physical methods provide a means by which the dispersion is derived from measurement of particle size. In actual fact, the value referred to as particle size when obtained by X-ray diffraction, should be termed crystallite size as the particle may contain several crystallites and the parameter being measured is the effective length in the direction of the diffraction vector, along which there is coherent diffraction. Additionally, for the case of X-ray diffraction, the value obtained is inherently an average value given that the sample will contain a distribution of particle sizes and the quantity to be determined from the XRD pattern is the... 

For some dosage forms and applications, microscopy with image analysis may be the method of choice. For instance, topical suspensions containing a number of sohd excipients make it difficult to directly measure the particle size of the drag substance. Measurement difficulties will be particularly severe, the more stable the suspension, since it will be quite difficult to mechanically... 

Measurement of particle size directly by visual observation is one of the oldest and most straightforward techniqnes available. Today, with widespread use of desktop image analysis equipment and other compnter-assisted techniques, direct estimation of size still holds considerable promise as a routine paleolimnological tool. 

A very direct way of measuring the fractal dimension is to have direct measurement of the size of individual aggregates and of the amount of solid that comprises them, a technique sometimes referred to as the particle concentration technique... 

In an inertial microfiuidic device, concentration, efficiency, and purity are the most broadly used parameters to quantitatively characterize device performance. For this, it is necessary to know concentration of separated particles at each outlet. The most common approach is to collect samples fi om each outlet and then to use flow cytometry to count and size particles. An alternative approach is to use a hemocytometer for direct measurement of particle concentration in each outlet. While hemocytometers offer a low-cost option, the increased error rate (as high as 10 %) will require a larger sample size to maintain the confidence interval. Once particle counts in each outlet are known, purity and efficiency of the separation can be calculated. Purity is calculated as the number of target particles over total particles in one outlet. Efficiency is calculated as the number of target particles from one outlet over total number of target particles from all outlets. Next, the modulated aspect-ratio device is used as an example to demonstrate these calculations. 

Instrumental techniques for measurement of particle size distribution of powders have had a tremendous advancement in recent times. Numerous methods and procedures have been developed at a steady pace over the years, and there is the possibility of covering the wide spectrum from nanosystems, to ultrafine powders, and to coarse particulate assemblies. Many instruments offer nowadays quick, reliable results for a wide variety of powders and particulate systems, and for a number of applications. There is still, however, the need to understand the basic principles under which sophisticated instruments operate, as well as to resource to direct measurements under some circumstances. Some of the most modern instrumental techniques are based on an indirect measurement and carry out transformations among the different ways of expressing particles size distributions, that is, by number, surface, or mass. Sometimes it is advisable to avoid transformations because instruments assume a constant shape coefficient on such transformation, which is not necessarily the case, and overestimation or underestimations of size of certain particles may arise. Also, in very specific applications, or in cases of basic or applied research, is better to measure directly the most relevant particle size and particle size distribution. For example, if research is carried out in modeling of solid-liquid separations, a direct measurement of the Stokes equivalent diameter would be most appropriate. The aim of the exercise is to measure the particle size distribution of a sample of medium-sized dolomite, and compare the results with those of the Andreasen Pipette method. 

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