Sieving methods

Sieving Methods and Classification Sieving is probably the most frequently used and abused method of analysis because the equipment, an ytical procedure, and basic concepts are deceptively simple. In sieving, the particles are presented to equal-size apertures that constitute a series of go-no-go gauges. Sieve analysis presents three major difficulties (1) with woven-wire sieves, the weaving process produces three-dimensional apertures with considerable tolerances, particularly for fine-woven mesh (2) the mesh is easily damaged in use (3) the particles must be efficiently presented to the sieve apertures. 

Special attention must be paid to the interpretation of particle size data presented in terms of either weight or number of particles. Particle weight data may be more useful in sedimentation studies, whereas number data are of particular value in surface-related phenomena such as dissolution. Values on the basis of number can be collected by a counting technique such as microscopy, while values based on weight are usually obtained by sedimentation or sieving methods. Conversion of the estimates from a number distribution to a weight distribution, or vice versa, is also possible using adequate mathematical approaches, e.g., the Hatch-Choate equations. 

Sieving methods have been discussed in the previous section. 

Separation depends on the selection of a process in which the behaviour of the material is influenced to a very marked degree by some physical property. Thus, if a material is to be separated into various size fractions, a sieving method may be used because this process depends primarily on the size of the particles, though other physical properties such as the shape of the particles and their tendency to agglomerate may also be involved. Other methods of separation depend on the differences in the behaviour of the particles in a moving fluid, and in this case the size and the density of the particles are the most important factors and shape is of secondary importance. Other processes make use of differences in electrical or magnetic properties of the materials or in their surface properties. 

The narrower the particle size distribution, the higher in theory is the potential theoretical plate number. A rough sieving is achieved by a water flow, air flow, or a vibration method. A common sieving method is Hamiltonian water flow (Figure 3.4). The particle distribution can be controlled within + 1 jum by this method. A slurry of stationary phase material is allowed to float in the cylinder, and a solvent flows from the bottom to the top. The smaller and lighter particles float to the top of the cylinder and the larger and heavier particles sink to the bottom. The required particles are collected at the top of the cylinder. The selection of suspension solvent and control of the temperature are important. 

Various grades of sorbitol are available that differ in their pcuticle size distribution. Most of the particle size measurements reported for sorbitol are by sieve methods [1,7,26,27], and permit direct comparison with the vendor s specifications. For the samples used in this study, the vendor specified sieve limits for the various grades are listed in Table 1. 

Wet sieving method of sieving coal that uses water as a medium for facilitating segregation of a sample into particle size (ASTM D-4749). 

Wet sieving is recommended whenever it is possible. Instructions for sampling with wet sieving method are presented in section 3.2.3.1. If it is not possible to wet sieve the stream sediment sample in the field, the collected stream sediment material should be dry sieved. Instructions for sampling and dry sieving are given in section 3.2.3.2. 

ISO 972 1997 Chillies and capsicums, whole or ground (powdered) - specification ISO 7540 1984 Ground (powdered) paprika (Capsicum annuum L.) - specification ISO 3588 1977 Determination of degree of fineness of grinding - hand-sieving method (reference method)... 

Microspheres intended for nasal administration need to be well characterized in terms of particle size distribution, since intranasal deposition of powder delivery systems is mostly determined by their aerodynamic properties and particle sizes. Commonly used methods for particle size determinations described in the literature are sieving methods [108], light microscopy [58], photon correlation spectroscopy [66], and laser diffractometry [25,41,53,93], The morphology of the microparticles (shape and surface) has been evaluated by optical, scanning, and transmission electron microscopy [66, 95],... 

Burt used 0.5 to 1.0 g samples for the micromesh sieves and found that the efficiency increased with decreasing sample size and then only when three felvation units were used. Separation efficiency increased if the fluid flow was pulsed two or three times a second so a pulsator was added to the equipment. The technique was found to be unsuitable with micromesh sieves, which were too fragile, but was more successful with woven wire sieves. The technique was not proposed as an alternative to standard sieving methods, but may be useful if only small quantities are available, or with hazardous materials, where small samples are desirable for safety reasons. The technique has also been used to grade 5 kg samples [109] in the size range 45 to 64 pm. 

STP 447B, Manual on testing sieving methods, (1998) Guidelines for... 

The different methods of particle size analysis can be grouped into several categories size range analyses (sieve methods), wet or dry methods, and manual or automatic methods (laser-light). All these methods are described elsewhere. 

The sieves method is exemplified by the refluxing, in benzene, of 6-meth-ylthio-8-azapurine with 2,3,5-tri-O-acetyl- D-ribofuranosyl 1-chloride and molecular sieves (Linde AW-500) (21 h). This gave a 2 1 ratio of the 9- and 8-triacetylribofuranosides of 6-methylthio-8-azapurine, separated on a silica column. Treatment with ammonia gave a 61% yield of pure 8-azaadenosine (20). The ratio of ribosides could be improved to 6 1 by an increase in time and temperature. 2-Acetamido-6-methylthio-8-azapurine was ribosy-lated similarly. 

The best method of the determination of the particle size distribution of the different crospovidone types is the light diffraction measurement (e.g. Malvern Master Sizer, see Fig. 63 and 64). The particle size distribution of normal crospovidone also can be measured by sieving or air jet screening. In this dry sieving method more than 50 % of the particles are coarser than 50 pm without any swelling. Therefore even with this dry method it corresponds clearly to the definition of Type A of the Ph.Eur. monograph. 

Table 157. Particle size of copovidone (sieving method, e.g. air jet screen)...

Evidence that most of the soil organic matter occurs in association with clay has come from studies where unbound organic matter, consisting of free humic and nonhumic substances plus undecayed or partially modified plant remains, is removed by flotation in a liquid of density intermediate between the free material and the clay-organic complex (see Greenland, 1965b). Solutions of density between 1.8 and 2.0 have been used, such as a benzene-bromoform mixture. Elutriation and sieving methods have also been applied. 

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