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| Particle Size Distribution | ||||||||||
| Methods for measuring Particle Size Distribution | ||||||||||
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A. Sieve analysis Traditionally, sieves are used for determining the PSD of lactose powders. They function very well in the size range of approximately 40 microns to millimetres. (larger lactose particles are seldom encountered). The sieves mostly used for analytical purposes are wire mesh screens. The apertures have square cross sections. All apertures of the same sieve are basically of the same magnitude. Nominal aperture size of the screen is commonly denoted in µ or mm.  Sieves with different aperture sizes The aperture size of analytical sieves used to be characterised by a mesh number. This number is related to the number of wires per surface area in the weave. Thus actual aperture size depends on the thickness of the wire. As a consequence, a high mesh number corresponds to a small aperture size, and vice versa. In different countries different systems were in use, e.g. ASTM (USA), BS (UK), Tyler (UK) and DIN (Germany). Often, sieve analyses and PSD specifications made up in one of these obsolete denotations are still encountered. Sieving is the most direct way of establishing particle size distributions. With one sieve, two fractions are obtained, a fine and a coarse fraction. The fractions are weighed accurately. An ideal, very sharp separation is never obtained. There are several reasons for this, of which the most obvious are
In other words, the sieving process does not have a clear end point. The end point has to be chosen arbitrarily. Usually when the sieving rate (the weight passing the screen per time unit) falls below a certain predetermined value, the sieving process is considered to be complete. For accurate determinations, sieves should be calibrated. This can be done microscopically or by using standard reference materials of an exactly known particle size distribution. Sieving has the advantage over modern instrumental techniques (e.g. light scattering techniques) that the sample on which the analysis is performed is actually physically subdivided into fractions of different size classes, after which they can be studied further. B. Laser diffraction techniques General Particles dispersed in a medium, e.g. in air or in a liquid, scatter the light beams by which they are hit. The light is not scattered equally in all directions. Some directions are preferred over others. A light scattering pattern emerges. This pattern is strongly related to the size and the size distribution of the particles. Complex theories have been developed that quantitatively relate the scattering pattern to particle size distribution. We will not discuss these theories any further here. Instrumentation Laser diffraction apparatus uses the scattering behaviour of light by dispersed particles. Essentially the equipment consists of:
It is important to grasp that this type of equipment essentially measures light intensities. Other than in sieving there is no direct measurement of particle size. The particle size distribution of the sample is constructed by putting the measured light intensities into the theoretical equations of either the Fraunhofer theory or the Mie theory and performing the calculations by computer. Nowadays, modern instruments that use the principle of laser diffraction are widely used. They offer enormous advantages over the more classical methods such as sieving, sedimentation and microscopy:
Various instruments are available commercially. All make use of the same principle, although differences exist between instruments of different producers. Differences may include the optical system (lenses), the number of detectors, the dispersion medium (air or liquid), the scattering model applied (Fraunhofer or Mie) and the software. Widely used laser diffraction instruments include those from Malvern, Sympatec and Beckman/Coulter. The results of sampling with instruments of different brands on the same sample will not necessarily be the same. Differences in results can have both theoretical and experimental causes. Important causes of the differences in resulting PSD are the dispersion medium, the number of detectors and the scattering model used. |
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