The physical and chemical properties of many consumer and industrial products are affected by their size distribution. For example, the rate of dissolution of sugar in water is not only determined by its physical solubility but it is also affected by the sugar crystal/particle size. To produce sugar of the correct size range, the manufacturing processes must be right and strict quality control must be in place. Quality control in sugar particle size range requires the analysis of the salt particle size distribution. Contemporary manufacturing is fast and product volume enormous. To keep up with the tempo of the production processes, particle size measurement will need to be accurate, rapid and preferably easy to use. There are a number of particle size measurement methods. They range from visual inspection of the particles, measurement by sieves, the use of sedimentation systems, and the application of optical/illuminating instruments. The use of light scattering instruments, especially those employing laser as the light source, has been gaining popularity. This is due to the good accuracy, short analysis time, ease of use and results presentation.

It has been found that there are quite a number of variables that will affect the laser scattering instrumental results, the first being the state of dispersion of the particles. There are no universal guidelines on the choice of appropriate dispersant or surfactants for a given system. In addition, the dispersion of the agglomerates is also affected by the morphology of the constituents, chemical interactions between the solids and the dispersion medium, and the presence of suspension medium within the agglomerates.

A good example (dispersion of color pigments) has been described by Li et al in Powder Technology 92 (1997). Moreover,the purity of the surfactant use may also affect the colloid stability and gives rise to false particle size distribution. The other factor concerns the actual particle shape. The particle shape affects the physical phenomena used for particle size measurement and therefore different shapes can give rise to totally different results for the same defined size.

Another variable to be taken into consideration is the difference in the sample preparation conditions. Deagglomerated sample may give a size distribution different from that of an undone sample. Last but not least is the theoretical models and mathematical techniques used in the various models of laser scattering instruments. We will expect the design of the instruments to differ between models and results thus follow. Since measurements from instruments employing the same basic principle of laser scattering can vary, more deviations are to be expected from instruments working on different basic theories.

Other than the physical and empirical conditions affecting the particle size distribution, the numerous definitions of particle size distribution must be noted. A simple example can quickly elucidate this point. We have 22 particles having the diameters shown in the first column of the table1. The lengths, areas, and volumes of the particles are shown on the same table. The next table, table 2 shows the % of particles at the 5 diameters according to the 4 definitions-number, length, area, and volume.

We are able to observe from the above tables that results are not the same. The plotted distributions are shown below. In particle size distribution analysis, the volume definition is often used.