By far the most important physical property of particulate samples is particle size. Particle size determinations is routinely carried out across a wide range of industries and is often a critical parameter in the manufacture of many products. Particle size has a direct influence on material properties such as:
- Reactivity or dissolution rate – e.g. catalysts, tablets
- Stability in suspension – e.g. sediments
- Efficacy of delivery – e.g. asthma inhalers
- Texture and feel – e.g. food ingredients
- Appearance – e.g. powder coatings
- Flowability and handling – e.g. granules
- Viscosity – e.g. nasal sprays
- Packing density and porosity – e.g. ceramics
Particle size distribution should be known – and controlled. Ultimately, this is in order to protect patients (to avoid changes in plasma profile), but also to avoid unpleasant surprises during manufacture. Measurement of particle size is not an exact technique and a “true” value does not exist. A range of analytical techniques is available for determination of particle size; these all have their strengths and weaknesses. In order to develop and validate analytical methods for determination of particle size, it is necessary to correlate several methods.
The goal of all particle-sizing techniques is to provide a single number that is indicative of the particle size. However, particles are three-dimensional objects for which at least three parameters (length, breadth and height) are required in order to provide a complete description.
Most particle-sizing techniques therefore assume that the material being measured is spherical and report the particle size as the diameter of the “equivalent sphere” which would give the same response as the particle being measured. But what is the right particle size?
The way the equivalent sphere approximation works is shown above for an irregularly-shaped particle. The diameter reported for this particle will be dependent on the physical property measured by the chosen technique.