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Particle size analysis: an expert on diffraction systems offers insight on particle size specifications and analysis.

Article Excerpt
Do I need particle size specifications for my pharmaceutical product?

Specifications define the critical to quality factors to which a pharmaceutical product must conform to be acceptable for its intended application. Quality by Design highlights the importance of identifying and controlling all the parameters that can influence final product performance: ICH Topic Q6A identifies particle size as a potentially important variable. (1) The specific need for a particle size specification is determined by assessing if and how this parameter affects performance.

For solid or suspension delivery systems, bioavailability is often directly related to particle size because it controls dissolution/solubility characteristics. Dissolution rate is directly proportional to particle surface area (Noyes-Whitney equation), so a finer particle size promotes faster drug dissolution. Particle size distribution is also relevant as a narrow distribution produces more uniform dissolution. Formulations with even a small number of relatively large particles may take some time to dissolve completely, but this may be the design intent.

For suspensions, stability is an important issue because if the active ingredient settles there is a greater chance of non-uniform delivery. Stokes' law relates settling velocity to the physical characteristics of the fluid and the size of particles in the suspension. The relationship here is a strong one: velocity correlates with the square of particle diameter. For suspension, stability, a very low settling velocity is preferable and is more easily achieved with finer particles.

Perhaps less obviously, particle size may also affect a formulation's behaviour during processing and, ultimately, its content uniformity, which is critical. Take, for example, the widespread operation of direct compression tableting; particle size can influence...

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