David Pugh
Dynamic Light Scattering (DLS) has been an important tool for determining particle size distributions in fine particulate material suspensions, micro emulsions and Nano-scale matter in general for 50 years. Many Nano scale materials are measured in non-aqueous media and as long as we know the viscosities in these media at 2 different temperatures, then the analysis are relatively simple. However most pharmaceutical suspensions are water based and the key danger is not in the sampling but the chemistry. When measuring the size of particles, a study of their zeta potential in advance is extremely important as a high zeta potential is indicative of a stable product whilst a low zeta potential is indicative of an unstable product likely to agglomerate. When two particles have a high zeta potential they repel each other like 2 negative or positive magnets so they are not attracted to each other thus leading to a stable solution. When a sample is diluted dramatic changes to its zeta potential can occur especially when the dilution causes the zeta potential to approach the Iso Electric Point (IEP-0 m Volts). There are a number of effects which may cause the zeta potential to decrease towards 0 mV after dilution; we will cover pH, salt concentration and polyelectrolyte or surfactant concentration. There are 2 approaches to take when encountering this problem, The first approach requires full knowledge of the suspensions pH, salt concentration and volume % concentration of the surfactant such that care in dilution may be effected (Prevention). The second involves technological advances (Prevention and Cure) which are unique and employs a method which in most pharmaceutical applications renders the need to dilute redundant. By measuring the suspension at full concentration, all potential chemistry problems are negated.
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