Comparing Different Particle Sizing Techniques

It is a reasonable expectation when first considering particle size analysis that different techniques should reported the same size for a given sample. However, due to the application of the equivalent sphere approximation, which is applied in order to derive a single diameter that is indicative of the size, different techniques will provide different size distributions unless spherical particles are measured. This was recognised in the 1940’s by Heywood who suggested that particle size analysis techniques generated results that were “dependant on the physical properties employed and the assumptions or conventions involved”. As different techniques measure different particle properties (sedimentation rate, light scattering patterns, projected image size) it is perhaps not unreasonable to expect them to generate different results. It is therefore important to consider the following when comparing the output of different methods:

• Non-Sphericity : Techniques will be affected by the presence of non-spherical particles in different ways dependant on the physical property being measured in order to derive the particle size. This is one reason why particle sizing standard reference materials tend to be spherical as similar results can then be generated by multiple techniques.
• Statistical Representations : Different techniques will apply different weightings to the particles within the distribution. For instance, one technique may measure the number of particles of a given size whereas another may measure the mass of particles. This will affect the shape reported size distribution and the technique’s sensitivity to changes in the distribution width.

In the end, no one representation of the particle size can be claimed to be more accurate than another. Instead, what is most important is that the measured parameter best relates to the application being developed. In many applications, such as in the production of pharmaceuticals, it is important to understand how the mass or volume of material is distributed between particles of different sizes. This is one reason why laser diffraction has become widely applied across industry.

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