10
Setting particle size specifications
The creation of a meaningful and product-appropriate particle size
specification requires knowledge of its effect on product performance
in addition to an understanding of how results should be interpreted
for a given technique. This section provides guidelines
for setting particle size
specifications on particulate materials—primarily when using the laser diffraction
technique, but also with information about dynamic light scattering (DLS) and image
analysis.
DISTRIBUTION BASIS
Different particle sizing techniques report primary results based on number, volume,
weight,
surface area, or intensity. As a general rule specifications should be based
in the format of the primary result for a given technique. Laser diffraction generates
results based on volume distributions and any specification
should be volume
based. Likewise, an intensity basis should be used for DLS specifications, volume
for acoustic spectroscopy, and number for image analysis. Conversion to another
basis such as number—although possible in the software—is inadvisable because
significant error is introduced. The exception to this guideline
is converting a number
based result from a technique such as image analysis into a volume basis (ref. 7).
The error involved is generally very low in this scenario.
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