Q. We need to measure the particle size of a moisture-sensitive powder. We would like to use dry analysis on our laser-diffraction system, but the particles are fragile and we’re struggling to get robust results. Is wet measurement with a nonaqueous solvent the only option?
A. Successful dry dispersion with pressurized air relies on applying enough energy to completely separate the particles without damaging the primary particle. For fragile particles such as yours, this can be challenging. It may be that wet measurement is the best answer. Before reaching that conclusion, though, you should rigorously assess the feasibility of dry dispersion.
First, carry out a pressure titration and measure particle size as a function of the pressure of the dispersing gas. Increasing pressure puts more dispersive energy into the system, and will therefore tend to decrease the measured particle size. Ideally, a plot of the resulting data will have a plateau, which is a region where increasing pressure has no impact on particle size, because complete dispersion has been achieved. If your data has a plateau, you have identified a condition for dry dispersion. However, as ISO13320 (2009) observes, such a plateau is seldom observed in the real world (1). In the absence of a plateau, you can compare your results with wet data. If, at a certain dispersion pressure, the match is very close across the complete particle-size distribution, you can use these dry-dispersion conditions. If not, it is possible that your disperser may be breaking up the primary particles.
Dispersers differ in their design. Some rely heavily on impaction, which is extremely efficient when the primary particles are strong enough to withstand the forces generated, but gentler dispersion can be a more effective strategy with more fragile materials. Some laser-diffraction instruments have dry systems that can be configured with different disperser types to accommodate the widest possible range of materials. Consider whether your disperser can be switched for something more suitable. If not, then wet measurement may be your only answer, unless your need is great enough to justify investment in a brand-new instrument.
—Carl Levoguer, product marketing manager for laser particle sizing and imaging at Malvern Instruments.
1. ISO, ISO13320:2009, Particle Sizing Analysis – Laser Diffraction Methods (Geneva, Switzerland, 2009).
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