Equipment and Processing Report: Invisible Particulate
Scott Stoeffler, McCrone Associates, Inc., Westmont, Illinois
Pharmaceutical Technology - February 16, 2011
Q: My drug failed a light-obscuration test, but after I filtered the drug for microscopic particle counting, I couldn’t see any significant particulate on the filter. What might have caused the failure, and why is it not visible?
A: One possibility is that what caused the failure was not solid particulate. A light-obscuration test can respond to droplets of suspended liquid (e.g., silicone oil in an aqueous product or water in an oil-based product), or even to air bubbles, as if they were solid, discrete particles. Air bubbles, of course, vanish completely upon filtration, water evaporates, and residual oil typically is not visible on the standard mixed-cellulose ester (MCE) filters used for USP microscopic particle counts. Even on a membrane filter made of a smooth material such as polycarbonate, oil residue from a filtered product may not be visible.