Acceptance Testing of Used Cascade Impactor Stages Based on Pressure Drop Measurements in a Flow System Managed with a Critical Flow Venturi: Part I-Laboratory Proof-of-Principle.

Background: Cascade impactors are essential for measuring the aerodynamic particle size distribution delivered by metered dose, dry powder, and similar inhalable drug products. For quality control of used impactors, periodic optical inspection of the nozzles of each impactor stage (stage mensuration) is currently the only method sufficiently precise to test whether used impactors are suitable for continued use, in accord with pharmacopeial standards. Here, we demonstrate a new method for quality control of used impactors. The method combines stage-wise pressure-drop measurement with a critical flow venturi (CFV) for air flow management. This technique avoids the unacceptably large uncertainty in conventional air flow rate measurements and instead relies on pressure and temperature measurement upstream of the CFV. These measurements can be made precisely with affordable equipment. Methods: We placed a toroidally shaped CFV downstream of a Next Generation Impactor™** (NGI) and precisely measured the stagnation pressure (±0.02%) and temperature (±0.03%) upstream of this CFV at impactor inlet flow rates close to 60 L/min. Pressure-drop measurements (±0.25%) at stages 3-7 and the micro-orifice collector were made with capacitive diaphragm transducers and with a special lid to the NGI that allowed pneumatic connection to the interstage passageways before and after each impactor stage. Results: The measured pressure drop values matched, to fractional percentage precision, those predicted by the incompressible flow theory through the nozzles and the compressible flow theory through the CFV. Conclusions: Practical equipment has been assembled that measures, to fractional percentage precision, the pressure drop through impactor nozzles at precisely managed flow conditions. The experimental results support the relevant flow principles. The results, thereby, support the use of this method for quantifying whether used impactor stages are suitable for continued use in the testing of registered inhalable drug products, in accord with pharmacopeial standards.