Background: Determination of aerosol aerodynamic particle size distributions (APSD) from dry-powder inhalers (DPIs), following quality control procedures in the pharmacopeial compendia, requires that the flow through the measurement apparatus, comprising induction port, optional pre-separator, and cascade impactor, starts from zero on actuation of the inhaler, using a solenoid valve to apply vacuum to the apparatus exit. The target flow rate, governed by the inhaler resistance, is reached some time afterward. Understanding the behavior of the DPI design-specific flow rate-rise time curve can provide information about the kinetics of the initial powder dispersion in the inhaler and subsequent transport through the APSD measurement equipment. Accurate and precise measures of the internal volume of each component of this apparatus are required to enable reliable relationships to be established between this parameter and those defining the flow rate-rise time curve. Methods: An improved method is described that involves progressive withdrawal of an accurately known volume of air from the interior passageways of the apparatus-on-test that are closed to the outside atmosphere. This approach is applicable for determining internal volumes of components having complex internal geometries. Filling some components with water, along with volumetric or gravimetric measurement, has proven valuable for the induction port and for checking other measurements. Results: Values of internal volume are provided for the USP (United States Pharmacopeia)/PhEur (European Pharmacopoeia) induction port, the Next-Generation Impactor (NGI™) with and without its pre-separator, and various Andersen 8-stage cascade impactor configurations with and without their pre-separators. Conclusion: These data are more accurate and precise, and therefore update those reported by Copley et al.
Keywords: cascade impactor; induction port; internal volume; pre-separator.