Cascade Impactor (CI) Mensuration—An Assessment of the Accuracy and Precision of Commercially Available Optical Measurement Systems

Multi-stage cascade impactors (CIs) are the preferred measurement technique for characterizing the aerodynamic particle size distribution of an inhalable aerosol. Stage mensuration is the recommended pharmacopeial method for monitoring CI “fitness for purpose” within a GxP environment. The Impactor Sub-Team of the European Pharmaceutical Aerosol Group has undertaken an inter-laboratory study to assess both the precision and accuracy of a range of makes and models of instruments currently used for optical inspection of impactor stages. Measurement of two Andersen 8-stage ‘non-viable’ cascade impactor “reference” stages that were representative of jet sizes for this instrument type (stages 2 and 7) confirmed that all instruments evaluated were capable of reproducible jet measurement, with the overall capability being within the current pharmacopeial stage specifications for both stages. In the assessment of absolute accuracy, small, but consistent differences (ca. 0.6% of the certified value) observed between ‘dots’ and ‘spots’ of a calibrated chromium-plated reticule were observed, most likely the result of treatment of partially lit pixels along the circumference of this calibration standard. Measurements of three certified ring gauges, the smallest having a nominal diameter of 1.0 mm, were consistent with the observation where treatment of partially illuminated pixels at the periphery of the projected image can result in undersizing. However, the bias was less than 1% of the certified diameter. The optical inspection instruments evaluated are fully capable of confirming cascade impactor suitability in accordance with pharmacopeial practice.