The Aerodynamic Particle Size Distribution of a MDI will determine where the particles in the aerosol cloud are likely to deposit following inhalation. If the particles are too large they will catch at the back of the throat. Too small, and they will be exhaled rather than deposited.
The instrument of choice for measuring the APSD of MDI products to meet both regulatory and pharmacopoeial requirements alike is the cascade impactor.
There are five different cascade impactors/impingers suitable for the assessment of APSD, however, only the Andersen Cascade Impactor (ACI), the Next Generation Impactor (NGI) and the Multi-Stage Liquid Impinger (MSLI) appear in both the US and European Pharmacopoeias.