Introduction to Aerodynamic Particle Sizing

Together with delivered dose uniformity, the Aerodynamic Particle Size Distribution (APSD) is widely recognised as a Critical Quality Attribute (CGA) in the in vitro characterisation of inhaled and nasal products since it is the APSD of an aerosol cloud that defines where the particles in that cloud are deposited following inhalation.

It is generally accepted that to be therapeutically effective, the particles should be in the range of 1 to 5 microns since particles > 5 microns will generally impact in the oropharynx and be swallowed whereas those < 1 micron may remain entrained in the airstream and be exhaled during the next breathing cycle.

The preferred instrument of choice for measuring the APSD of inhaled and nasal products is the cascade impactor because:

• Cascade impactors measure aerodynamic particle size
• Cascade impactors allow measurement of the active pharmaceutical ingredient (drug)
• Cascade impactors measure the entire dose

Between them, the US and European Pharmacopoeias list no less than five different cascade impactors/impingers suitable for the aerodynamic assessment of fine particles.

However, only the Andersen Cascade Impactor (ACI), the Next Generation Impactor (NGI) and the Multi-Stage Liquid Impinger (MSLI) appear in both Pharmacopoeias.

When selecting an impactor, much will depend on the product to be tested, the data that is required, the geographical location where the product is to be marketed and whether the unit is to be used for product development or quality control.

In research applications,in vitro in vivo correlation and bioequivalence may be important and so detailed particle size data may be required. In routine quality control, where the concern is batch-to-batch variation, a coarser test may be acceptable.

To see an animation of how a cascade impactor works, please click on the following link: How Does a Cascade Impactor Work?

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