cascade impactor

A cascade impactor is an instrument used to sample, fractionate, and collect aerosol particles into discrete size classes based on their aerodynamic diameter. It operates on the principle of inertial impaction through a sequential series of stages. As an air or gas stream is drawn through progressively narrower nozzles, particle velocity increases; particles with sufficient inertia deviate from fluid streamlines to strike and deposit onto collection plates, while smaller particles remain suspended in the flow and proceed to downstream stages. Cascade impactors are standard tools in atmospheric chemistry, occupational hygiene, air pollution monitoring, and pharmaceutical testing, particularly for the determination of aerodynamic particle size distribution (APSD) in orally inhaled drug products such as metered-dose inhalers (MDIs) and dry-powder inhalers (DPIs).

A cascade impactor is an instrument used to sample, fractionate, and collect aerosol particles into discrete size classes based on their aerodynamic diameter. It operates on the principle of inertial impaction through a sequential series of stages. As an air or gas stream is drawn through progressively narrower nozzles, particle velocity increases; particles with sufficient inertia deviate from fluid streamlines to strike and deposit onto collection plates, while smaller particles remain suspended in the flow and proceed to downstream stages. Cascade impactors are standard tools in atmospheric chemistry, occupational hygiene, air pollution monitoring, and pharmaceutical testing, particularly for the determination of aerodynamic particle size distribution (APSD) in orally inhaled drug products such as metered-dose inhalers (MDIs) and dry-powder inhalers (DPIs). $$\mathrm{Stk} = \frac{\rho_p d_p^2 C_c v}{9 \eta W}$$ where: $\rho_p$ is the particle density, $d_p$ is particle diameter, $C_c$ is the Cunningham correction factor, $v$ is mean jet velocity, $\eta$ is gas dynamic viscosity, $W$ is nozzle diameter or slit width. Each stage is defined by a cut-off diameter ($d_{50}$), which corresponds to the aerodynamic diameter collected with 50% efficiency. Because nozzle diameters decrease at each successive stage, fluid velocity ($v$) increases, yielding progressively lower $d_{50}$ values.

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