Current Approaches to APSD Measurements of OIPs Based on Inertial Impaction - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 24

Fig. 2.7 Hypothetic single stage impactor collection efficiency curve

when E is 50% (Fig. 2.7 ). At this location, defined as the cut size ( d p,50 ) or effective
cutoff diameter,

12 /

=

9 h

r

nW
U

(2.7)

Cd St

c , 50 p , 50 50

0

or in terms of volumetric flow rate ( Q ),

12 /

=

3

9 pm

r

nW
Q

(2.8)

Cd St

c , 50 p , 50 50

4

0

for a multiorifice stage comprising n circular nozzles. Equations (2.7) and (2.8) can
be written in terms of the more usually encountered aerodynamic diameter scale
( d ae,50 ), substituting (2.3) for d p .
The assumption is often made that the mass of particles larger than d 50 (the size
corresponding to E 50 ) that penetrate the stage is exactly compensated by the mass
associated with particles finer than this size that are collected. Thus, the cut size can
be defined as a single-valued constant for a given stage at a fixed flow rate. Particles
with d ae ≥ d 50 are assumed to be fully collected, whereas all particles finer than d 50 are