Current Approaches to APSD Measurements of OIPs Based on Inertial Impaction - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 29

Table 2.6 Flow Reynolds numbers (Ref f dimensionless) associated with
the four-stage MSLI at different flow rates ( Q )

Stage 1 2 3 4

S / W 0.38 0.39 0.50 2.22

Q (L/min) X c N/A N/A N/A 0.31

20 1,132 2,018 3,552 1,485

30 1,701 3,030 5,334 2,250

40 2,267 4,043 7,115 3,002

50 2,835 5,049 8,886 3,749

60 3,401 6,061 10,667 4,500

70 3,962 7,073 12,449 5,252

Table 2.7 Stage d 50 values ( m) for the four-stage MSLI at Q = 60 L/min

( From [ 26 ] —used with permission )
Stage

μ

1 2 3 4

d 50 13.0 6.8 3.1 1.7

Overall, the available literature indicates that some historical differences exist
between commercially available ACIs and some uncertainties exist in the perfor-
mance characteristics of the basic ACI design.
In the case of the multistage liquid impinger (MSLI), cited as one of the standard
apparatuses for aerodynamic assessment of OIP aerosols in the European
Pharmacopoeia the values of S / W are lower than the recommended range for all
except stage 4 and values of Ref f are in general much higher than their equivalents
with the ACI at ca. 30 L/min. Table 2.6 summarizes the design properties of the
MSLI at different flow rates [ 33 ].
Particle bounce and re-entrainment are avoided altogether because particles are
captured in the impingement fluid ready for recovery and subsequent assay.
However, this design results in shallower collection efficiency curves that are indic-
ative of poorer size selectivity, and the d 50 sizes (Table 2.7 ) are therefore likely to be
more dependent upon small changes in either S or W compared with either the ACI
or NGI systems.
The Marple-Miller series of five-stage impactors (MMI), cited in the US
Pharmacopeia, are currently available in three sizes, all based on the original work
of Miller [ 34 ] and Marple et al. [ 35 ]. A low-flow rate version (model 150P) was
developed in the late 1990s to permit MDIs with add-on devices intended for low-
flow patients to be tested at more appropriate conditions of use (4.9 and 12 L/min)
[ 36 ]. This addition enables measurements to be made by MMIs at flow rates ranging
from 4.9 to 90 L/min with d 50 values that are all located within the useful range for
all types of OIP testing (Table 2.8 ).
Stage collection efficiency curves for all versions of this CI expressed either in
terms of d ae or

St are steep and associated with GSD stage values that are close to or
below 1.2 (Table 2.9 ).

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