Current Approaches to APSD Measurements of OIPs Based on Inertial Impaction - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 31

Table 2.10 Values of X c , S / W , Re f , and d 50 ( μ m) for the NGI size-fractionating stages at flow rates

( Q ) associated with its archival calibration

All flow rates
Stage 1 2 3 4 5 6 7 MOC a

X c N/A b N/A b 0.35 0.41 0.61 0.84 0.85 0.94

S / W 1.0 2.0 3.0 3.0 3.0 3.1 4.9 7.1

Q = 15 L/min
d 50

14.1 8.61 5.39 3.30 2.08 1.36 0.98 Do not use

Re f 1,482 724 404 338 230 167 166

Q = 30 L/min
d 50

11.8 6.40 3.97 2.30 1.36 0.83 0.54 0.36

Re f 2,938 1,435 801 669 455 328 324 149

Q = 60 L/min
d 50

8.06 4.46 2.82 1.66 0.94 0.55 0.34 0.14

Re f 5,876 2,870 1,602 1,339 909 757 647 298

Q = 100 L/min
d 50 6.12 3.42 2.18 1.31 0.72 0.40 0.24 0.07
Re f 9,793 4,783 2,671 2,231 1,515 1,095 1,079 496
a MOC micro-orifice collector—used as an alternative to a backup filter; note that the size corre-
sponding to 80% collection efficiency ( d 80 ) rather than d 50 values are quoted for this equivalent to
a backup filter, rather than an impaction stage
b Single-nozzle geometry; therefore, cross-flow parameter does not apply

In summary, there is some freedom in establishing the precise location of the
collection surface beneath the nozzle plate with many impactor designs except per-
haps the MSLI. This flexibility is available for the ACI and NGI, which are the
apparatuses most likely these days to be used in OIP aerosol characterization. More
importantly from a user's standpoint, small variations in the precise thickness
(depth) of a tacky coating that could be applied under normal circumstances to the
collection surfaces, to improve particle adhesion, are unlikely to affect stage perfor-
mance. As will be seen in Chap. 10 , the ability to mitigate bias due to particle
bounce and blowoff from the collection surface is particularly important with abbre-
viated systems when used to characterize both MDI- and DPI-generated aerosols.
The NGI represents the state of the art for CI design. Based on their experience
with the Marple-Miller series of CIs, the developers of the NGI took the opportunity
to ensure that the criterion for S / W was met for all stages (Table 2.10 ). Likewise, the
multinozzle designs of stages 3-7 and the MOC were arranged such that values of
X c were significantly <1.2. Some values of Ref f are <500 for certain stages, particu-
larly those that size fractionate the finer particles and when operating this CI at the
lower flow rates within its range [ 38 ]. Nevertheless, all stage collection efficiency
curves for this impactor throughout the original design range of operation from 30
to 100 L/min were shown to be sharply defined with minimal overlap [ 21 ]. Even at
the lower flow rate extension at 15 L/min for nebulizer testing, these curves are still
acceptable for size-fractionating purposes [ 22 ]. Note, however, that the MOC
should be followed by a final filter or the internal filter option be used to replace the
MOC when used at 15 L/min [ 22 ].