Good Cascade Impactor Practices - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 105

Fig. 4.12 Near-normal to lognormal APSD from a pMDI product presented in log-probability
scaled format

droplets, each with its own APSD. The elimination of one of these modes (e.g.,
the coarser primary droplets) by impaction on baffles within jet nebulizers is one
way to present a unimodal APSD to the CI system.
2. If auxiliary moments, such as GSD, are being used to define the spread of the
APSD, check for deviations from lognormal by plotting the cumulative mass on
log-probability scaling and ensuring that the relationship between mass of API
and aerodynamic diameter is both symmetric about the MMAD and linear or
near to linear (Fig. 4.12 ).
3. The GSD is determined from the relationship

d
d

84 1 .

GSD =

(4.6)

15 9 .

in which d 15.9 and d 84.1 are the sizes corresponding to the 15.9th and 84.1st percentiles
of the cumulative mass-weighted APSD. If the plot similar to that illustrated by
Fig. 4.12 is not linear or near to linear in shape, then it is recommended that for regu-
latory reporting, at least, that the fact that the APSD is not lognormal be acknowl-
edged and simply the spread of the distribution ( S APSD ) reported as a ratio, where

d
d

90

S = (4.7)

APSD

10