Physical Causes of APSD Changes in Aerosols from OIPs and Their Impact on CI Measurements - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 70

Fig. 3.4 Predicted droplet size-time profiles for two different sampling flow rates using an evapo-
ration model developed by Rao et al. ( From [ 75 ]— used with permission )
saturated atmospheric conditions in the laboratory, which is why RH amb is normally
subsaturated. However, it follows that good controls of both ambient temperature
and in particular relative humidity are important to obtain reproducible CI-measured
APSDs for these systems.
Water vapor absorption onto hygroscopic powders is a well-known source of
aerosol APSD shifts, predominantly in the direction of larger sizes, as the locally
bound water increases particle-particle cohesiveness [ 78 ]. The processes involved
include local dissolution and recrystallization, leading to irreversible aggregation
through solid bridge formation [ 79 ]. While such processes are most likely to take
place in storage, they may need to be considered with highly hygroscopic particles,
such as cromolyn sodium, even when sampling the resulting aerosol [ 80 ]. The best