Validating AIM-Based Instrumentation and Associated Measurement Techniques - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 309

Fig. 10.30 Size-related metrics from FSI with uncoated collection surfaces with equivalent NGI-
generated metrics ( From [ 38 ]— courtesy of D. Russell-Graham )

interpolated to give an equivalent FPF <5.0μm assuming a unimodal and lognormal
APSD; since the NGI has no stage with a cut-off diameter located at precisely
5.0 m aerodynamic diameter. Their metrics obtained with the particle collection

surfaces of the FSI uncoated were in reasonably close agreement with those mea-
sured using their full-resolution NGI (Fig. 10.30 ), but there was a small but sys-
temic bias toward higher values of FPF <5.0μm with the FSI.
These findings are consistent with particle bounce and re-entrainment and were
largely eliminated, using the silicone fluid coating (Fig. 10.31 ).
Similar trials were carried out with other DPIs at flow rates determined via the
standard pharmacopeial method for DPI testing, using an appropriate insert to
maintain the 5

μ

m stage cut-off at each flow rate. These results confirm the initial
conclusion that a coated FSI produced FPF <5.0μm values that were closely compara-
ble to those obtained using the full-resolution NGI. However, the magnitude of
minor discrepancies between the NGI and FSI varied from product to product, serv-
ing to highlight the necessity of justifying the use of AIM techniques through suit-
able method development and comparative testing against a full-resolution
instrument, for each inhaler product.
A further study was investigated by this group to assess the value of AIM as a
screening tool during early-stage formulation screening [ 38 ]. This investigation was
structured to simulate a Design of Experiments (DoE) of the type routinely applied
during early-stage product development. Here, the goal was to investigate the ability
of the FSI to correctly identify trends in FPF resulting from changes in percentage

μ