Improving Flow Distribution and Analytical Repeatability with a Novel Pre-Separator Base for the Andersen Eight-Stage, Non-Viable Cascade Impactor (ACI)

Daryl L. Roberts, Diem Ngo, Bill Doub

Background: Flow uniformity is assumed in cascade impactors (CIs) used for assessment of particle sizes dispensed by inhaled drug products. However, this assumption is invalid for stages 0 and 1 of the Andersen 8-stage non-viable impactor (ACI; [1]) and may affect test results generally for dry powders with this CI [2]. There is a need to improve aspects of the ACI to overcome this issue.*
Materials and Methods: We made a prototype pre-separator (PS) base for the ACI that preserves the impaction aerodynamics of the conventional PS, but that distributes the flow through a circumferential slit rather than through the three standpipes of the conventional PS. We tested the ACI with a commercial dry-powder inhaler (DPI) at 28.3 L/min and 60 L/min with the conventional and the prototype PS base.
Discussion: There was generally less variability in the mass-per-stage impactor data with the prototype PS base. With the conventional PS base, unusual particle deposits were visible on the first collection stage, right in line with the three standpipes. With the prototype PS base, the deposits on the first collection stage were more evenly distributed, indicating more uniform air flow.
Conclusion: More even flow distribution leading into the ACI size-fractionating stages reduced the variability in the mass-per-stage results under common compendial test conditions. Because the conventional ACI PS base does not appear in pharmacopeial guidelines, an alternative PS base is a valid option for ACI users and may for some drug products result in reduced test-to-test variability.

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