Reducing bounce effects in the Andersen cascade impactor

The collection efficiency of the Andersen cascade impactor (ACI) can be affected by particle bounce, overload and re-entrainment (or blow-off), collectively referred to as bounce effects. Reduction of bounce effects in the ACI operated at 60 LPM was investigated for placebo large porous particles. Aerodynamic particle size distributions (aPSDs) obtained with the ACI and multi-stage liquid impinger (MSLI) were compared by observation of modes and statistical comparisons of the mass median aerodynamic diameter (MMAD) and geometric standard deviation (σg). Particle bounce effects were prevalent in the ACI with uncoated plates, i.e., bi-modal distribution with statistically significant differences in MMAD and σg (P < 0.05). Coating the impaction plates with a thin layer of vacuum grease and decreasing the ACI stage jet velocities reduced, but did not minimize bounce effects. Bounce effects were minimized using 20-μm pore glass fiber filters saturated in water placed on inverted impaction plates, with good agreement obtained between the ACI and MSLI aPSDs, i.e., mono-modal with no statistically significant differences in MMAD and σg (P > 0.05). Selection of the impaction substrate material and solvent must be evaluated with the drug product and analytical methods to minimize bounce effects and obtain an accurate measure of the aPSD.