Effect of formulation factors on the observed bounce in cascade impactors used to measure the spray particle size of metered dose inhalers

Abstract

This study was conducted to investigate the extent of particle reentrainment, or bounce, in cascade impactors used to measure the size of the spray from single and multiple doses of metered dose inhalers (MDIs), and to determine the effect of some common formulation properties on the observed bounce. MDIs were formulated as suspensions or solutions, with varying levels of surfactant. These were fired into two impactors (the Andersen Sampler Mark II and the Marple-Miller model 150), the collection surfaces of which were uncoated or coated with silicone oil or glycerin. The amount of drug from a formulation collected on each stage was determined and comparison was made between results with coated and uncoated stages. Efficacy of the coating for eliminating bounce was demonstrated by comparing single shots with multiple shots. Different ways of describing bounce effects offer varying indications of its significance. The suspension formulations (containing surfactant) all showed evidence of bounce, but solution formulations showed little or none. Silicone oil and glycerin were equally effective in eliminating bounce. Results with impactors of two different designs were similar. Particle bounce was shown to occur with a variety of formulations when impactor stages are not coated.