Comparison of two methods to prevent evaporation of aqueous droplets in the Andersen Cascade Impactor

The measurement of the particle size distribution by means of cascade impaction is a common and important method to characterize medical aerosols. However, particle size distributions of aqueous droplets (e.g. generated by nebulizers or the Respimat ® Soft Mist ™ Inhaler) are difficult to determine because of evaporation of water after aerosolization. The aim of this study was to compare two different methods which reduce droplet shrinkage between the points of aerosol generation and droplet impaction. A method based on impactor refrigeration was compared against a method using humidified air. It was shown that both methods are capable of preventing evaporation of aqueous droplets after aerosolization and that they are suitable to measure reproducible particle size distributions of aqueous medical aerosols