The influence of sampling chamber dimensions on aerosol particle size measurement by cascade impactor and twin impinger.

The influence of the sampling chamber dimensions upon particle size estimation by cascade impaction has been investigated and compared with measurements by the twin impinger. Aerosols of salbutamol and disodium fluorescein (DF) were generated from pressurized metered dose inhalers. The mass median aerodynamic diameter (MMAD) by the Andersen Impactor for salbutamol ranged from 2.0 to 2.8 microns with geometric standard deviations (g.s.d.) of 1.7 to 2.4. These observations were independent of the distance travelled to the first impaction surface of the impactor and volume of the sampling chamber. The DF MMAD ranged from 5.0 to 6.9 microns with g.s.d. values of 1.7 to 1.8. Changes in droplet size within the sampling chamber may cause significant differences in particle size estimates as indicated by the cascade impaction data for DF. The respirable fraction of the salbutamol samples was similar whether determined by impaction or using the impinger. The latter device has previously been indicated to give clinically relevant estimates of respirable fraction for commercial inhalation aerosol devices.