Characterisation of solution-based pressurised metered-dose inhaler aerosols with an optical particle counter

The White Light Aerosol Spectrometer (welas(R)) is an alternative method to multistage cascade impaction in the analysis of particle-size distribution (PSD) that allows measurements in high concentrations by single particle measurement. Correspondence of the PSD measured by the welas(R) system in combination with a new aerosol sampling system has been discussed before for aqueous aerosols from nebulisers [M. Kuhli, M. Weiss, H. Steckel, A sampling and dilution system for droplet aerosols from medical nebulisers developed for use with an optical particle counter, Journal of Aerosol Science 40 (2009) 523-533]. For aerosols from solution-based pressurised metered-dose inhalers (pMDI), both the apparent density and the dynamic shape factor of the dry solid particles come into account for correlation of aerodynamic to scattered light equivalent diameter. The aim of this study was to enable welas(R) measurements for pMDI aerosols. Equal particle drying properties in cascade impaction (Next Generation Impactor, NGI) and the aerosol sampling system for the welas(R) should be assured. Therefore, an additional, optionally preheated, extension device was included to modify the aerosol sampling system. The PSD measured with the aerosol sampling system that allowed most complete particle drying was compared to the PSD measured by NGI. The introduction of an empiric calibration allows correlation of NGI and welas(R) measurements for the investigated solution-based pMDIs.