Characterisation of a carrier-free dry powder aerosol formulation using inertial impaction and laser diffraction

The purpose of the study was to examine the suitability of using laser diffraction to measure the fine particle fraction (FPF) of drugs emitted from carrier-free dry powder aerosol formulations.

Particle size distribution of terbutaline sulphate from Bricanyl Turbohaler, which contained loose agglomerates of drug particles only, was measured separately by laser diffraction apparatus equipped with a metal throat and a twin-stage, multi-stage liquid impingers, or Andersen cascade impactor at flow rates ranging from 28.3 to 100 l min(-1). In-line measurements were then conducted which allowed the same aerosolised particles to be measured first by laser diffraction then captured by an impactor or impinger for subsequent chemical analysis.

A significant linear correlation (p < 0.001, R2 = 0.96, ANOVA) existed between the results obtained from two techniques when measurements were conducted independently. There was little difference in FPFs measured by inertial impaction and laser diffraction at the same flow rate. When in-line measurements were conducted, the FPFs measured by inertial impaction were approximately 0.7-0.9 times the aerosol FPFs measured by laser diffraction. This linear relationship was statistically significant and had a statistically insignificant y-intercept, regardless of inhaler batches, impinger types and measuring position of the laser beam.

Laser diffraction could prove to be a reliable technique for development, evaluation and quality control of carrier-free, dry powder aerosol formulations.