Correlation between inertial impaction and laser diffraction sizing data for aerosolized carrier-based dry powder formulations.

Purpose: The purpose of the study was to determine whether the drug fine particle fraction (FPF) from different dry powder aerosol formulations measured by laser diffraction at a range of flow rates correlated with that measured by inertial impaction.

Materials And Methods: Ten binary formulations were prepared containing 1.5% w/w salbutamol base or sulphate, blended with the sieved (63-90 microm) fraction of different sugars (regular lactose, spray-dried lactose, sorbitol, dextrose or maltose). A further six ternary formulations were prepared containing 1.5% w/w salbutamol sulphate, 97% coarse lactose (63-90 microm) and 1.5% micronised or intermediate-sized lactose (1-50 microm). The FPF particles (< 5 microm) of these formulations were measured by laser diffraction and inertial impaction at flow rates between 28.3 and 100 l min(-1).

Results: When only the particles with diameter < 60 microm obtained by laser diffraction were considered the FPF (< 5 microm) could be determined and this enabled the aerosolisation of all 16 blends to be feasibly compared at flow rates ranging from 28.3 to 100 l min(-1). A significant linear correlation was found between the fine fractions measured by laser diffraction and the salbutamol fine fractions determined by inertial impaction (r2 = 0.934). Such correlation was also confirmed for formulations containing added fine lactose.

Conclusion: Particle size measured by laser diffraction under the employed conditions reflected the aerodynamic properties of the drug. Laser diffraction can be used as on-, in- and/or at-line measurements and controls for dry powder aerosol formulations.