Comparison of nebulized particle size distribution with Malvern laser diffraction analyzer versus Andersen Cascade Impactor and low-flow marple personal cascade impactor

Particle size of nebulized aerosols can be measured directly using laser di ffraction or by evaluating aerodynamic properties by cascade impaction. As of today, there are no generally accepted standards for measuring particle size distribution from nebulizers. Laser diffraction has been questioned be cause of potential evaporative losses of the small particles at the edge of the plume, causing an apparent shift in the particle size distribution and thus a larger mass median diameter (MMD). When particle-sizing wet aerosol s, cascade impaction may give rise to an apparent shift in the distribution , resulting in a smaller mass median aerodynamic diameter (MMAD) due to eva porative losses of aerosol droplets as they enter the impactor at ambient t emperature. The modified low-flow Marple 296 Personal Cascade Impactor (MPC I) is currently being proposed as the European standard for wet aerosol ana lysis to minimize evaporative losses during sampling. The present study com pared the particle size distribution of salbutamol and sodium cromoglycate aerosols nebulized by the Pari LC Star, using laser diffraction (Malvern Ma stersizer X; MMX) and cascade impaction (Andersen Cascade Impactor [ACI] an d the commercially available MPCI), which was either at ambient temperature or cooled to the nebulized aerosol temperature (10 degreesC). MMDs obtaine d with the MMX were virtually identical to the MMADs measured with both imp actors when cooled with no significant differences in geometric standard de viation (sigma (g)). When the impactors were operated at ambient temperatur e, MMADs were smaller (18 to 30%) with a significantly larger sigma (g) (p < 0.05) compared to the MMX. These findings suggest that droplet distributi on data for wet aerosol where evaporation process has not been minimized mu st be viewed with caution. There was no evidence suggesting a significant e vaporative loss of small droplets from the edge of the plume during laser p article sizing. The MPCI does not minimize evaporative losses of aerosol pa rticles during sampling.