Optical measurement of nebulizer sprays: A quantitative comparison of diffraction, phase Doppler interferometry, and time of flight techniques

Five spray analysis instruments are compared by measurement of identical medical nebulizer sprays. The instruments tested include a phase Doppler interferometer (Aerometrics), a time-of-flight instrument (Amherst Process Instruments) and three diffraction instruments (all by Malvern). Two different jet nebulizers were used for testing. Each was tested with both water and propylene glycol. Volume distributions, mass median aerodynamic diameters (MMAD), and ninety-percent volume aerodynamic diameter measurements (d.(90)) are compared. MMAD results from the Aerosizer were consistently higher than those from the diffraction instruments. MMAD and d.(90) results From the Phase Doppler system agreed well with the diffraction instruments for water sprays, but varied from the other instruments for propylene glycol sprays. This occurred because the Phase Doppler Interferometer reported larger droplets present in the propylene glycol sprays that in some cases significantly affected the otherwise smaller sized volume distributions. These droplets were not apparent in the results from the other instruments bur were noted during visualization studies. Volume distribution results demonstrate that the Malvern 2600 is inappropriate for use with sprays in the examined size range. The other volume distributions were similar with the exception of the Aerosizer, which reported higher, more normally distributed volume distributions compared to the other instruments. (C) 1999 Elsevier Science Ltd. All rights reserved.