Aerosol characterization aerodynamic diameter

Both from deposition studies and force balances it can be derived that the optimum (aerodynamic) particle size lies between 0.5 and 7.5 pm. Within this approximate range many different subranges have been presented as most favourable, e.g. 0.1 to 5 pm [24], 0.5 to 8.0 pm [25], 2 to 7 pm [26] and 1-5 pm [27-29]. Particles of 7.5 pm and larger mainly deposit in the oropharynx [30] whereas most particles smaller than 0.5 pm are exhaled again [31]. All inhalation systems for drug delivery to the respiratory tract produce polydisperse aerosols which can be characterized by their mass median aerodynamic diameter (MMAD) and geometric standard deviation (oq). The MMAD is the particle diameter at 50% of the cumulative mass curve. 

Another example is the aerodynamic diameter this diameter is used to characterize aerosolized particles for which the density is difficult to determine. Thus, one assumes that the particle density equals 1 and does the Stokes" Law calculations for the sphere of unit density and having the same settling velocity as the particle in question (8). 

As a result of the complex aerodynamics of the particle beam and associated skimmers, the size distribution of the particles that reach the ion source differs significantly from that in the gas samples from outside the system. To relate the measured chemical compositions to the outside aerosol, it is necessary to correct for this effect. This can be accomplished in principle by determining the elficiency of transmission of particles from the exterior into the chamber. It is also possible to use data for particle size distributions measured outside the spectrometer to characterize the external aerosol. Because the particle size distribution measured with an optical particle counter does not correspond to the aerodynamic diameter, there will be some difficulties of interpretation. 

Very often inertial deposition in impactors is used to characterize the aerodynamic behavior of aerosol particles. However, much larger inertial forces are applied for particle deposition in impactors than are available for particle deposition in the human respiratory tract. The particle size obtained by this technique is the inertial diameter. This diameter is defined in the same way as the aerodynamic diameter but based on inertial rather than gravitational particle transport. When a particle is not only inertially but also gravitationally transported its inertial diameter is identical with its aerodynamic diameter. 

Measurements of the quantity and quality of the aerosolized drug allow characterizing the dosing properties of inhalation devices in vitro. Multistage im-pactors are used to assess particle mass and mass distribntion of an aerosol, and methods are available to estimate the mass median aerodynamic diameter (MMAD) of the aerosol as well as the dose of delivered from and retained within an inhalation system. 

DeCarlo, P. E, Slowik, J. G., Worsnop, D. R., Davidovits, P., and Jimenez, J. L. Particle morphology and density characterization by combining mobility and aerodynamic diameter measurements. Part 1 Theory, Aerosol Sci. Tech., 38, 1185-1205, 2004. 

Thus, for spherical particles having a density equal to Ig/cm, the aerodynamic and eqnivalent volume diameters are mathematically equal, hi practice, all medical aerosols comprise particle size distributions thus, the value mass median aerodynamic diameter (MMAD) is freqnently used to characterize an aerosol. 

A comparative study of the spray-dried nanoparticulate budesonide versus micronized budesonide was performed Each powder was blended with lactose and then filled into a Clickhaler (ML Laboratories) DPL and the aerodynamic diameters of the dehvered powders were characterized on an Andersen eight-stage cascade impactor. The results, which are summarized grcq)hically in the following section, show that the MMAD of the nanoparticulate aggregate aerosol was much smaller than the MMAD of the micronized drug aerosol. A substantial fraction of... 

The question of particle shape is a complex problem and we are still at the sti e where we are developing methods to see if we can characterize adequately the range of shapes within a powder and their effect on the powder system and/or the aerosol system. It is becoming apparent that some complex problems will require more than one method of characterization thus if one was inhaling a complex soot particle the aerodynamic diameter which governs the penetration of the lung is one parameter whereas the fractal structure is another needed to assess the potential health hazard of the inhaled aerosol particle. 

Besides mass concentration, atmospheric particles are often characterized by their size distribution. Aerosols are typically sized in terms of the aerodynamic equivalent diameter (dae) of the particle, usually expressed in micrometer (pm) or nanometer (nm) (Mark, 1998). Atmospheric particles are usually nonspherical and with unknown density. Therefore, the r/ae of a particle is usually defined as the diameter of an equivalent unit density sphere (p = 1 gctrf3) having the same terminal velocity as the particle in question (Mark, 1998 Seinfeld and Pandis, 1998). 

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