Cascade impactor measurement

Van Vaeck, L., and K. Van Cauwenberghe, Cascade Impactor Measurements of the Size Distribution of the Major Classes of Organic Pollutants in Atmospheric Particulate Matter, Atmos. Environ., 12, 2229-2239 (1978). 

In addition, the cumulative size distribution obtained for pure micron-ized and supercritically produced powders, combined with computation of the aerodynamic diameter according to Eqs. (2) and (3), shows that FPF of supercritically produced particles is about 50%, twice as much as FPF for micronized powder and close to the dispersion efficiency found with lactose (76). The correct values for the total emitted dose of drug particles also were confirmed. In all cases, the state of dispersion for the different formulations was correctly predicted and the results obtained showed good agreement with the cascade impactor measurements made on the same samples. Therefore laser diffraction has distinct advantages over impactor techniques with respect to the speed and reproducibility of measurements and can be used as a complementary method for analysis of aerosol behaviour of different formulations. 

A deconvolution problem in general does not have a unique solution. Instead there are an infinite number of possible solutions that can fit the same set of cascade impactor measurements. It is well recognized that, for most engineering applications, actual particle size distributions can be reasonably represented by a set of log-normal distributions. With this concern, in the following, a deconvolution method (chi-squared method) to extract particle size distributions from cascade impactor data is introduced, which is based on multimodal log-normal size distributions (Dzubay and Hasan, 1990). 

Figure 6 Sketch depicting technique for bench measurements of inhaled mass and particle distribution. Breathing pattern defined by settings on Harvard pump. Particles presented to patient are captured on the inhaled mass filter. In separate experiments, the cascade impactor measures inspired aerosol isokinetically. (From Ref. 10.)...

The activity median aerodynamic diameters (AMADs) of Pb (Table 2.3) and mass median aerodynamic diameters (MMADs) of SO (Table 2.4) determined from a series of low- pressure (LPI) cascade impactor measurements made during the period January to October (1985) by Papastefanou and Bondietti (1987) are illustrated in Figure 2.7. The Pb data were derived from measurements made at the same time as and from measurements made to compare Pb versus Pb. The mean aerodynamic diameter of Pb was about three times smaller than that of SO ". Much less sulfate was found in the aerosol fraction below 0.08 pm... 

Fig. 2.21. Aerodynamic size distribution and the fitted log-normal distribution from the cascade impactor measurement of particles resuspended from the fixed air filter samplers (FAS) filter.

Cascade impactors—measures one-half to two-thirds of accumulation mode, all of coarse particle mode particle bounce is a serious 0 (filter) 0.3 (last stage) 30 8 1 pg/m /stage Inertial impaction... 

Markowski, G.R. (1984). Reducing blow off in cascade impactor measurement. Aerosol Sci. Technol. 3, 431 39. Marley, N.A., Gaffney, J.S., Drayton, RJ. (2000). Measurement of Pb, Po, and Bi in size-fractionated atmospheric aerosols An estimate of fine-aerosol residence times. Aerosol Sci. Technol. 35,569-583. 

Pb-210, Be-7, P-32, S-35 (as So ), and stable so - were measured using cascade impactors. The activity distribution of Pb-212 and Pb-214 was largely associated with aerosols smaller than 0.52 pm. 

Aerodynamic Size Distributions of Naturally-Radioactive Aerosols. Measurements of radionuclide distributions using cascade impactors indicate that Be-7 and Pb-210 are associated with larger aerosols than Pb-212 and Pb-214 (Robig et al., 1980 Papastefanou and Bondietti, 1986). Measurements of Pb-210 associations over oceans indicated activity median aerodynamic diameters (AMAD) near 0.6 pm (Sanak et al., 1981). The impactor measurements of Moore et al. (1980) on Pb-210, Bi-210, and Sr-90 sizes in continental air indicated that about 80% of the activity from all three nuclides was associated with aerosols below 0.3 pm. That work also determined that the mean age of aerosol Pb-210 was about a week. Knuth et al. (1983) compared Pb-210 and stable Pb sizes at a continental location and found that 78% of the Pb-210 found below 1.73 pm was smaller than 0.58 pm. Young (1974) reported that the most of the Be-7 in the atmosphere was associated with submicron aerosols. 

Table III. Median Aerodynamic Diameters (MAD) and Geometric Standard Deviations (Og) of Pb-210 and Be-7 Based on Radioactivity and SO Measured with High-Volume Cascade Impactors...

The aerodynamic size distributions of Pb-214, Pb-212, Pb-210, Be-7, P-32, S-35-SoJ , and stable SO4 were measured using cascade impactors. Pb-212 and Pb-214, measured by alpha spectroscopy, were largely associated with aerosols small than 0.52 11m. Based on over 46 low-pressure impactor measurements, the mean activity median aerodynamic diameter (AMAD) of Pb-212 was found to be 0.13 11m, while for Pb-214 the AMAD was larger—0.16 lim. The slightly larger size of Pb-214, confirmed with operationally different impactors, was attributed to a-recoil-driven redistribution of Pb-214 following decay of aerosol-associated Po-218. A recoil model was presented that explained this redistribution. Low-pressure impactor measurements indicated that the mass median aerodynamic diameter of SoJ ... 

Better collection methods are needed for stack measurements. Virtual Impactors or other devices that avoid the bounce-off problems of cascade Impactors should be developed. Methods for collection of very large amounts of size-segregated particles suitable for detailed organic analyses are needed. Better low-blank filter and collection-surface materials that can withstand high stack temperatures are needed. Careful attention should be given to the determination of vapor-phase species of volatile elements and compounds. 

Most of the published composition/size distribution data have been obtained by analyzing cascade impactor samples. Some of these data suffer from poor size classification as a result of particle bounce or reentrainment, seriously limiting size resolution. Even when this problem is overcome, the data obtained with conventional cascade impactors are not capable of resolving many details of the distribution of submicron particles. These instruments typically classify only those particles larger than 0.3-0.5 tam aerodynamic diameter. All smaller particles are collected on a filter downstream of the impactor. Some measurements of the variation of composition with size below this limit have been attempted by aerodynamically classifying resuspended ash ( ). These data suffer from incomplete disapregation as well as poor classification of the smaller particles. 

Van Vaeck, L K. Van Cauwenberghe, and J. Janssens, The Gas-Particle Distribution of Organic Aerosol Constituents Measurement of the Volatilization Artifact in Hi-Vol Cascade Impactor Sampling, Atmos. Environ., 18, 417-430 (1984). 

In an attempt to increase the amount of particles retained in the lungs, large porous particles with low density (p < 0.1 g/cm2) have been designed (Edwards et al. 1997). The particles were composed of 50% lactide and 50% glycolide. Porous and nonporous particles loaded with testosterone were aerosolized into a cascade impactor system from a dry powder inhaler (DPI) and the respirable fraction was measured. Nonporous particles (d = 3.5 pm, p = 0.8 g/cm3) exhibited a respirable fraction of 20.5 3.5%, whereas 50 + 10% of porous particles (d = 8.5 pm, p = 0.1 g/cm3) were respirable, even though the aerodynamic diameter of the two particle types were nearly identical. Porous particles as a consequence of their large size and low mass density can... 

Stein SW. Size distribution measurements of metered dose inhalers using Andersen Mark II cascade impactors. Int J Pharma 1999 186(1) 43—52. 

Particle Number Concentration and Size Distribution. The development of aerosol science to its present state has been directly tied to the available instrumentation. The introduction of the Aitken condensation nuclei counter in the late 1800s marks the beginning of aerosol science by the ability to measure number concentrations (4). Theoretical descriptions of the change in the number concentration by coagulation quickly followed. Particle size distribution measurements became possible when the cascade impactor was developed, and its development allowed the validation of predictions that could not previously be tested. The cascade impactor was originally introduced by May (5, 6), and a wide variety of impactors have since been used. Operated at atmospheric pressure and with jets fabricated by conventional machining, most impactors can only classify particles larger... 

Chemical Composition Aerosol composition measurements have most frequently been made with little or no size resolution, most often by analysis of filter samples of the aggregate aerosol. Sample fractionation into coarse and fine fractions is achieved with a variety of dichotomous samplers. These instruments spread the collected sample over a relatively large area on a filter that can be analyzed directly or after extraction Time resolution is determined by the sample flow rate and the detection limits of the analytical techniques, but sampling times less than 1 h are rarely used even when the analytical techniques would permit them. These longer times are the result of experiment design rather than feasibility. Measurements of the distribution of chemical composition with respect to particle size have, until recently, been limited to particles larger than a few tenths of a micrometer in diameter and relatively low time resolution. One of the primary tools for composition-size distribution measurements is the cascade impactor. 

Suspensions contain micronized drug for proper delivery to and absorption in the respiratory system. Typical particle size of the micronized drug is from 2 to 5 microns [5], Aerodynamic mean particle size as measured by cascade impactor or direct method of microscopic analysis is usually from 0.5 to 4 microns [5], Additional particle-sizing techniques such as light scattering can be used [6],... 

Particle Size Analysis Many particle-size-analysis methods suitable for dry-dust measurement are unsuitable for liquids because of coalescence and drainage after collection. Measurement of particle sizes in the flowing aerosol stream by using a cascade impactor is one... 

Certain measurements cannot be made in the field, and for these it is necessary to collect samples for transport to the sorts of instruments that are available only in laboratories. Collection of size-segregated samples can be achieved by the use of a cascade impactor, which consists of a series of apertures of decreasing diameter, each of which is above a matched, stacked collection... 

The impacted drug fractions are normally measured via a specific chemical assay, although mircoweighing the plates has also been investigated. A number of actuations are normally accumulated for each cascade impaction determination to collect sufficient drug for accurate assay. The United States Pharmacopoeia includes a test for MDIs, using a cascade impactor coupled to a throat of specified dimensions. ... 

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