Cascade impactor particle analysis

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

Figure 4. Qualitative fluorescence analysis of a 5 n fly ash particle trapped on the first stage of the Cassella cascade impactor used to sample the precipitator inlet...

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... 

Most cascade impactors do not give data in real time. The collection surfaces must be removed from the device and subjected to chemical or gravimetric analysis. However, one impactor does give data in real time. The Model PC-2 Air Particle Analyzer (California Measurements, Inc., Sierra Madre, CA) achieves a real-time measurement by using piezoelectric quartz crystal microbalance (QCM) mass sensors to electronically weigh particles at each impactor stage [62,63], The device has 10 stages and separates the aerosol into... 

Mitchell JP, Nagel MW, Wiersema KJ, et al. Aerodynamic particle size analysis of aerosols from pressurized metered-dose inhalers comparison of Andersen 8-stage cascade impactor, next generation pharmaceutical impactor, and model 3321 aerodynamic particle sizer aerosol spectrometer. AAPS PharmSciTech 2003 4 E54. 

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. 

In nearly all cases, cascade impactors with inlet throats are used for the analysis of DPI clouds. In most instances this is because DPI clouds contain excipients as well as drug particles, and chemical drug-specific assays are required in order to measure the drug size distribution. Although inlet bends are not strictly necessary, because the aerosol particles are moving at the same speed as the inhaled air, they are usually used as a matter of experimental convenience. 

A cascade impactor consists of a series of orifices, of decreasing opening size, which lie ahead of collecting plates (see Figure 2.22). For each orifice size, particles that are too large to remain with the gas stream are deposited onto the collection plates. With each subsequent stage, smaller and smaller particles are trapped and collected. Since the collection is due to sedimentation, the results are usually reported in terms of equivalent aerodynamic diameter. Since each size fraction is collected on a separate collection plate, they are available for individual chemical analysis. Cascade impactors are applicable to the collection and classification of particles down to about 10 nm and up to about 25 pm (aerodynamic diameter) [128,129]. 

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