The Mercer-style impactor

The use of an impactor with a single circular orifice for each jet provides a convenient method for determining the aerodynamic size distribution of an easily assayed aerosol, such as of a radioactive aerosol. This type of impactor is preferred when small sample rates and short sampling periods are desired since the stage characteristics can be accurately prescribed and particle bounce or re-entrainment can be minimised with the use of an appropriate liquid film on the collectors. 

The so-called 100 cm min impactor is normally operated at flow rates from 50 to 150 cm min The effective aerodynamic cutoff diameters are 0.23, 0.50, 0.75, 1.12, 1.53, 2.13 and 2.90 pm. The 1 Lmin unit is normally operated from 300 to 1000 cm min . The effective aerodynamic cutoff diameters are 0.37,0.81,1.18,1.78,2.38,3.34 and 5.10 pm. Operation of the impactor depends critically on maintenance of a constant sampling rate. Normally, the impactor should be operated for no more than 10 min to avoid alteration of the effectiveness of the liquid film on the stage collectors in preventing bounce and to avoid re-entrainment caused by sample build-up depending on the aerosol concentration. 

Using a method of weighed least-squares, a log-normal function describing the aerosol mass and activity size distribution with respect to aerodynamic (resistance) diameter has been routinely fitted to the data. It is also possible to approximate a log-normal function for the mass and activity distribution with the use of log-probability paper. The distribution is then describable by a mass or activity median aerodynamic diameter, MMAD or AMAD, and associated geometric standard deviation, Og. 

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The design of the Mercer-style impactor is illustrated in Figure 6.24 (In-Tox Products, L.L.C., 101 Wilderness Ct. S., Moriarty, NM 87035). Each stage of the impactor has a collector cover-slip holder which can accommodate a standard 22 mm diameter glass cover slip... 

Raabe, O.G. (1979). Design and use of the Mercer-style impactor for characterization of aerosol aerodynamic size distributions. In Aerosol MeasuremerU. University Presses of Florida, Gainesville, FL, pp. 135-140. 

Fig. 6.24. A schematic of the cross-section of the Mercer-style cascade impactor.

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