There are several types of sieve impactors; one-stage impactors have only one perforated plate set in front of an agar plate, while stacked sieve samplers can have up to six stages of perforated plates and agar plates.
When considering sieve impactors, it is important to note that the six stage impactor (with six perforated plates and corresponding agar plates) has an approximately 40 percent higher recovery rate when compared to single and two-stage versions.
In stacked sieve, or cascade samplers, each perforated plate is held above an agar plate with successive plates having smaller holes. At a constant flow, larger particles impact on the first stage whereas smaller particles impact on the last impaction stage. The major advantage of a stacked sieve impactor is that it can provide data on particle size.
Sieve impactors, however, are not without their own set of specific problems. Unless sampling times are short or the relative humidity high, the areas of nutrient agar directly under each hole of a stage can rapidly dry out, adversely affecting the growth of fastidious microorganisms. The agar-filled Petri dishes must be very specific for efficient counts. The agar must be completely level in the dish and filled with precisely the suggested amount of agar to give the correct plate-to-agar surface distance.
Another problem is presented by the plastic Petri dish itself. Using plastic dishes may cause electrostatic effect, deflecting particles away from the dish to deposit on other internal parts of the sampler. While these problems are significant, much like the slit-to-agar sampler, the major problem with sieve impactors is their inefficiency at collecting smaller particles.
From: “Selecting and Active Air Sampling Methodology”