Impaction

Microbes and Atmosphere
Part-1
Relationship between Microbes and Atmosphere
Atmosphere as a Habitat
Transport of Spores
Sampling of Air
Launching
Transport of Microorgannisms in Air
Deposition and Adhesion
Types of Deposition
Gravitational Settling
Surface Impaction
Rain and Electrostatic Deposition
Methods of Air Sampling
Impingement
Impaction
Centrifugation
Filtration and Deposition
Sampling Methods and their characteristics

Impaction

	Impaction The Anderson six-stage impaction sampler (Fig. 10.5) provides accurate particle size discrimination in contrast to the impinger. This device was developed by Anderson in 1958 and the general operating principle is that air is sucked through the sampling port and strikes agar plates. Impaction procedure depends upon the internal properties of the particle (size, density), on the physical parameters of the impactors (inlet nozzle dimensions) and the airflow pathway.
The principle that underlies this sampling device is simple and ingenious. Air impinging onto the lop agar plate is travelling at relatively low speed, and is deflected around the agar plate. Only the larger (heavier) airborne particles will have sufficient momentum (defined as mass x velocity) to break free from this air current and impact onto the top agar surface. But then the same volume of air is sucked through a series of small holes, so its velocity is increased and this enables smaller particles to impact onto the second agar plate and so on, down the series of plates with increasingly smaller holes, so that the momentum of the airborne particles is increased at each stage.
The result is a size (mass) separation of the airborne particles, which is remarkably similar to that which occurs in the human respiratory tract (Fig. 10.5c), as explained later. Large spores such as those that impact onto a rotorod tape (greater than 7-10 micrometres) will impact onto the topmost agar plate. Smaller spores (3-7 micrometres) will impact on the middle agar plates, and even very small spores (e.g. the spores of actinomycetes, 1-2 micrometres diameter) will impact on the lowest agar plates.
When the apparatus is running (see Fig. 10.5c) the incoming air impinges onto the topmost agar plate, where airborne particles can impact on the agar surface. Then the air is drawn round this first agar plate, and through the first set of perforations, so that particles can impact on the second agar plate, and so on down the stack. Larger particles are collected on the first layer, and each successive stage collects smaller and smaller particles by increasing the flow velocity and consequently the impaction potential. The particle size distribution of the air particles can be directly related to the particle size distribution that occurs naturally in the lungs of animals.
	The lower stages correspond to the alveoli and the upper stages to the upper respiratory tract. The biological sampling efficiency is somewhat lower because of the method of collection which is impaction on an agar surface (on a solid surface).

Impaction

After this apparatus has run for some time (a few minutes or several hours, depending on the likely spore load in the air), the agar plates are removed and incubated, to identify the organisms that grow on them. Fig. 10.7a shows a plate from the lowest part of the Anderson sampler. The colonies are of thermophilic actinomycetes (Faenia rectivirgula or Thermoactinomyces vulgaris) that are common causes of Farmer's lung disease.

Figures above shows agar plates from the middle part of the Anderson sampler, where several species of Aspergillus and Penicillium have developed from spores about 3-5 micrometres diameter.

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