What is an impactation?

Answer 1

2) Impactors

Impactor samplers use a solid or adhesive medium, such as agar, for particle collection and are much more commonly used in commercial applications than impingers, largely because of their convenience. In a typical impactor sampler air is drawn into a sampling head by a pump or fan and accelerated, usually through a perforated plate (sieve samplers), or through a narrow slit (slit samplers). This produces laminar air flow onto the collection surface, often a standard agar plate or contact plate filled with a suitable agar medium. The velocity of the air is determined by the diameter of the holes in sieve samplers and the width of the slit in slit samplers. When the air hits the collection surface it makes a tangential change of direction and any suspended particles are thrown out by inertia, impacting onto the collection surface. When the correct volume of air has been passed through the sampling head, the agar plate can be removed and incubated directly without further treatment. After incubation, counting the number of visible colonies gives a direct quantitative estimate of the number of colony forming units in the sampled air.

Impaction samplers offer benefits in terms of convenience and pre-poured, gamma-irradiated contact plates and standard petri dishes from specialist suppliers can be used with them to minimise the risk of contamination and variation. They are also able to handle higher flow rates and the large sample volumes necessary to monitor air quality in clean rooms where the number of microbes present is likely to be very low. However, care must be taken not to allow agar plates to remain in the sampler heads for too long, or the medium may dry out and deteriorate. Microbial cells may also be damaged by mechanical stress during the sampling process and lose viability.

Most impaction samplers also do not allow the use of rapid methods to enumerate and characterise microorganisms, but rely on conventional culture for several days to obtain a result. This problem can be overcome to some extent by the use of a water-soluble polymer gel instead of agar. This allows the sample to be analysed by rapid techniques such as PCR or cytometry.

A wide variety of instruments have been developed using the impaction principle. One of the best known is the Andersen sampler, a multi-stage 'cascade' sieve sampler that uses perforated plates with progressively smaller holes at each stage, allowing particles to be separated according to size. Another well known instrument is the Casella slit sampler, in which the slit is positioned above a turntable on which is placed an agar plate. As air is drawn through the slit, the agar plate rotates, so that particles are deposited evenly over its surface.

Both of these instruments have been used for many years, but more recently a number of highly portable and convenient impaction samplers have been developed specifically for monitoring the air in production facilities and other sensitive areas. Most of these are sieve samplers, such as the Surface Air System (SAS) samplers made by pbi International in Italy, and use agar contact plates or full-sized culture plates as the collection surface. However, some types, such as the RCS samplers from Biotest Diagnostics, use a centrifugal impeller to accelerate air onto a dedicated agar-coated strip that can be incubated directly. These portable samplers can be hand-held, or mounted on a tripod during sampling, and can be programmed to sample a specific volume of air, or sequential samples at pre-set times. Samplers specifically designed to monitor the microbiological quality of compressed gases are also available.

Answer 2

An impaction is a condition where there is a blockage or obstruction preventing the normal flow or movement of something, like feces in the colon or earwax in the ear canal. Treatment may involve removal of the impacted material through various means such as manual removal or irrigation.