Air sampling Part 3 - Impaction

Impaction

Impaction is of two types. One based on the principle of centrifugal action and another based on impingement onto solid.

a)         Centrifugal action

Egs: 1. Air Centrifuge

        2. Reuter centrifugal air sampler

b)      Impingement on solids

Egs: 1. Hollaender & Dalla valle sampler

        2. Slit sampler

        3. Sieve sampler

        4. Andersen six stage or multistage sampler

 Air sampling by Centrifugal action

1.         Air Centrifuge

It is a primitive type of air centrifuge defined by Wells in 1993. In this method, particles from air are centrifuged onto culture medium. Inner surface of the instrument is lined with culture medium. Sampled air passes along a tube which rotates rapidly on its long axis and acts as the inlet. Bacteria  containing particles get deposited on the medium during this centrifugation and develop into colonies on incubation.

2.         Reuter centrifugal air sampler

A modern version of air centrifuge is Reuter centrifugal air sampler. This device is portable and battery operated. This resembles a large cylindrical torch with an open-ended drum at one end.  This open end acts as both inlet and outlet. Drum end encloses an impeller blade rotated by battery power.A plastic strip coated with medium is inserted along the inner side of the drum. Air entering the drum is subjected to centrifugation by the rotation of the impeller blade when switched on. Suspended particles in air attach to the medium on plastic strip and can be removed and  incubated after sampling period at required temperature.

Advantages

❖         Simple to use

❖         Convenient for transportation

Limitations

❖         Less efficient than Slit sampler in detecting particles below 5mm diameter

❖         Volume of air sampled can not be accurately measured and controlled

 

Air sampling by Impingement on solids

Impingement on solids is based on the principle that when air is drawn from the environment at a fixed rate and suspended particles are allowed to impinge on the surface of an agar plate, each particle containing viable bacteria will form a colony on incubation.

 1.         Hollaender & Dalla valle sampler

Consists of a brass container with a removable bottom. It is fitted with an inverted glass funnel which will act as the inlet. Inside of the funnel and rim is to be wiped with alcohol before use. At the lower part of the container, a petri dish base with medium is placed and screwed tightly against the funnel. Funnel is kept just above the media, approximately at 2mm above and should not touch the media. Chamber has an outlet that can be connected to a vacuum pump. When the pump is switched on,  and air borne microbes are impinged upon the agar medium. Vacuum pump can be connected in series with a flow meter to measure the exact volume of air  sampled. Effective sampling rate is 28 lit/minute.  It is a simple, efficient and portable air sampler.

 

 2.   Slit sampler (Bourdillon slit sampler)

An efficient and convenient device for enumeration of bacteria carrying particles in the unit volume of air. This method was introduced by Bourdillon et al  in 1941 and hence also called Bourdillon slit sampler. Instrument consists of an enclosed box with an air tight door. It has a narrow slit on the top measuring 0.33 mm width, 27.5 mm length and vertical parallel sides of about 3 mm depth. Slit acts as the inlet. Outlet of the box is connected to a vacuum pump and maintains a negative pressure of 22.6 mm Hg inside. At correct negative pressure, air at a rate of 1 cubic foot/min enters through slit. At the bottom, a rotating platform is present to keep the agar plate. This platform is usually coated with adhesive or gripping material to prevent slipping out of the agar plate during sampling. Slit is exactly 2mm above the plate. As the plate is rotating with the platform, particles are deposited over the entire surface of the plate.

In this method, a sterile,dry agar plate with an even surface is taken. Sampling area, date, time, duration  of sampling, volume of air sampled etc are marked. Slit is made free from dust with alcohol and by inserting the edge of a stiff paper. Petri Plate is placed on the platform through the door and height is adjusted to be 2mm below the slit. The motor that rotates the plate and the vacuum pump are switched on simultaneously. After the sampling period, both are switched off and the plate is taken out carefully, incubated to get the colonies. It can be also used to count spores and hence useful in plant disease forecasting.

Limitations

❖         Chance  of contamination from skin and clothes of the operating person while handling the plate.   

❖         Unnecessary movements should be avoided during sampling.

A modification is Size Grading Slit Sampler with slits of different sizes kept in series. Sets of plates used to collect particles of different sizes. It is highly efficient and found to collect 95% of water droplets sprayed into air. It can even collect respiratory secretion droplet nuclei of 0.2µm diameter.

2.1. Hirst Spore trap

Hirst spore trap is an example of a slit sampler. Was developed by Hirst in 1952. Also known as Burkard Spore Trap. It is a volumetric air sampler that is one of the standard devices for monitoring airborne pollen and spores. Widely used by the plant pathology community.

It is a type of suction slit impactor used for pollen and spore sampling. First sampler of this type was designed  in 1952 byDr James Hirst, a plant pathologist in England. So samplers of this type are usually referred to as Hirst spore traps.

In  Burkard trap, air is drawn into a 14 mm X 2 mm orifice at a rate of 10 lit/ min and any air borne particles with sufficient inertia are impacted on either a greased tap or a greased microscopic slide beneath the orifice. & days continuous sampling is possible with removal of slides occasionally.

3. Sieve sampler

This is a mechanically simpler instrument. More or less similar to the slit sampler.  Particles containing microorganisms are distributed over the plate as separate air jets through several holes in the sieve plate. Perforated sieve is acting as  the inlet and outlet connected to the vacuum pump. After sampling, petridish is removed carefully and incubated to get the colonies.

4. Anderson sampler

Also known as Anderson six stage/eight stage/ multistage sampler. Here several impingers (6 to 8) with holes of different sizes are arranged in a series. This is an indigenous device that selectively trap different  particles  according to their size. It consists of a stack of 6 to 8 metal sections that are fitted together with ring seals to form an airtight cylinder. Each metal section has a perforated base. Number of perforations in each section is the same, but the size of perforations progressively reduces from top of the column to the bottom. Open petri plates with media are placed between each metal section, resting on three studs. When fully assembled, an electric motor sucks air from the bottom of the unit making air to enter through the inlet at the top and passes down through the cylinder. Air sucked in the top of the column travels relatively at a low speed towards the first agr plate and so only the largest particles impact on the first agar plate. The air travels round the edge of the agar plates and through the perforations to the second agar plate and so on. As this continues down the stack, same volume of air is forced to travel through successively smaller perforations, so air speed is progressively increased. Thus even smallest particles can impact onto the lower agar plates. After 5-15 min, metal plates are separated and petri dishes are incubated. Anderson sampler mimics the deposition of air borne particles in the human respiratory tract.