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
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.
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.
