The following points highlight four sampler that are conventionally used for trapping pollen grains. The samplers are: 1. Rotorod Sampler 2. Burkard Personal Volumetric Sampler 3. Burkard Seven Day Volumetric Sampler 4. Andersen Two Stage Sampler.
1. Rotorod Sampler:
The rotorod sampler (Fig.10.10) was originally devised by Perkins (1957) and has been somewhat modified by Harrington (1959). The device relies upon the high efficiency with which small airborne particles are deposited on narrow cylinders oriented at right angles to high velocity winds.
The sampler consists of a small constant speed, battery operated motor, which makes the sticky coated brass rod to whirl around its axis. The collecting arms are made up of brass rod having 0.15 cm cross sectional area, which were bent in the form of ‘U’. The two vertical arms measuring 7 cm long and 3.5 cm apart from the axis.
The arms are bent together slightly inwards so that they come to a vertical position while in rotation. This is welded to a brass rod of 1 cm in diameter and 1cm in length in an upright position. A hole is drilled at the bottom of the rod to keep it firmly on the motor shaft with a screw device. Each D.C. motor with 6 volt battery gives rotation speed of 2500 r.p.m.
Sampling Method:
The sections facing the wind are stuck with sticky cellotape along the full length and edge trimmed uniformly with a sharp blade. The surface of the trap is uniformly coated with glycerine jelly as a mountant.
Air sampling is carried out at the desired site at regular intervals. The samplers are kept vertically above ground level and generally operated for half an hour.
Mounting and Scanning of the trace:
After sampling, the cellotape with spore catches is removed from the rods and placed on clean glass slide. The glass slide is warmed a little and covered by cover slip. The slide is labelled mentioning the exposure site, date and time.
The counting of pollen and spores is done by examining the slide in narrow horizontal traverses across the width of slide until the entire surface is covered. The counting is normally done in lower magnification of microscope, but higher magnifications are also used when required.
The counts are converted to concentration/m3 of air by multiplying with an appropriate conversion factor which is calculated by following the method as suggested by The British Aerobiological Federation (1995).
Calculation of Conversion Factor:
When a rod rotates at high speed the arms may be thrown out slightly. To calculate the volume swept by the rotating arms the radius of the top of the arm (r2) must be estimated.
If S = width of the rod (say, 0.1 cm)
h = length of the strips (say 6.5 cm)
Area of strips counted (a) = field width (say, 350 x 10-4 cm.) x width of strips (say, 0.16cm.) x number of strip sections per traverse (say, 8) x number of traverses (say, 5).
Total area of strips (A) = 2 x width of strips (say, 0.16 cm.) x length of one strip (say, 6.5 cm.)
2. Burkard Personal Volumetric Sampler:
This sampler is made by Burkard Manufacturing Co., Herts, England. This trap is made up of light alloy and plastic material (Fig 10.10). The height of this trap is 96 mm with a diameter of 88 mm and net weight is 600 gm. This apparatus can be operated either on mains or with batteries. The impaction of orifice is mounted in the vertical plane.
The instrument has a suction rate of 10 litres of air per minute. An ON/OFF switch is mounted on the underside of the sampler. When the upper ring assembly is rotated until the red dots are in one line, the aperture at the side gets exposed to accept the slide. The instrument is provided with slide holes to move the exhaust air. Airborne particles are collected directly on the glass slides.
Sampling Methods:
A clean glass slide measuring 75 x 25mm of 0.8mm thickness, coated with melted glycerine jelly in the middle is inserted through the slide slit. Then the air sampling chamber is closed by rotating the upper ring in either direction at least one inch ensuring that the slots are fully covered.
At the end of the sampling, the slide is removed by rotating the upper ring and with the help of a suitable spatula or forceps. A sample of 2 x 14mm get deposited on the glass slide. The slide is mounted in DPX by placing a 14 sq.mm microscopic cover slip. This is allowed to dry for two to three days and then scanned.
Scanning of exposed slides:
In each slide several scans (say 20) are made across the trace with the use of a compound microscope under low (10x) and high (40x) magnifications. The width of each scan is measured (say 230pm).
The counts are converted to number per cubic meter of air by multiplying with an appropriate conversion factor which is calculated as follows:
Conversion Factor:
Total area of trace = 2x 14 = 28mm2
Scanning width = 230µm
No. of scans = 20
Area scanned in
one trace = 230 x 20pm
= 4600pm = 4.6mm
Total area scanned = 4.6 mm x 2mm
= 9.2 mm2
Suction rate = 10 liters/minute
Sampling time = 10 minutes
Volume of the air sampled = 10 x 10=100 liters = 0.1m3
Fraction of total deposit scanned:
Volume of air sampled in scanning area = Volume of the air sampled x Fraction of total deposit scanned
Conversion factor for estimating number/m3 = 1/0.0328
= 30.48 = 30 (rounded)
3. Burkard Seven Day Volumetric Sampler:
It is an instrument using the volumetric method (Fig.10.10). It resembles Hirst trap. It can trap samples of airborne particles such as fungus spores and pollen grains continuously for a period of seven days. This apparatus runs on electric power supply and provides a continuous sampling of air for seven days. An automatic clock fitted in the instrument is synchronized with the air. This suction trap provides data on rapid changes in the composition of airspora.
The air is sucked through at the rate of 10 litre/min with the help of a vaccum pump. The transparent cellophane tape coated with glycerine jelly is sticked on the rotating drum having a circumference of 345 mm. The drum rotates at 2mm/hr in front of a 2mm/4mm orifice through which the air is aspirated.
The adhesive tape is changed in every week. The mounting of cellophane tape is done in glycerine jelly. Scanning is done by dividing the tape into seven sections which are mounted on seven separate slides. The tape can be further divided into hourly intervals for microscopic examination.
Calculation of Conversion Factor:
[The method of Burkard sampler as suggested by The British Aerobiological Federation (1995) is followed].
Daily average concentrations.
If F = the flow rate of trap in 1 min-1 (about 10)
W = the width of the trap orifice, mm (14mm)
f = microscopic field width counted (say 350µm)
NC = total number of spores counted in one traverse, the concentration of spores (N) per m3 is:
Hourly average:
As the rate of rotation of the Burkard trap is 2 mm h-1 and the length of the orifice is 2mm. Each piece of tape is exposed for one hour. This means that a single traverse across the tape will give a measure of the hourly average concentration of spores. Time averages less than this cannot be measured.
Hourly average spore concentrations are calculated as follows:
NC = Spore count from one traverse across the tape
f = field width of the trace in pm
F = flow rate of the trap 1 min-1 (~10)
N = the spore concentration
4. Andersen Two Stage Sampler:
This is the two stage and bijet sampler for airborne particles which runs on electric power supply. Here after entering the circular orifice air is drawn through a series of two circular plates each perforated with 400 holes.
The plates in series have progressively similar holes and so the largest particles being deposited in the first while the smallest in second petridish containing appropriate media. The air to be sampled enters the jet orifices (at a rate of 28.3 litre per minute) of stage I and cascades through the jet orifices of stage II with a higher orifice velocity ( Fig.10.10).
Fig. 10.10: Volumetric samplers (A = Burkard one day personal sampler, B = Andersen sampler, C = Burkard sevenday volumetric sampler, D = Rotorod sample
Sampling Methods:
The petridishes are thoroughly washed after rinsing with chromic acid solution and dried in oven. The petridishes are wrapped by brown paper and sterilized by autoclaving at 15 lbs for 20 minutes. The appropriate media are poured in conical flask and plugged by non-absorbent cotton.
The media are sterilized by autoclaving. After sterilization the media are poured in sterilized petridishes within UV or inoculating chamber. Generally antibacterial supplements like Penicillin and antifungal agent like Mycostatin are added.
Two petridishes are placed in the respective plates. The lid of orifice is opened. At the end of the sampling, the petridishes are removed by opening the lids of the plates which are brought to the laboratory and incubate at 28° – 30°C for 3-4 days.
The developing fungal colonies are counted and their characteristic features are noted. Fungi from the developing colonies are also placed on a clean slide, mounted and stained. The fungi are examined under microscope. Identification is made by observing vegetative and reproductive characters of the fungi.
Calculation of Conversion factor:
Suction rate = 28.3 litres/min.
If operated for 10 minutes, air taken in is = 0.283 m3
If 0.283 m3 of air forms one colony, then lm3 of air forms 1 /0.283 = 3.53 colonies
So, the conversion factor for 10 minutes operation is 3.53.
Accordingly, if operated for 3 minutes the conversion factor will be 11.77.