The following points highlight the four main methods of air sampling. The methods are: 1. Gravity Sedimentation Methods 2. Inertial Methods 3. Filtration 4. Precipitation.
1. Gravity Sedimentation Methods:
a. Sedimentation from still air:
Alvarez and Castro (1952) constructed a simple box for the study of airborne fungi, has two hinged slides and a covered tray at the bottom for inserting a microscopic slide or petridish. During air sampling the hinged slides are raised horizontally and wind is allowed to blow through the box.
The slides are then closed and the entrapped spores are sedimented under gravity. Here sampling is discontinuous and a small volume of air being sampled at a time. An improved form of this model was later described by Ogden (1974).
b. Sedimentation from wind:
The method of examining air from deposits on a freely exposed, horizontal surface, such as microscopic slides was used by earlier scientists like Pasteur, Pouchet and even can be traced back to van Leeuwenhock but it fails to examine air quantitatively.
The different methods are:
i) The gravity slide was used by Blackley (1873), Durham (1946), Hyde and Williams (1944), Gregory (1961), Tauber (1974). Some of the common samples are Tauber trap, Durham Sampler, Individual Pollen Collector etc. These are cheap, simple and operate continuously.
But they fail to estimate the airspora quantitatively and gives a distorted picture of airspora, because they preferentially select the larger particles. To correct this distortion Scheppegrell (1922) tried to calculate the volumetric concentration using a formula based on particle diameter which was later corrected by Cocke (1937) to particle radius. But inspite of these defects it is widely used by aerobiologists and has contributed much knowledge to the air spora.
ii) Frankland and Hart (1887) introduced “Gravity Petridish” in which the petridish containing sterile nutrient media were exposed in open air for 1-10 minutes, incubated and after development of colonies these are identified, counted and subcultured for further study.
Gregory (1973) pointed out the defects of such trapping procedure which are – sensitivity of particle size, wind speed and aerodynamic effects; moreover, small volume of air sampled intermittently and so diurnal changes of airspora are not revealed and only cultivable molds or bacteria can be trapped.
c) Sedimentation from artificially moving air:
Hesse (1884, 1888) constructed a narrow horizontal tube of 70 cm long and 3-4 cm wide containing a layer of Koch’s nutrient gelatine. A known volume of air is aspirated slowly through the tube and microbes settled and grew on the medium. Funnel device of Hollander and Dalla Valle (1939) was also based on same principle. In this process very small air is sampled and which is difficult to express quantitatively, so it is rarely used by workers.
2. Inertial Methods:
In this method the particles may be retained on filters, on flat surface or on liquids. Air sample may be drawn through a jet tube or apparatus may move the trapping surface through the air.
a) Impaction using wind movement:
i) Vertical and inclined microscope slides are used for trapping pollen grains and fungal spore by many scientists namely, Blackley (1873), Ward (1882), Mehat (1952), etc.
ii) Vertical Cylinder:
A removable sticky coating like vaseline or cellotape or cellophane paper is applied on a cylinder which hang vertically from a support of desired height. This was first used by Rempe (1937) for trapping airborne pollen. Counting is done by traversing the cellotape/cellophane tape i.e. in effect following circumference around the cylinder.
The number of traverses necessary to obtain a reliable count will depend on the number deposited. Spores counted in the area scanned can be then converted to number per square centimeter for comparison with other catches on cylinder traps.
The vertical trap is convenient and cheap but the main drawback is that it catches a negligible amount of spore in still air and small spores at ordinary wind speed and so changes in catch occurs with alteration in wind speed.
This was first used by medical workers and later by plant pathologists. This was first introduced by Salisbury (1866) and developed more fully by Maddox (1870, 1871). Such wind operated aeroconicoscope are fully qualitative and gives no idea about the number of organisms. So this sampler is now mainly of historical importance.
b. Forced air impactors:
Sampler through which air is drawn by pumps, fans, etc., is dependent of changes in wind speed and difference in particle size, and they can give a volumetric reading under field condition.
i) Sieving filters draw air through a filter with too small pores for organisms sought to penetrate but is of little use because of insufficient rate of air flow through small pores. Impaction filters differ from sieving filters in that they consists of deep layer of fibres or granules separated by relatively wide air spaces.
ii) Multistage liquid impinger developed by May (1966) was devised to separate the collected particles into three fractions approximately corresponding to the sizes retained in the upper respiratory tract, bronchi and bronchioles and penetrating to the alveoli of the lung respectively.
iii) The Slit sampler was developed by Bourdillon (1941). In this a rotating petridish containing suitable nutrient agar media is placed under a slit through which air is drawn. Sampling is done for short time to avoid the interference of growth of one colony with another.
iv) Andersen Sampler: Andersen (1958) developed a sampler similar in principle to slit sampler but here after entering the circular orifice air is drawn through a series of six circular plates each perforated with 400 holes. The plates in series have progressively similar holes and so the largest particle being deposited in the first while the smallest in last petridish.
v) Hirst (1952) trap: The suction trap provides data on rapid changes in the composition of airspora. It is a power driven trap for continuous operation in field. The spores in a measured volume of air are drawn through an orifice and are impacted on sticky surface on a slowly moving microscope slide which moves 2mm/hr. The suction rate is 10 lit/min. The spore free air passes out through the instrument into the pump. Thus it leaves a trace at the end of 24 hours.
vi) Burkard trap (made by Burkard Manufacturing Co., Rickmansworth, Herts, England) is similar to Hirst trap which collects and deposits particles on a plastic band on a clock driven drum rotating once in seven days. This trap is much more advantageous because, records on seven day airspora may be obtained which can be further expressed as hourly concentration of airspora.
vii) Other continuously operating suction traps deriving from Cascade impactor designed by Husain (1963), Tilak sampler by Tilak and Kulkarni (1970), etc., are also used.
The advantages of volumetric trap are its robustness, simplicity and continuous operation with minimum of capital cost and power requirement.
viii). Whirling Arm sampler- In this, instead of increasing the spread of air-flow towards the trapping surface to impact particles, the surface is rapidly moved through the air. Based on this principle, Perkins (1957) developed Rotorod sampler, Its trapping efficiency is nearly 100% for particle larger than 15pm in diameter in still air. May (1967) found that for a wind speed of 1-6 mile/sec., the efficiency is 80- 100% for particles larger than 20pm in diameter.
In this method, the particle is removed from the air by suction. The air is allowed to pass through fibrous or porous medium that sieves the particles. In such a case filters with smooth surface like molecular membranes are suitable for the microscopic examination of the entrapped particles.
i) Electrostatic precipitation:
It is very useful for small particles. Air is drawn through the sampling unit and the particles are charged near the entrance and then attracted to an electrode of opposite charge inside the instrument.
ii) Thermal precipitation:
These are similar to electrostatic precipitations through the electrostatic charges. From the air which flows through the sampler the particles are driven away from a hot surface to a cooler one.