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Methods of Air Sampling and Analysis

  • Botany

The following article highlights the methods of air sampling and analysis. The methods include: air sampling techniques, Inertial Methods, Filtration, Precipitation.

Air Sampling Techniques and Analysis

Gravity Sedimentation Methods

Still air

Alvarez Castro and Castro (1952) created an easy box for the study of fungi that live in air and has two slides hinged and a tray covered on the bottom to insert tiny slides or petridish. In air sampling, the slides that are hinged are raised horizontally, and the wind is allowed to flow throughout the.

Slides are shut and the spores entrapped are then sucked up by gravity. This air sampling techniques model is discontinuous in sampling and only a small amount of air is collected at one time. A more refined version that was based on this concept was explained in the work of Ogden (1974).

Wind

This air sampling techniques 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.

1. It was employed in Blackley (1873), Durham (1946), Hyde and Williams (1944), Gregory (1961), Tauber (1974). The most common examples include Tauber traps, Durham Sampler, Individual Pollen Collector and so on. They’re inexpensive, simple and work constantly. But they fail to estimate the airspora quantitatively and give a distorted picture of airspora, because they preferentially select the larger particles. To fix the distortion, Scheppegrell (1922) attempted to determine the volumetric concentration by using a formula that was based on particle diameter, but this then was corrected Cocke (1937. In spite of these shortcomings, it is extensively used by aerobiologists and has provided significant knowledge to the field of air spora.

2. Frankland as well as Hart (1887) introduced “Gravity Petridish” where the petridish that contained sterile nutrient media were exposed to the open air for 1-10 mins then incubated, and following the development of colonies, they are determined, counted, and subcultured to further research. Gregory (1973) identified the flaws of this trapping method, which are sensitiveness of particle size as well as aerodynamic and wind speed in addition to the fact that a small volume of air samples are taken intermittently, and thus diurnal fluctuations of airspora cannot be observed and only molds that are cultivable or bacteria are able to be caught.

Artificially moving air

Hesse (1884) created a narrow horizontal tube that was 70cm long and 3-4 centimetres wide that contained a layer of Koch’s nutrient-rich gelatine. A certain volume of air is slowly aspirated through the tube, and microbes settle and developed in the gelatine. Funnel technology of Hollander as well as Dalla Valle (1939) was similar to the same concept. In this procedure, very little air is sampled, and this is difficult to quantify and is therefore rarely used by people working.

Air sampling methods

Methods of Air Sampling and Analysis. Image Source: http://www.nationalpetrographic.com/

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.

Wind movement

Microscope slides that are inclined or vertical are utilized for capturing pollen grains as well as fungal spores by a variety of scientists, including Blackley (1873), Ward (1882), Mehat (1952) and many more.

i) Vertical Cylinder

A removable, sticky coating such as cellotape, vaseline or paper is sprayed on an cylinder, which is suspended horizontally from a structure with the desired height. The first time this was used was in the work of Rempe (1937) to capture pollen that is borne by air. 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 that are counted within the area that is scanned may be converted into numbers per square centimeter, to be compared against other catches caught on the cylinder traps.

Vertical traps are convenient and affordable, however the primary disadvantage is that it only catches only spores in the still air, and tiny spores in normal wind speeds and therefore it changes the amount of spores caught depending on the speed of wind.

ii) Aeroconicoscope

This was initially utilized by medical practitioners and later , plant pathologists. It was first used in Salisbury in 1866 and further developed more extensively by Maddox (1870 and 1871). The wind-operated aeroconicoscopes are completely qualitative and give no information about the amount of organisms. This is why this instrument is now mainly of historical importance.

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.

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

Filtration

This method is where particles are removed from the air via suction. The air is then allowed to flow through a porous or fibrous material which separates the particles. In such cases, filters that have smooth surfaces like molecular membranes work well to examine microscopically the particles that are trapped.

Precipitation

Electrostatic precipitation

It’s extremely beneficial for tiny particles. The air is drawn through the sampler and particles are then charged at the point of entry and then attracted by an electrode with a charge of opposite within the instrument.

Thermal precipitation

They are like electrostatic precipitations due to Electrostatic forces. As the air flows through the sampler, particles are pulled away from hot surfaces towards a cooler.

Further Readings