AIR SAMPLING TECHNIQUE 

 SEDIMENTATION FROM ARTIFICIALLY MOVING AIR 



Hesse's method of sedimentation in long horizontal tubes has been 

 described in Chapter I. Cocke (1938) used the same principle for visual 

 microscopic examination of airborne pollen. A small chamber was lined 

 top and bottom with eight glass slides, leaving a passage of only 1-2 mm. 

 between floor and roof. With 1-4 cu. metres of air per 24 hours drawn 

 between the slides, Cocke reported good agreement with the gravity slides 

 exposed simultaneously out-of-doors. 



Similar in principle are the funnel device (Hollaender & Dalla Valle, 

 1939), in which air enters the stem of an inverted conical funnel suspended 

 3 mm. above a Petri dish of medium, and the bottle device of Scharf (in 

 duBuy et al., 1945), in which air plays from the end of a tube onto the 

 surface of a culture medium in a horizontal medical-flat bottle. As the 

 volume of air sampled in a given time is often inconveniently small, and 

 they are not easily made quantitative, none of these methods has passed 

 into routine use. 



Intrtial Methods 



In the 'inertial methods' the particles may be retained on filters, on 

 flat surfaces, or in liquids. The air may operate by being drawTi through a 

 jet or tube, or by being spun to produce centrifugal separation; or, al- 

 ternatively, the apparatus may move the trap surface through the air as 

 in the whirling arm devices, or in sampling from aircraft. The grouping 

 adopted below is convenient rather than fundamental. 



impaction using wind movement 



(i) Vertical and inclined sticky microscope slides have been used for 

 catching pollen in hay-fever research and spores in cereal rust studies. 

 Blackley (1873) exposed four slides at a time, facing different points of the 

 compass; but commonly a single slide is exposed for 24 hours in a pivoted 

 vane shelter which swings to face the wind — cf. Craigie (1945), Clark 

 (1951), and Mehta (1952). Slides were fixed in trees, in positions compar- 

 able with leaves, in order to trap spores oi Hemileia vastatrix, a rust fungus 

 devastating coffee, in Ceylon by Ward (1882) and in India by Mayne 



(1932). 



Various methods are used to test for the spore liberation period of a 

 green plant or fungus, but this is a different problem from air sampling 

 (Pettersson, 1940; Oort, 1952; Hopkins, 1959). 



Vertical slide traps have been sent aloft fixed to kites, first apparently 

 by Blackley (1873). ^ variety of such devices were used by Mehta (1933, 

 1940, 1952) in his extensive studies of rust dissemination in India. During 

 1930-32 he used free hydrogen balloons to carry vertical slides, the cylinder 

 containing the slide being opened and closed again by burning fuses after 



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