AIR SAMPLING TECHNIQUE 



(Andersen, 1958) resembles the Cascade Impactor; after entering a 

 circular orifice, air is drawn in succession through a series of six circular 

 plates, each perforated with 400 holes through which spores are impacted 

 directly on to sterile medium in Petri dishes (Fig. 18). Succeeding plates 

 in the series have progressively smaller holes; the largest particles are 

 deposited in the first dish and the smallest in the last; different media can 

 be used for the different size-fractions. Air is sampled at the rate of 28-3 

 litres per min.; wall losses are claimed to be negligible, and retention 

 is said to be 100 per cent, even for single bacterial cells. 



In our work the Andersen sampler has proved very convenient and 

 has given good results with bacteria, actinomycetes and moulds, though 

 for particles larger than about 8-10 /x the addition of some form of pre- 

 impinger seems desirable for avoiding losses on the front of the first plate. 



Reference should be made here to the use of the animal lung as a 

 sampling device. Lurie & Way (1957) injected macerated lung tissue 

 of various animals intrapcritoneally into mice, from whose livers and 

 spleens two dermatophytes {Trichophyton mentagrophytes and Microsporon 

 gypsemn) were subsequently isolated in culture. 



(xii) Whirling arm. The principle of moving an object through air on 

 a rotating arm has been developed in aerodynamics laboratories as an 

 alternative to the wind-tunnel. As a device for air sampling it has been 

 developed in the United States — beginning w ith the 'airw hip' of Durham 

 (1947), who used a 36 in. aluminium rod to swing a forward-facing, sticky 

 glass slide in a circle at 100 r.p.m. Near a stand of flowering ragweed 

 {Ambrosia spp.), Durham recorded a maximum pollen concentration of 

 about 10 million per cubic metre of air. 



A high-speed whirling arm trap was developed by Perkins (1957) as 

 the so-called 'Rotorod sampler'. It has been used in plant pathological 

 studies by Asai (i960), and has been somewhat modified by Harrington 

 et a I. (1959). The Rotorod sampler consists of a length of i/i6th in.-square 

 cross-section brass rod, bent to form a vertical U-shaped collector with 

 arms 6 cm. high, 8 cm. apart, and fixed to the shaft of a miniature electric 

 motor running at 2,520 r.p.m. over the range 9 to 15 volts from dry 

 batteries. The sticky arms effectively sample air at 120 litres per min. 

 Rotating at a peripheral speed of about 10 metres per sec, high collecting 

 efficiency would be expected for pollens and spores down to about 12 ju. 

 diameter; for smaller spores, efficiency would fall below 50 per cent. This 

 sampler has proved reliable in use, light in weight, and self-contained; the 

 batteries are cheap, and will run continuously for two or three days. 



ADHESIVES 



The choice of adhesives for samplers is limited. Petroleum jelly and 

 glycerine jelly have many advantages. Glycerine jelly has the best optical 

 properties for visual examination, but it is hygroscopic and dissolves in 

 damp weather. Petroleum jelly (e.g. 'Vaseline') is reliable for coating 



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