AIR SAMPLING TECHNIQUE 



to accelerate air through an orifice to a speed at which deposition effi- 

 ciency becomes high (80-100 percent) and airborne particles are deposited 

 on a solid surface.* The protot^'pes of these are^ series for the study of 

 industrial dusts and atmospheric nuclei, such as the Owen jet dust- 

 counter, the Aitken nuclei counter, and various forms of koniometer 

 {see p. 96). These instruments are not very suitable for microbial work, 

 but Durham (1947) found such a pump effective for spot samples of 

 pollen clouds in high concentration. Power-driven prototypes for aero- 

 biological study include the impactor of Hawes et al. (1942). 



(vii) The slit sampler (Bourdillon et al, 1941) was designed primarily 

 for indoor studies with bacteria. A stream of air is dra^^TL through a 

 narrow slit placed just above the surface of sterile medium in a slowly 

 rotating Petri dish. Particles, spores, etc., are impacted on the medium 

 and, after a few minutes' running time, the Petri dish is removed and incu- 

 bated so that colonies may develop and be counted. The 'duplex radial- 

 jet air sampler' of Luckiesh et al. (1949) is similar in principle and allows 

 sterilization by autoclaving. Retention in the slit sampler is good; identi- 

 fication is made in culture; the apparatus is suited for occasional or 

 regular 'spot' samples, but not for continuous use. It is best used indoors 

 as it is not suitable for isokinetic sampling, and collection errors would be 

 high when sampling large particles in a wind. 



(viii) The Cascade Impactor (K. R. May, 1945) is a highly efficient 

 suction trap. Used isokinetically, it serves as the standard of reference 

 against which other devices may be calibrated. The Cascade Impactor 

 consists of a folded tube through which the air to be sampled is drawn 

 by suction. During its passage, the air is accelerated through a series of 

 four progressively narrower jets, and airborne particles are impacted on 

 sticky slides placed close behind the jets. The first jet (the intake orifice) 

 faces the wind. In the original model, at the sampling rate of 17-5 litres 

 per min., the speeds through the four jets are: 2-2, 10-2, 20-4, and 34 

 metres per sec, permitting 50 per cent of the particles whose diameters 

 are 12 ^, 4 ju., 2 /x, and i-i /x to penetrate the respective jets. Later models 

 give good retention down to 0-5 ju.. 



The Cascade Impactor thus fractionates the catch into four sizes and 

 it avoids breaking up clumps. It is ideal for short sampling periods, but 

 if used for sampling the air-spora out-of-doors in summer in England the 

 trace becomes dense after an hour or two. A model of the Cascade Im- 

 pactor with moving slides was developed by K. R. May (1956) to over- 

 come this kind of difficulty and to give time-discrimination. A five-stage 

 Cascade Impactor has been designed by Wilcox (1953). 



(ix) The automatic volumetric spore-trap (Hirst, 1952) is a power-driven 

 trap designed for operating continuously in the field (Fig. 17). It consists 

 essentially of a single impactor slit (actually this has the same dimensions 



* Impaction and impingement are nearly synonymous. In the context of air samp- 

 ling, devices putting particles into a liquid are distinguished as 'impingers', 



99 



