the surface of collection suspensionsand are re-entered into the exit airflow
(Grinshpunet al., 1997; Lin & Li, 1999a). The samplingtimeand flowrate
influence the CE of hydrophobicspores.
Thereis also a newdevelopedversion of AGI-30impinger, BioSampler,
wherenon-evaporating,higher-viscosity liquids can be usedas collection liquid
(Linet al., 1999).Theapparatushas equivalent or bettermicrobial relative
recoverythanthe conventionalAGI-30,when the collection liquidis water(Lin
et al., 2000). The non-evaporatingliquidenables longer samplingperiodsas it
doesnot kill microbesor allowthemto growand doesnot evaporate evenwith
longersampling times.The construction differenceis that the downwardaerosol
flowis splitintothreenozzleflows.Eachnozzlehas a sonic orifice that is
directedat an identicalangletoward the curved innersurface. Thus,the aerosol
particles are thrownat an angletoward the surface and are removedby the
combined forcesof impaction and centrifugal motion.The presenceof three
angularnozzlesestablishesswirling air motionin the collectionvessel. The
swirling air flowentrainsthe liquidand swirlsit upwardinto the region where
the aerosolflowsfromthe nozzlesreachthe innervessel surface.Thus,the
aerosolparticles are removedinto the liquidfilmwhichcarriesthe removed
particles downinto the liquidreservoir(Linet al., 1999).
Anotherapplicationof impinger principle is the multistage liquidimpinger
withthree stages that are intendedto correspond to the principaldeposition sites
in the human respiratory system:the upperrespiratorytract,the bronchiolesand
the alveoli(Griffiths & DeCosemo,1994).
Impingementis useful for samplingheavily contaminatedair, sincethe liquid
samples can be dilutedto the appropriatelevel for subsequentgrowthculture
analysis (Linet al., 2000).The impinger is inexpensive and simpleto operate,
but viability loss mayoccurowingto the amount of shearforcesinvolved in
collection.Anotherlimitationis that the sampler shouldbe sterilizedbeforeeach
sampling(Kang & Frank,1989a).Samples collected by impingementmethod
shouldbe refrigerated and processed as soonas possible to avoidthe increaseof
bacterialculturability (Li and Lin, 2001).Dataobtainedusing the AGI-30 must
be usedwith cautionin food-processing environments containinglargeviable
particles, because the agglomeratedmicrobes will be separated into suspension,
whichincreasesthe cfu number(Kang & Frank,1989c;Lin & Li, 1999a).
37.5.3 Centrifugalsamplers
Centrifugalsamplers havea propeller that pullsair into the samplingunit and
pushesthe air outward to impacton a tangentially placedstripof nutrientagar
set on a flexible plasticbase.Particlesin the incomingair maybe thrownout of
the air streamby centrifugalforceto be caughtagainst the peripheralsurface
(Ljungqvist & Reinmu»ller, 1998).Centrifugalsamplersdo not generate a high-
velocity jet flowduring sampling, and less stressis imposed on airborne
microbes comparedwithimpaction methods. These samplersare simple and
easyto operateand can rapidlysample a highvolumeof air, resultingin more
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