of the sampler of impaction devices(Li & Lin,1999). Impactorswithseveral
collection stages,i.e. cascade impactors, give informationin addition on the size
distribution of the aerosol (Henningson& Ahlberg, 1994).
There are twotypes of solid-surfaceimpactors: slit samplers and sieve
samplers. The slit sampler usuallyhas a 0.2±1.0mm-wide taperedslit, which
produces a jet streamwhenthe air is sampledby vacuum(Kang& Frank,1989a;
Ljungqvist & Reinmu»ller,1998).Theslit samplermayhavea turntablefor
rotatingthe agar plateso that aerosol particles are evenly distributed on the agar
surface(Kang & Frank,1989a). The velocityof the air variesaccordingto the
slit widthof the air sampler used.A linearvelocity of 20±50m/s is typical,since
particles witha minimumdiameter of 0.5±1.0m do not followthe deflecting
streamof air but impactagainst the collection surface.The smaller the size of a
particle, the higher the velocity needed in the impaction (Ljungqvist &
Reinmu»ller, 1998).
Sievesamplersare operatedby drawing air througha largenumber of small,
evenlyspacedholesdrilled in a metalplate.The particlesare impactedon an
agarsurfacelocated below the perforatedplate(Kang & Frank,1989a). The
impaction velocity is dependent on the size of the perforations, distanceto the
impaction surface and performance of the vacuum pump (Ljungqvist &
Reinmu»ller, 1998). Whenthe concentrationof viable particlesin an aerosolis
high,one sieveholemayallowmorethanone viable particle to passthrough,
resultingin the formation of a single colonyfromtwo or moreviableparticles.
Thisinaccuracy can be correctedby reducing samplingtimeor by using either
the microscopicmethodor a positive-holemethodfor enumeration(Kang&
Frank,1989a). Normally the positive-hole correctiontablesare included in each
commercialsievesampler.
In 1958a cascade-sieveimpactor, the Andersensampler,was developedand
is nowprobablythe best-knownsampler for bioaerosols(Andersen, 1958).It is
one of the few samplers witha solidcollectionsurfacethat givesinformationon
the size distribution of the bioaerosol.The sampler collectsairborne particles
ontoa seriesof agarplatesat a flowrate of 28.3 L/min(Griffiths& DeCosemo,
1994).Eachsievehas successivelysmallerholes. Thiscausesincreasedparticle
velocityas air flowsthroughthe apparatus.Largeparticlesimpactat the initial
stageand smallparticlesfollow the airflow untilacceleratedsufficiently to
impactat a laterstage(Kang& Frank,1989a).Detection of microbes relieson
theirability to increasefollowing sampling (Griffiths& DeCosemo, 1994).The
CE of the Andersen samplerwas foundto be greaterthan 90%for particles with
aDae50of <2.5m. Thisefficiency wasnot affected by air velocity;at air
velocities <1 m/s,particles as largeas 20mDae50werecollected witha near
100% efficiency. The efficiency, however, fell rapidly with increased air
velocity.The CE of the Andersen sampleris moreaffectedby air velocitythan
by particle size (Griffithset al., 1993).The calculatedDae50valueis 0.57m
(Nevalainenet al., 1992).TheAndersen6-stagesampler is oftenusedas a
reference sampler (Henningson & Ahlberg, 1994). The 2-stage Andersen
sampler is usedto collect and separate respirable fromnon-respirable particles.
Improving air sampling 625