as biologicalefficiency that refers to the ability of the samplerto maintain
microbial viabilityand preventcell damage during sampling.CE is usually
expressed as the 50%aerodynamic cut-offdiameter (Dae50), i.e. the particlesize
collected to 50% diameter (Henningson & Ahlberg, 1994). For efficient
collection it is crucialto choosean impactorwith aDae50belowthe meansize of
the particlesbeingsampled (Jensenet al., 1992).It is not possibleto determine
the PE of a sampler with the methods on hand. TE can be estimated as
(Henningson& Ahlberg,1994):
TEàCEPE Ö 37 : 1 Üwhere TEàtotalefficiency, CEàcollectionefficiency and PEàpreservation
efficiency.
Nevalainenet al.(1992)determined CE as a measure of how well the inletof
a sampler deposits the particles without being affected by their physical
properties.Henningson and Alberg(1994)expressed the collection efficiency
CE as:
CEàIEÖ 1 ˇWLÜCSE Ö 37 : 2 Üwhere IEàinletefficiency, CSEàcollectionstage efficiency and WLà
sampler walllosses.
37.5 Air samplingmethods...
37.5.1 Sedimentationplates
Sedimentation,also referred to as settle platetechnique,has traditionallybeen
usedin the foodindustry. The platesare easy,inexpensive and collect particles
in theiroriginalstate (Kang& Frank,1989a); however, theyhaveserious
drawbacks.The ability of settle platesto collect airborne particles is governed
by the gravitational forceon the particle whichdecreases witha velocity
dependent on its mass.Hence, settleplatesare biasedtowards collecting larger
particles and are sensitiveto air movement (Griffiths& DeCosemo, 1994).The
methodis not quantitativeand in highaerosolconcentrationsthe uncountable
numbers of colonies can be a problem(Holahet al., 1995).
37.5.2 Impactors and impingers
Impactionis the mostcommonactive samplingtechnique for collectionof
airborne viable particles (Ljungqvist & Reinmu»ller,1998).In impactors the
inertial forcesare usedto collect particles. The inertiaof a particleis determined
by its mass and velocity.In the collection stageof impactors the air streamis
forced to changedirection, and particles withtoo higha levelof inertiaare
impacted on eithera solidor liquidsurface. Liquid-usingimpactors are called
impingers (Henningson& Ahlberg, 1994).The aerodynamic particlediameterof
the targetaerosol is one of the mostimportantphysicalfactors determining CE
624 Handbookof hygiene controlin the foodindustry