TheDae50valueof the first stage is 8.0m and theDae50valuefor the second
stage is 0.95m (Andersen,1958). For thisreason viable particles above
0.95m in an aerosolcan be separated into two size ranges:0.95to 8m on the
first stageand 8m and aboveon the second stage.
Thereare numerousdifferent 1-stageimpactorsamplers but onlya few are
describedhere.The surface air system(SAS) sampleris a portable, single-stage
sieve samplerthat collects particles ontoa contact plateor standardPetri plate.
The sampler operates witha flowrate of 180 L/min (Griffiths & DeCosemo,
1994).The calculatedDae50value is 1.45m (Nevalainenet al., 1992).The
microbial air sampler MAS-100 is an impactor that aspirates air either
horizontallyor vertically througha perforatedplatewith400 holes 0.7 mm in
diameter (Meier & Zingre, 2000).The resultingairstream containingparticles is
directedontothe agarsurfaceof a standardPetriplate.The impaction speedof
the airborne microbeson the agar surface is approximately 11 m/s,which
according to the manufacturer corresponds to stage fivein the Andersen
sampler.Thisspeedguarantees that all particles over 1 m are collected. At an
aspirationvolumeof 100 L/minand with 400 holes 0.7 mmdiameterholes
serving as a catchfor the lid, the MAS-100attainsa collision speedof 10.8 m/s.
The calculatedDae50valueis 1.62m (Meier& Zingre,2000)or 1.72m (Li &
Lin, 1999).Air samplers withaDae50of less than 2 m shouldtheoretically be
able to precipitate practically any airborne microbiological contaminant-
carrying particle(Meier & Zingre, 2000).
Impingementmethodsare highly efficientfor particles greaterthan 1 m
when high jet velocities are used(Kang & Frank, 1989a). The all-glass
impinger-30sampler is a widely usedhigh-velocityimpinger(Kang& Frank,
1989a;Griffiths& DeCosemo, 1994 ). The sampleroperates by drawing aerosols
throughan inlettubecurvedto simulate the nasal-passagerespiratoryinfection
potentialof airbornemicrobes(Nevalainenet al., 1992).The jet is held30 mm
abovethe impingerbaseand consists of a shortpiece of capillarytubing
designedto reduce cell injury (Kang& Frank,1989a). ThecalculatedDae50
value is 0.31m (Nevalainenet al., 1992). Whenthe sampleris used for
recoveringtotalamountof airborne microbes fromthe environment, the curved
inlettubeshouldbe washed witha known amount of collectionfluidafter
sampling,sincelargerparticles (i.e. diameter>15m) are collected on the tube
wallby inertial force(Kang& Frank,1989a). The agglomeratedmicrobes are
separated into suspension,whichincreases the numberof colony-forming units
(CFUs) (Lin& Li, 1999a). Thebubbles, risingthrough the liquid, entrain
previouslycollected particles and create new aerosols by bursting at the liquid±
air surfacewhenthe impingersoperateat a high-level collectionfluidand
sufficientlyhighsamplingflow rate.Thenumber of reaerosolizedparticles
increased as samplingtimeincreased (Linet al., 1997).The theoretical overall
inletsamplingefficiencyincluding wallloss is closeto 100%for 1m particles
and is significantlyreducedfor 5m and largerparticles (Willekeet al., 1992).
Hydrophobicity mayplayan importantrole in the CE of AGI-30 impingers.
Spores frommanyfungiare hydrophobicand maybe lost because theyfloatto
626 Handbookof hygiene controlin the foodindustry