day mailing delay before plating, while concentrations of mould and
thermophilicbacteriaremainedsteady(Thorneet al., 1994).The material of
the membranedoesnot significantlyaffect the amount of cells,sinceequivalent
numbers of bacteriaand yeasts couldbe collected on nitrocellulose,polyamide
and gelatine filters analysed with 4^0 ,6-diamidino-2-phenylindole (DAPI)
counting(Neefet al., 1995).
Thegelatinefilter membrane is composedof gelatine foamdesigned to
preventvegetativemicrobesfrombeing inactivatedby desiccation when air is
drawnthroughthe filter (Parkset al., 1996). The gelatinemembrane is water-
solubleso that it can easilybe dilutedfor platingor be solubilizedon top of a
agar,resultingin colonies on the agar surface (Kang& Frank,1989a). If gelatine
filtersare to be placedon agarplates,thenthe platesshouldbe carefullydried
beforehand, because wet platescan makecolony countingdifficult. The gelatine
filtersdo not meltintothe agarif theyare incubated at lowtemperature.
Therefore,the filterplatesshouldfirst be incubatedat 26 ÎC for 1 h to ensure
meltingof the gelatinefilterinto the agar(Parkset al., 1996).
37.5.5 Electrostaticprecipitation
Theairborne microbes can be collected by impartingthem withelectrical
chargesand thendepositing themin an electricalfieldontoa growthmedium.
Electrostaticprecipitation can efficiently removethe airborne microbes. Over
70% of viable Bacillus subtilis var. niger spores and Penicillium brevi-
compactumfungalsporesentering an electrostaticprecipitator wereenumerated
as colony formingunits.The sensitivePseudomonasfluorescensvegetative cells
wereenumerated twice as efficiently in the precipitationas in the impinger
BioSampler. For thisreason it may be especially useful for collectionof
sensitive microbial strains(Maineliset al., 2002).
37.5.6 Particle samplers
Opticalparticlecounters are basedon laser-lightscattering by a single particle
and are widely usedfor measuring the concentrationand size distribution of
airborne particles. The crucial responsecharacteristics of a particle counter
includethe sizing accuracy, countingefficiency and detectionlimits (Yooet al.,
1996).Thedetectionlimit of the smallest detectable particles is a crucial
characteristicof the counter.Light-scattering countersare usuallycalibrated by
meansof polystyrenelatex(PSL) spheres. Owing to the differencesbetween the
opticalproperties of the calibrated PSL spheresand actual particles, errors in the
resultsmayoccur(Yooet al., 1996).Opticalcounterson the marketinclude
laserdiffractometers,phase-dopplersystems,intensity-deconvolutionsystems,
and laser±particle interaction system/imageanalysers.Othertechniquesusedin
particlecountersincludethe electricalmobilitytechniquesusedin electrical
aerosolanalysersand differential mobility analysersas wellas lightdetection
and ranging(LIDAR) technology,which useslightwavesin the same way that
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