of voltagenecessaryto keepa constantpresetdraincurrent,i.e. both,operating
temperature as wellas metallayer thickness/kindof metal, will influencethe
sensitivityand selectivity.
A number of feasibility studies havebeenperformed withMOS and/or
MOSFETtechnologyon different foodapplicationscomprising lipidoxidation
in meats, fishand oils,freshness of fish, poultryand meat,or have been
compared withstandard techniques(e.g.Schalleret al., 1998;Haugen,2001;
Bu»ckinget al., 2002;Miettinenet al., 2002).
Acousticwave devices
Thistechnique is baseduponpiezoelectric sensors (vibrating quartzcrystals)
whichare madeof quartz,lithiumniobateor lithiumtantalate (Schalleret al.,
1998).Thesecrystals are an inherent part in electronics,electricdata processing
and high-frequency technology.
The generalprincipleis basedupona physicalcharacteristicof the quartz
crystal,the essential partof the system:piezoelectricity.This piezo-effect' refersto the production of electricalchargesby the impositionof mechanical stress, i.e. thesesystems applyan oscillatingelectric fieldto createa mechanical wave.Theirtransformationtowards a sensor is achievedby coating witha polymermaterial, e.g. known fromchromatographic stationary phases. The coatingis achievedby dissolving a chosenpolymerin an organic solvent; after this airbrush or spincoatingtechniquescan be usedto coatthe sensor surface. The two aspectsof this technique, bulkacoustic wave(BAW)and surface acoustic wave (SAW), have different modes of oscillations: the three- dimensionalBAW(alsocalledquartz microbalances', QMBs) travelsat 5±
30 MHz through the crystal, while the two-dimensional SAW range at
frequencies ranging from10 MHz to 2 GHz.Thesensinglayerconsists of a
polymer, which can interactwitha given analyte,i.e. the majormeasurement
parameter is the changein the resonancefrequency due to the changein mass,
whichresults fromthis.Practicalconsequences of the different layouts are a
highersensitivityof SAWs combinedwiththe needfor a moresophisticated
controlset-up (Ampuero and Bosset, 2003). A sensor array willdelivera
`fingerprint' of the sample if the differentpolymersattractdifferentvolatiles and
therefore the variationin mass willinduce different frequency shiftsin a
reproducible way, as longas temperature and humidity are kept constant
(Nakamotoet al., 1990;Rappet al., 1995).
To overcome the disadvantagesof relatively insufficientsensitivitypre-
concentratingunitswerecombinedwiththesedevices,e.g. Tenaxtubes(Bender
et al., 2003) or solid phase micro-extraction (SPME). This SPME±SAW
couplingwas usedfor proofof principle experiments withinprocess control,off-
flavourrecognition and fruitstoragecontrol (Bu»cking et al., 2005).Other
research studiesusedBAWsas a tool to predictthe optimalharvestdateof
apples(Saevelset al., 2003)or to monitor the ripeningprocessof Emmental
cheeses, during which the concentration of 2-heptanone changes
characteristically(Bargonet al., 2003).
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