are relativelylow costsfor the device itself,its maintenance, as wellas its easy
handling. The disadvantagesare effects suchas sensor signalshiftand sensor
drift (i.e. phenomenathat mightchangesensorsignals unpredictably)and its
often unspecific detection abilities. However,theseissueshavepartlybeen
solved recentlyby computationalmethods(Haugenet al., 1999;Arturssonet al.,
2000;Tomicet al., 2002,2004).An overviewlistingof sensorsystemsand
applicationscan be foundin Table19.2.
Metal oxidesensors and metaloxide semiconducting field effect transistors
The metaloxidesemiconductors(MOS) are the mostfrequentlyusedsensorsfor
gas sensingand belong to the groupof solidstatebasedchemosensors. First
developedin the 1960sas detectors for liquidpetroleumgasestheyconsist of a
metaloxidelayeron top of a semiconductor. The gas sensing principle is based
on the reactionbetween adsorbed oxygenon the oxidesurfacewithincoming
molecules (Nantoand Stetter, 2002).The outputsignalis derivedfroma change
in conductivityof the oxidecaused by the reactionwiththe incomingmolecule.
There are two typesof MOS sensors,the n-type(SnO 2 , ZnO,Fe 2 O 3 , WO 3 ),
whichrespond to reduciblegases, and the p-type (CuO,NiO, CoO), which
respond to oxidisable gases.The sensors operate at hightemperaturesbetween
300 and 450 ÎC. The metaloxidein the surface layerdetermines the selectivity
of the sensors, an optionaladdedcatalyst(i.e. noblemetals,mostly platinum)
will also influenceselectivityas well as theiroperating temperature. Theyhavea
sensitivityrangefrom5±500 ppm,and are relativelyinsensitiveto waterin the
humidity rangefrom30±80%relative humidity(Tomicet al., 2002).
Metal oxidesemiconducting fieldeffecttransistors (MOSFET) also operate
at elevatedtemperatures(100±200 ÎC). Theyconsistof threelayers: a doped
silicon semiconductor, an oxidelayer(silica)as insulatorand on top a catalytic
metallayer(Lundstro»met al., 1990).Thiscatalytic surfacewill interactwith
volatiles by a changeof potential± the outputsignalcorresponds to the change
Table19.2 Overviewof advantages/disadvantagesand applicationexamplesof the
differentsensortypes
Metaloxide Acousticwave ConductingpolymersExample Lidenet al.(2000), Benderet al.(2003), Gibsonet al.(1997),
On-linemeasurement At-linemeasurement Off-linemeasurement
Advantages Sensitive(ppm) Lowenergy Sensitive
Fastresponse consumption Selective
Cheapsensors Highstabilityof signal
Dis- Sensorsignalshift Lesssensitive Reproducibilityof
advantages and sensordrift fabrication
Poorselectivity Sensorshift
Highenergy (oxidation)
consumption
290 Handbookof hygiene controlin the foodindustry