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TheMembraneInterfaceProbewasdevelopedbyGeoprobe Systems.ItissoldanddistributedsolelybyGeoprobe Systems.Itisprotectedunderpatentnumber5,639,956bytheUnitedStatesPatentOffice.TheMembraneInterfaceProbe(MIP)isascreeningtoolwithsemi-quantitativecapabilitiesactingasaninterfacebetweenthecontaminatesinthesubsurfaceandgasphasedetectorsatthesurface(Figure1).Themembraneissemi-permeableandiscomprisedofathinfilmpolymerimpregnatedintoastainlesssteelscreenforsupport.Themembraneisapproximately6.35mmindiameterandmaybeeasilyreplaced.Themembraneisplacedinaheatedblockattachedtotheprobe.Thisblockisheatedtoapproximately100-120degreesCandisraisedattheleadingedgetoprotectthemembrane.Heatingtheblockhelpsacceleratediffusionofthecontaminatethroughthemembrane.Diffusionoccursbecauseofaconcentrationgradientbetweenthecontaminatedsoilandthecleancarriergasbehindthemembrane.Aconstantgasflowof35-45mL/minsweepsbehindthemembraneandcarriesthecontaminantstothegasphasedetectorsatthesurface.Traveltimefromthemembraneinterfacetothedetector(s)isapproximately30-45sec(dependingonthelengthoftrunklineandflowrate).TheMIPPrincipleofOperation.Thedownhole,permeablemembraneservesasaninterfacetoadetectoratthesurface.Volatilesinthesubsurfacediffuseacrossthemembraneandpartitionintoastreamofcarriergaswheretheycanbeswepttothedetector.ThemembraneisheatedsothattravelbyVOCsacrossthisthinfilmisalmostinstantaneous.MIPacquisitionsoftwarelogsdetectorsignalwithdepth.Theabilitytodetectacontaminantisdeterminedbythetypeofdetectorsbeingused.Anylaboratorygradegasphasedetectorwithananalogoutputof1-5Vmaybeused.Mostcommonlyuseddetectorsincludephotoionizationdetector(PID),electroncapturedetector(ECD)andtheflameionizationdetector(FID).Eachdetectorisdesignedforsensitivitytoagrouportypeofcontaminant.TheECDisusedforchlorinated(TCE,PCE)contaminantdetection,PIDisbestusedforthedetectionofaromatichydrocarbons(BTEXcompounds),andtheFIDisbestusedforstraightchainedhydrocarbons(methane,butane).Thesedetectorsmaybeusedinserieswiththeleastdestructivedetectorbeingfirstandthemostdestructivedetectorcominglast.TheMIPsystemcanprocessfourdetectorsignalsatonetime.Thesedetectorsignals,inconjunctionwiththetimeinwhichacontaminanttakestoreturntothesurface,aregraphedversusdepth.ThedetectorinformationandtheelectricalconductivityofthesoilaregraphedbytheFC4000fieldinstrument.Thisallowstheoperatortodeterminethelocationofthecontaminant,therelativeconcentrationofthecontaminantandthesoilinwhichthecontaminantislocated.TheMIPlogcanbeusedtodeterminethedepthatwhichamonitoringwellshouldbeplaced,atwhatdepthsamplesneedtobecollected,ortheintervalforinjectionofremediationmaterials.