2002Fielddetectionandmonitoringofexplosives

FielddetectionandmonitoringofexplosivesJehudaYinon*NationalCenterforForensicScience,UniversityofCentralFlorida,Orlando,FL32816-2367,USADetectionofexplosivesinsituisofmajorimportanceinseveralapplications:findinghiddenexplosivesinairportluggageandinmail;screeningofpersonnelforconcealedexplosives;environmentalmonitoringofexplosives-contaminatedsites;and,detectionofburiedlandmines.Mobileandhand-helddetectorshavebeendevelopedforthevariousapplications.Theycanbesub-dividedintothreeclasses:vaporandparticledetectors;radiationdetectors;and,biochemicaldetectors.Anoverviewofthevariousexplosivesdetectorsandtheirprinciplesofoperationispresented.#2002PublishedbyElsevierScienceB.V.Allrightsreserved.Keywords:Hiddenexplosives;Environmentalmonitoring;Landmines1.IntroductionFielddetectionofexplosivesisanextremelyrelevantanalyticalissueinlawenforcementandenvironmentalapplications.Withtheincreasinguseofexplosivesbyter-roristgroupsandindividuals,lawenforcementandsecurityagentsarefacedwiththeproblemofdetectinghiddenexplosivesinluggage,mail,vehicles,aircraft,ontravelers,andsoon.Inbombing-sceneinvestigations,itisimpor-tanttofinddebristhatincludesexplosiveresi-dues.Mobileandhand-heldexplosivesdetectors,similartothoseusedfordetectinghiddenexplosives,canbeofgreathelpindetectingsuchresidues.On-siteenvironmentaldetectionandmon-itoringoftracesofexplosivesandtheirdegra-dationproductsisnecessaryinareassuspectedofbeingcontaminatedbytoxicexplosivesinordertomonitorthequalityofgroundwaterandpreventpoisoningofpopulationsofhumansandanimals.Thedetectionoflandminesisanacute,urgentworldwideproblemthatneedsspecificandeffectivefield-detectionmethods.Thecommonissueinthevariousapplicationsisthatonehastodealwiththedetectionandidentificationofverysmallamountsofexplo-sives.However,specificinstrumentsandtech-niqueshavebeendevelopedformostapplications.2.Detectionofhiddenexplosivesinairlineluggageandmail[1,2]AlistofthemostwidelyencounteredhighexplosivesisshowninTable1.Itincludesthevaporpressuresofthevariousexplosivesinpureform[2].Plasticizedexplosiveshavealowervaporpressure.2.1.VaporandtracedetectorsThesedetectorsmeasuretracesofcharacter-isticvolatilecompoundsthatevaporatefromtheexplosiveorarepresentasparticlesontheexplosive’scontainersurface.Vaporsamplesarecollectedfromthetargetareaorobjectbydrawingambientairintothedetector.Particlesamplesarecollectedbywip-ingasurfacewithapaperfiltertraporwithhand-heldvacuum,followedbydesorptionintoananalyzer/detector.Inaddition,chemicalpreconcentratorshavebeendevelopedinordertoincreasedetectionsensitivity.Mostpreconcentratorsarebasedondrawinginalargevolumeofair,collectingorganiccompounds–thatincludetheexplo-sives–fromtheairstreamontoachemical0165-9936/02/$-seefrontmatter#2002PublishedbyElsevierScienceB.V.Allrightsreserved.PII:S0165-9936(02)00408-9292trendsinanalyticalchemistry,vol.21,no.4,2002*Tel.:[phone];Fax:[phone].E-mail:[e-mail]filter,followedbyvaporizingtheseorganicsintothedetector.Instrumentsusedinthefieldinclude:gaschromatograph(GC)withelectroncapturedetector(ECD)[3];GCwithchemilumines-cencedetector,alsoknownasthermalenergyanalyzer(TEA);ionmobilityspectrometer(IMS);and,massspectrometer(MS).TheTEAisanitrogen-specificdetector[4–6],inwhichthenitroexplosiveanalyte,elutingfromthefastGC,isintroducedintoapyrolyzer.Compoundscontainingnitrogroupsdecom-posetoproduceanitrosylradical,NO
,bycat-alyticreductionoftheliberatedNO2.TheNO
passesintoareactionchamber,whereitisoxi-dizedbyozone,formingelectronicallyexcitednitrogendioxide,NO2*,whichdecaysbacktoitsgroundstatewithemissionofchemilumi-nescentlightinthenear-infraredregion.Theemittedlightisdetectedbyaphotomultiplier.ThelightintensityisproportionaltotheNO
concentrationandhencetothenitrocompoundconcentration.Aredfilterisplacedinfrontofthephoto-multipliertoblockanylightwithaspectralfre-quencyhigherthanthenear-infrared.Analysistakesabout18s.Althoughthereisnomanu-facturerdataonsensitivitiesandfalse-alarmratesoftheTEAexplosivesdetector[7],laboratorytestsshowedthatthesensitivityoftheGC-TEAforexplosiveswasinthelowpicogramrange.TheIMS[8]comprisesasample-inletsystem,anatmosphericpressureionsourcefollowedbyanion-moleculereactor,ion-driftspectrometerandadetector.Analyteionsareformedinthereactorandinjected,byanelectricfield,intothedriftregion,wheretheyareseparatedaccordingtotheirmobility.Theion-mobilityspectrumconsistsofaplotofioncurrentasafunctionofdrifttime.Thedrifttimedependsontheionicmass:heavierionsmoveataslowerspeedandthereforehavealongerdrifttime.Ionsareformedinanatmosphericpressureionsource,byelectronsemittedfroma63Nibetasourceorbyelectricaldischarge.IMShasbecomeawidelyusedtechniqueforthedetec-tionoftracesofhiddenexplosives[9].Forthedetectionofexplosives,airornitrogenatatmo-sphericpressureisusedasbothcarrieranddriftgases.Introductionofareagentgaswillincreasesensitivityandselectivity.Forexample,hexa-chloroethane(C2Cl6),willproduceareagention,Cl,whichwillundergoion-moleculereactionswithanexplosivemoleculetoformadductionsofthetype(M+Cl).Formationofreagentions,suchasNO2