当前位置:首页 > 商业/管理/HR > 经营企划 > [核磁共振波谱学讲义]第三章—NMR实验技术基础(1NMR仪器)
第三章NMR实验技术基础1NMR仪器如图,现代超导核磁谱仪的主要组成部分包括:1.超导磁体Magnet包括FieldLock,ShimCoils2.探头Probe内有RFCoils,GradientCoils3.脉冲编程器及射频放大器4.接收器5.数据采集及处理计算机Atthetopoftheschematicrepresentation,youwillfindthesuperconductingmagnetoftheNMRspectrometer.ThemagnetproducestheBofieldnecessaryfortheNMRexperiments.ImmediatelywithintheboreofthemagnetaretheshimcoilsforhomogenizingtheBofield.Withintheshimcoilsistheprobe.TheprobecontainstheRFcoilsforproducingtheB1magneticfieldnecessarytorotatethespins.TheRFcoilalsodetectsthesignalfromthespinswithinthesample.ThesampleispositionedwithintheRFcoiloftheprobe.Someprobesalsocontainasetofgradientcoils.ThesecoilsproduceagradientinBoalongtheX,Y,orZaxis.Gradientcoilsareusedforforgradientenhancedspectroscopy,diffusion,andNMRmicroscopyexperiments.Theheartofthespectrometeristhecomputer.Itcontrolsallofthecomponentsofthespectrometer.TheRFcomponentsundercontrolofthecomputeraretheRFfrequencysourceandpulseprogrammer.Thesourceproducesasinewaveofthedesiredfrequency.Thepulseprogrammersetsthewidth,andinsomecasestheshape,oftheRFpulses.TheRFamplifierincreasesthepulsespowerfrommilliWattstotensorhundredsofWatts.Thecomputeralsocontrolsthegradientpulseprogrammerwhichsetstheshapeandamplitudeofgradientfields.Thegradientamplifierincreasesthepowerofthegradientpulsestoalevelsufficienttodrivethegradientcoils.Theoperatorofthespectrometergivesinputtothecomputerthroughaconsoleterminalwithamouseandkeyboard.Somespectrometersalsohaveaseparatesmallinterfaceforcarryingoutsomeofthemoreroutineproceduresonthespectrometer.Apulsesequenceisselectedandcustomizedfromtheconsoleterminal.Theoperatorcanseespectraonavideodisplaylocatedontheconsoleandcanmakehardcopiesofspectrausingaprinter.1.超导磁体MagnetMagnet主要要求:a高磁场强度,分辨率与B0成正比,而灵敏度与B032成正比,故750MHz较600MHz的分辨率提高25%,而灵敏度提高40%b高均匀性,目前可达10-9c高稳定性TheNMRmagnetisoneofthemostexpensivecomponentsofthenuclearmagneticresonancespectrometersystem.Mostmagnetsareofthesuperconductingtype.Asuperconductingmagnethasanelectromagnetmadeofsuperconductingwire.Superconductingwirehasaresistanceapproximatelyequaltozerowhenitiscooledtoatemperatureclosetoabsolutezero(-273.15Cor0K)byemersingitinliquidhelium.Oncecurrentiscausedtoflowinthecoilitwillcontinuetoflowforaslongasthecoiliskeptatliquidheliumtemperatures.(Somelossesdooccurovertimeduetotheinfinitesimallysmallresistanceofthecoil.Theselossesareontheorderofappmofthemainmagneticfieldperyear.)Thelengthofsuperconductingwireinthemagnetistypicallyseveralmiles.Thiswireiswoundintoamulti-turnsolenoidorcoil.Thecoilofwireandcryroshimcoilsarekeptatatemperatureof4.2Kbyimmersingitinliquidhelium.Thecoilandliquidheliumarekeptinalargedewar.Thisdewaristypicallysurroundedbyaliquidnitrogen(77.4K)dewar,whichactsasathermalbufferbetweentheroomtemperatureair(293K)andtheliquidhelium.Thefollowingimageisanactualcut-awayviewofasuperconductingmagnet.Themagnetissupportedbythreelegs,andtheconcentricnitrogenandheliumdewarsaresupportedbystackscomingoutofthetopofthemagnet.Aroomtemperatureboreholeextendsthroughthecenteroftheassembly.Thesampleprobeandshimcoilsarelocatedwithinthisborehole.Alsodepictedinthispictureistheliquidnitrogenlevelsensor,anelectronicassemblyformonitoringtheliquidnitrogenlevel.Goingfromtheoutsideofthemagnettotheinside,weseeavacuumregionfollowedbyaliquidnitrogenreservoir.Thevacuumregionisfilledwithseverallayersofareflectivemylarfilm.Thefunctionofthemylaristoreflectthermalphotons,andthusdiminishheatfromenteringthemagnet.Withintheinsidewalloftheliquidnitrogenreservoir,weseeanothervacuumfilledwithsomereflectivemylar.Theliquidheliumreservoircomesnext.Thisreservoirhousesthesuperconductingsolenoidorcoilofwire.Takingacloserlookatthesolenoiditiscleartoseethecoilandtheboretubeextendingthroughthemagnet.FieldLockInordertoproduceahighresolutionNMRspectrumofasample,especiallyonewhichrequiressignalaveragingorphasecycling,youneedtohaveatemporallyconstantandspatiallyhomogeneousmagneticfield.ConsistencyoftheBofieldovertimewillbediscussedhere;homogeneitywillbediscussedinthenextsectionofthischapter.Thefieldstrengthmightvaryovertimeduetoagingofthemagnet,movementofmetalobjectsnearthemagnet,andtemperaturefluctuations.HereisanexampleofaonelineNMRspectrumofcyclohexanerecordedwhiletheBomagneticfieldwasdriftingaverysignificantamount.Thefieldlockcancompensateforthesevariations.ThefieldlockisaseparateNMRspectrometerwithinyourspectrometer.ThisspectrometeristypicallytunedtothedeuteriumNMRresonancefrequency.ItconstantlymonitorstheresonancefrequencyofthedeuteriumsignalandmakesminorchangesintheBomagneticfieldtokeeptheresonancefrequencyconstant.Thedeuteriumsignalcomesfromthedeuteriumsolventusedtopreparethesample.Theanimationwindowcontainsplotsofthedeuteriumresonancelockfrequency,thesmalladditionalmagneticfieldusedtocorrectthelockfrequency,andtheresultantBofieldasafunctionoftimewhilethemagneticfieldisdrifting.Thelockfrequencyplotdisplaysthefrequencywithoutcorrection.Inreality,thisfrequencywouldbekeptconstantbytheapplicationofthelockfieldwhichoffsetsthedrift.OnmostNMRspectrometersthedeuteriumlockservesasecondfunction.Itprov
本文标题:[核磁共振波谱学讲义]第三章—NMR实验技术基础(1NMR仪器)
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