核磁共振波谱学讲义第三章NMR实验技术基础1NMR仪器知识讲解.docx
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核磁共振波谱学讲义第三章NMR实验技术基础1NMR仪器知识讲解.docx
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核磁共振波谱学讲义第三章NMR实验技术基础1NMR仪器知识讲解
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,ShimCoils
2.探头Probe内有RFCoils,GradientCoils
3.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Magnet
Magnet主要要求:
a高磁场强度,分辨率与B0成正比,而灵敏度与
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10-9
c高稳定性
TheNMRmagnetisoneofthemostexpensivecomponentsofthenuclearmagneticresonancespectrometersystem.Mostmagnetsareofthesuperconductingtype.Asuperconductingmagnethasanelectromagnetmadeofsuperconductingwire.Superconductingwirehasaresistanceapproximatelyequaltozerowhenitiscooledtoatemperatureclosetoabsolutezero(-273.15︒Cor0K)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.
FieldLock
InordertoproduceahighresolutionNMRspectrumofasample,especiallyonewhichrequiressignalaveragingorphasecycling,youneedtohaveatemporallyconstantandspatiallyhomogeneousmagneticfield.ConsistencyoftheBofieldovertimewillbediscussedhere;homogeneitywillbediscussedinthenextsectionofthischapter.Thefieldstrengthmightvaryovertimeduetoagingofthemagnet,movementofmetalobjectsnearthemagnet,andtemperaturefluctuations.HereisanexampleofaonelineNMRspectrumofcyclohexanerecordedwhiletheBomagneticfieldwasdriftingaverysignificantamount.Thefieldlockcancompensateforthesevariations.
ThefieldlockisaseparateNMRspectrometerwithinyourspectrometer.ThisspectrometeristypicallytunedtothedeuteriumNMRresonancefrequency.ItconstantlymonitorstheresonancefrequencyofthedeuteriumsignalandmakesminorchangesintheBomagneticfieldtokeeptheresonancefrequencyconstant.Thedeuteriumsignalcomesfromthedeuteriumsolventusedtopreparethesample.Theanimationwindowcontainsplotsofthedeuteriumresonancelockfrequency,thesmalladditionalmagneticfieldusedtocorrectthelockfrequency,andtheresultantBofieldasafunctionoftimewhilethemagneticfieldisdrifting.Thelockfrequencyplotdisplaysthefrequencywithoutcorrection.Inreality,thisfrequencywouldbekeptconstantbytheapplicationofthelockfieldwhichoffsetsthedrift.
OnmostNMRspectrometersthedeuteriumlockservesasecondfunction.Itprovidesthereference.Theresonancefrequencyofthedeuteriumsignalinmanylocksolventsiswellknown.ThereforethedifferenceinresonancefrequencyofthelocksolventandTMSisalsoknown.Asaconsequence,TMSdoesnotneedtobeaddedtothesampletosetreference;thespectrometercanusethelockfrequencytocalculatereference.
ShimCoils
ThepurposeofshimcoilsonaspectrometeristocorrectminorspatialinhomogeneitiesintheBomagneticfield.Theseinhomogeneitiescouldbecausedbythemagnetdesign,materialsintheprobe,variationsinthethicknessofthesampletube,samplepermeability,andferromagneticmaterialsaroundthemagnet.AshimcoilisdesignedtocreateasmallmagneticfieldwhichwillopposeandcanceloutaninhomogeneityintheBomagneticfield.Becausethesevariationsmayexistinavarietyoffunctionalforms(linear,parabolic,etc.),shimcoilsareneededwhichcancreateavarietyofopposingfields.Someofthefunctionalformsarelistedinthetablebelow.
ShimCoilFunctionalForms
ShimFunction
Z0
Z,Z2,Z3,Z4,Z5
X,XZ,XZ2,X2Y2,XY,Y,YZ,YZ2
XZ3,X2Y2Z,YZ3,XYZ,X3,Y3
BypassingtheappropriateamountofcurrentthrougheachcoilahomogeneousBomagneticfieldcanbeachieved.Theoptimumshimcurrentsettingsarefoundbyeitherminimizingthelinewidth,maximizingthesizeoftheFID,ormaximizingthesignalfromthefieldlock.Onmostspectrometers,theshimcoilsarecontrollablebythecomputer.Acomputeralgorithmhasthetaskoffindingthebestshimvaluebymaximizingthelocksignal.
2.探头SampleProbe
Thesampleprobeisthenamegiventothatpartofthespectrometerwhichacceptsthesample,sendsRFenergyintothesample,anddetectsthesignalemanatingfromthesample.ItcontainstheRFcoil,samplespinner,temperaturecontrollingcircuitry,andgradientcoils.TheRFcoilandgradientcoilswillbedescribedinthenexttwosections.Thesamplespinnerandtemperaturecontrollingcircuitrywillbedescribedhere.
ThepurposeofthesamplespinneristorotatetheNMRsampletubeaboutitsaxis.Indoingso,eachspininthesamplelocatedatagivenpositionalongtheZaxisandradiusfromtheZaxis,willexperiencetheaveragemagneticfieldinthecircledefinedbythisZandradius.Theneteffectisanarrowerspectrallinewidth.Toappreciatethisphenomenon,considerthefollowingexamples.Inpictureanaxialcrosssectionofacylindricaltubecontainingsample.InaveryhomogeneousBomagneticfieldthissamplewillyieldanarrowspectrum.InamoreinhomogeneousfieldthesamplewillyieldabroaderspectrumduetothepresenceoflinesfromthepartsofthesampleexperiencingdifferentBomagneticfields.Whenthesampleisspunaboutitsz-axis,inhomogeneitiesintheXandYdirectionsareaveragedoutandtheNMRlinewidthbecomesnarrower.
Manyscientistsneedtoexaminepropertiesoftheirsamplesasafunctionoftemperature.Asaresultmanyinstrumentshavetheabilitytomaintainthetemperatureofthesampleaboveandbelowroomtemperature.Airornitrogenwhichhasbeenwarmedorcooledispassedoverthesampletoheatorcoolthesample.Thetemperatureatthesampleismonitoredwiththeaidofathermocoupleandelectroniccircuitrymaintainsthetemperaturebyincreasingordecreasingthetemperatureofthegaspassingoverthesample.
RFCoils
RFcoilscreatetheB1fieldwhichrotatesthenetmagnetizationinapulsesequence.TheyalsodetectthetransversemagnetizationasitprecessesintheXYplane.MostRFcoilsonNMRspectrometersareofthesaddlecoildesignandactasthetransmitteroftheB1fieldandreceiverofRFenergyfromthesample.YoumayfindoneormoreRFcoilsinaprobe.
EachoftheseRFcoilsmustresonate,thatistheymustefficientlystoreenergy,a
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