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

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