©A.‐F.Miller2010 T1Measurement 1

Practicum 3, Fall 2010 Measuring the longitudinal relaxation time: T1.

Strychnine,dissolvedCDCl3TheT1isthecharacteristictimeofrelaxationofZ‐magnetizationKnowledgeofT1enablesyoutomakeaninformedchoiceofthedurationofdelaybetweenscans.Iftooshortarelaxationdelayischose,thenthepeaksofslowlyrecoveringprotonswillbesmallerthantheyshouldbe,anditwillnotbepossibletocomparepeakareastolearnaboutprotonnumbers.Also,weusetheT1asourfirstestimateforchoosingmixtimesinNOESYspectra,and3xT1asourrelaxationdelayinNOESYspectra.HencetheneedtoknowT1.

Startwithabeautiful1dspectrum,andacalibratedpw90.(Itturnsoutthatthedefaultvalueintheprobefileisagoodcompromiseforpeopleinarushornotbotheredaboutperfection.)

UndertheExperimentschoose'convertcurrentparameterstodo...>Relax.Measurements>T1'

Thiswillretainyourcurrentsw,tofandpw90,butitwilladdafewfeaturestotheexperimenttosupportmeasurementofthemagnetization'srecoveryafterinversion.ToseethepulsesequenceclickSequence[1]inFigure1.

Thefirstpulseisa180°pulse(p1=2*pw90)thatwillinvertmagnetization[2].Thisplacesitinahigh‐energystatewithrespecttothefield,soitwill'decay'backtotherestingstate.Timeisallowedforthisprocess,intheformofadelaycalledd2[3].SofarallmagnetizationisstillalongZ,butitsamplitudeisrecoveringwithtime.Todeterminewhattheamplitudeisafteragiventimed2,weusea90°pulse(pw)[4]to'read'ourmagnetizationbyrotatingitintotheXYplanewhereourreceivercoilsdetectit[5].Wecannotmonitorrecoverycontinuously,insteadwerepeattheexperiment6ormoretimes,waitingadifferentamountoftimed2ineachrepeatbeforeexecutingthereadpulse.Thusweendupwithaseriesof1dspectra,eachrepresentingadifferentdurationofrecoveryaftertheinversionpulse.

InAcquire>Defaults,asetofthreeboxesinvitesyoutoenterfirst‐guessesoftheT1syousamplemayhave,andVarianusesthesetosetthedelaysintheexperimenttoperformameasurement(youwillhavetousethescrollbarontheRHSofthepaneltorevealthese)[6].SetT1Modetoinversionrecovery[7](don'tworryifthebarstaysblank),thensetT1mintotheshortestT1youexpecttobepresentamongyourresonances(say0.2sec.:enter0.2)[8],thelongestT1youanticipate(erronthelongsidehere,say3(seconds))andthetimeyouarewillingtodevotetotheexperiment(eg.0.1(hours)).

Themin.andespeciallythemaxtimeswillbeusedtochooseanarrayofd2valuesrangingfromveryshorttosomethingontheorderofyourmaximumestimate.Youcanseethesebyclickingon'Arrays'.[9].Theexperimenttakesalongtimeinpart

©A.‐F.Miller2010 T1Measurement 2

duetothelongrelaxationtimeallowedbetweenscans[10].Thisischosentobe≈5timesyourestimatedlongestT1.Thedurationoftheexperimentalsoincreasesinproportiontothenumberofscanscollectedforeachvalueofthedelayd2.Therefore,wewillcutbackonthenumberofscansifpossible(Iwillchange4to1[11]).

Youmayalsowanttogointothe'Array'panelanddecreasethenumberofvaluesofd2used.Becauseweareexpectinganexponentialrecovery,weanticipatethatanexponentialarray[1]willbeabetterchoicethanalineararray(Figure2).Thusinthiscasesuccessivevaluesofd2are2xthepreviousvalue,ratherthanbeingsomevalueplusthepreviousvalue.Theincrementistheln(2).Ifyouwanttotripleeachvaluetogetthenextone,useanincrementofln(3)=1.1.IfIamrushingIchooseanarraysizeof6(theminimumallowed)andanincrement1.1.Theresultingarraycoversafactorofapproximately700.Thusforalongestestimateof3secondsIwantashortestvalueof0.004.IgobackandmakeadjustmentstothefirstvalueandincrementasneededtogetthelastvalueIwant.

MakesurethatGainisanumber,notautogain(Acquire>Acquisition)

Asalways,closetheArraybox,checkSequence,ShowTimeandthenAcquire.

Theresultshouldbeaseriesofspectrainwhichweseethatspinsareinitiallyinvertedandthengraduallyrecoverasthedelayd2ismadelonger(Figure3).

IntheLHSpanel,withthe'ArrayedSpectra'tabactive,changevaluesin'ChartDimensions'.ForVert.Pos.use20[1](from90)VertHeight300[2](from900).UnderOffsetVerticalchangethisinteractively(stepsizeof10)[3]with+10,itgives17.7(agooddisplay)andin"Show"checkthe'values'box[4].

Youseethatdifferentpeaksrecoverindifferenttimes.ThenullrepresentsapproxT1*0.7.Dividenulltimeby0.7toestimateT1≈1sec/.7=1.4secforpeakat6ppm.0.5s/.7=0.7Sforaliphatics.

InProcess>T1Analysis(Figure4[1]),clickonDisplayLastSpectrum[2].Itshouldbeuprightasshown.Ifitisnot,youwillhavetophaseitmanually(afteractivatingtheadditionaloptionsontheright‐hand‐sideofthegraphicaldisplaywindow[3]).Figure4showsacasewherethesoftwarepickedaridiculousnumberoflines.ActivatetheactionbuttonsontheRHSofthegraphicsdisplaywindow[3]andactivatethreshold(Figure5[1]).Movethethresholduptobemoreselective[2].Youcanalsousethecursorstorestrictyouranalysistooneregionofthespectrumbyplacingcursorsarounditandclickingthe'+'magnifyingglass[3].Thenclickon'DoT1Analysis'[4].AllthelinestheinstrumentpickedwillbeinthetableintheRHSbox[5].Ifyouscrollallthewaytotherightedgeoftheboxwiththenumbersinit[6],asecondverticalscrollbarwillappearinsidetheonevisibleinthefigure.Youmayonlybeabletoseethesmalldownarrowheadatthebottom.Clickonthattoscrollupanddown.First,allthepickedpeaksarelistedandeachisassignedan'index'number,whichwillbeusedbelowtoprovidetheT1forthecorrespondingpeak.Nextthereisasummaryoftheanalysisinwhicheachpeak'sT1isprovided

©A.‐F.Miller2010 T1Measurement 3

alongwiththeassociatederror.Finallyadetailedanalysisfollowsforeachpickedpeak.

Usingthechemicalshiftsprovidedwiththespectrum(Figure6[1])inconjunctionwiththenumericaloutputtellingyouwhichindexnumbercorrespondstoeachchemicalshiftvalue[2]youcanselectafewlinesandforthemplotthepeakamplitudevs.durationofrecoveryd2.Nextto'DisplaySelectedFits'enterthelinenumbersforthelinesyouwouldlikeinformationon,forexample"1,10,36"[3].Then<return>.

Atimecourseofmagnetizationrecoveryisproduced(Figure7).

Don'tbelieveT1valuesderivedfromcurvesthatdon'tfitthedatawell.Don'tbelieveanyvalueswhoseerrorsaremorethan10%thevalue(readthenumericaloutputinthesummaryofanalysissection).Don'tbelieveaT1thataremorethanonethirdoftherecoverytimeyouusedbetweenscans(=d1+at).IftheT1syougetarelongerthan(d1+at)/3,repeattheexperimentwithalongerd1,roughly5timesthelongestT1yougot.

Togetacopyofthegraphicaloutput,usetheLHpanel's1Dtab(Figure8[1]),click'PrintScreen..."[2].

Inwindowthatopens(Figure9)selectPSland,(•)file,givenamewithpath(eg.che555/data/yourname/yourfile(omitthe.ps),POSTSCRIPT,Mono/Color...Save,thenClose

Asusual,youcansaveyourdatasetasa.fidfileusingthedisketteiconinupperleft.(Figure10)

Illustration of why the relaxation delay must be longer than T1 Ifyoudonotallowsufficienttimebetweenscansforresonancestorecover,theywillbesmallerinsubsequentscans.Ifthetotalrecycletime(d1+at)isshorterthantheT1sofsomeresonancesbutlongerthantheT1sofothers,thentheformerresonanceswillbeshrunkenmorethantheothers,inyourspectra.Thus,peakareaswillnolongerbeproportionaltothenumbersofprotonsassociatedwitheachpeak.Figure11comparestheresultsofusingfourdifferentd1valuesallincombinationwithanatof0.5.Thus,thetotalrecycletimeswere11.5,3.8,1.5and0.8s(ford1valuesof11,3.3,1.0and0.3s).Theresultsshowthatthe0.8srecycletimecausesaromaticresonances,inparticular,tobeproportionaltelysmaller(Figure12).NotethattheTMSlineisalsomuchsmallercomparedtotheotherlineswhenrecyclingisfast.

Theabovepartialsaturationofslow‐relaxingresonancescanbepreventedeitherbychoosingalonger(d1+at)orbyusingasmallertip‐anglepulse.Ifyouusea45°pulsethenyoudonothavetowaitaslongbetweenscans,becausemagnetizationdoesnotneedtorecoverasmuch.

©A.‐F.Miller2010 T1Measurement 4

Usingasmallertipangleincombinationwithashorterrecycletimeallowsyoutogetthebestdataperhourofspectrometertime.Thisisinvaluableforweakspectrasuchas13C1ds,orsamplesthatarenotstable.

OnceyouknowT1thesoftwarecancalculatetheoptimalpulsewidthandtipanglegivenachoiceofd1.Inthecommandlinetype'ernst(3.4,13.6)'fortheexampleofacasewheretheT1is3.4s.andthepw90is13.6us.Thusthegeneralformofthecommandis'ernst(t1_est,pw90)'.Thiscausesthesoftwarewillupdatethepulsewidthandtipangleoftheexperimentformaximalsensitivityperhourofspectrometertime,giventhed1valueinyourparameterset.

© Anne-Frances Miller, 2010

T1, Figure 1

[1]

[2]

[3] [4] [5]

[6]

[7]

[8]

[9][10][11]

© Anne-Frances Miller, 2010

T1, Figure 2

[1]

[2]

© Anne-Frances Miller, 2010

T1, Figure 3

[1]

[2]

[3]

[4]

© Anne-Frances Miller, 2010

T1, Figure 4

[1]

[2]

[3]

© Anne-Frances Miller, 2010

T1, Figure 5

[1][2]

[3]

[4]

[5]

[6]

[7]

© Anne-Frances Miller, 2010

T1, Figure 6

[1]

[2]

[3]

© Anne-Frances Miller, 2010

T1, Figure 7

© Anne-Frances Miller, 2010

T1, Figure 8

[1]

[2]

© Anne-Frances Miller, 2010

T1, Figure 9

© Anne-Frances Miller, 2010

T1, Figure 10

© Anne-Frances Miller, 2010

T1, Figure 11

© Anne-Frances Miller, 2010

T1, Figure 12