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Mass Spectroscopy Lecture 4

Prof. Nizam M. El-Ashgar

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Detectors vIonbeamsafterpassingthroughmassanalyzer, strikesondetector. vTheionscanbeelectricallydetectedbydetector whichhavebeenseparatedaccordingtotheir mass/charge ratiointhesystem. vThechoiceofdetectoris basedontherequired detectionsensitivityandthespeed anditisalso determined by other application specific requirements, suchasthethermalandchemical stabilityandtheamountofspaceavailable.

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Desirable Detector Properties SomeidealcharacteristicsofIonCollectorsandDetectors v Highamplification. v Fasttimeresponse. v Lownoise. v Highcollectionefficiency. v Lowcost. v Narrowdistributionofresponses. v Sameresponseforallmasses. v Largedynamicrange(maxlinearresponse/detectornoise). v Longtermstability. v Longlife. v Mountedoutsideofthevacuumifpossible. Notpossibletomeetallofthesecharacteristicsatthesametime ٣

Ion Beam Measurement Characteristics • Theoreticallyatleasttherateofionsenteringthedetector isameasureoftheamountofthatisotopeinthesample. • Theoreticallyifweratiotheratesoftwoisotopebeamsthis ratioistheratioofthetwoisotopesinthesample:

R S 1 = 1 R 2 S 2

• WhereR1istherateofisotope1,R2istherateofisotope 2,S1istheamountofisotope1inthesampleandS2isthe amountofisotope2inthesample.

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Measurements Howdowemeasuretherates? – Wecanmeasuretheratedirectly: thatis, directlycountthenumberofionsappearingatthe detectoroveracertainperiodoftime. • Thisisknownasionorpulsecounting. – Theionbeamisanelectriccurrent. • Wecanmeasuresomeparameterassociatedwith electriccurrentsandusethisasaproxyforthe rate. • Thisisknownasanalogmeasurement. • Wecanmeasurecharge,currentorvoltage.

Types of detectors Therearevarioustypesofdetectors whichareusedinmassspectrometry. v ElectronMultipliers. Dynode: Surfacethatis v FaradayCups. heldathighpotentialso v PhotographicPlate. that ions striking the surface produce secondary charged .ﻋداد اﻟوﻣﺿﺎت v Scintillation Counter v Channel Electron Multipliers. particles that are v Resistive Anode Encoder Image subsequently detected. Detectors. v High mass detection Detector. Note:Forpositiveions,the potentialisnegativeand v Conversion dynodes. thesecondaryparticlesare v Helium Leak Detector. electrons and negative v Advanced Detectors. ions.Fornegativeions,the potentialispositiveand v Cryogenic Detectors. thesecondaryparticlesare v Multi -Pixel Photon Counter. positiveions. .Other Detectors v٦

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Electron Multiplier Detector v Forioncurrentlessthan10-15 ampanelectronmultiplier detector(EMT)isused. v Anelectronmultiplierdetectorisusedtodetecttheion signalsemergingfromthemassanalyzerofamass spectrometer. Principle: Ø Thebasicprinciplethatallowsanelectronmultiplier detectortooperateis calledsecondaryelectronemission. Ø Whenachargedion(particleorelectron)strikesondetector surfaceitcausessecondaryelectronswhicharereleased fromatomsinthesurfacelayer. Ø Thenumberofsecondaryelectronsproduceddependson thetypeof incidentprimaryparticle, itsenergyand characteristicoftheincidentsurface. ٨

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Features of ETP Electron Multipliers • Ithasspecializedsurfacematerialresultingin veryhighsecondaryelectronemission. • Airstable. • Discrete dynode design results in extended operatinglife. Detectordesign: Therearetwobasicdesignsof electronmultipliersthatarecommonlyusedinmass spectrometryare: 1)Thediscrete-dynodeelectronmultiplierand 2)ThecontinuousdynodeElectronmultiplier.

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Discrete -dynode electron multiplier v Inthisdevice,positiveionsstrikeaconversioncathode liberatingelectronswhicharethenacceleratedand multiplied’ viaaseriesofuptotwentydynodes. v Secondaryelectrons areconstrainedbyamagneticfield tofollowincircularpath,causingthemtostrikeonthe dynode. v Thistypeofdetectorisextremelysensitive,havingagain ofupto108. v Aluminium-baseddynodeshaveimprovedperformances ofthetraditionalmaterials(Cu/Bealloys)whichagerather badlyintheresidualatmosphereofthespectrometers,or duringnonworkingperiods(returningtoatmospheric pressure).

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Electron Multiplier

Discrete dynode electronmultiplier

Working of EMT vEachdynodeisconnectedtoaresistorchainand thefirstdynodeisatgroundpotential, sothatboth positiveandnegativeionsmaybedetected. vThelastdynodecanbebetween +1500to+3500 Vdepending onthetypeandageofelectron multiplier. v Whenanion(electron,neutral,etc.)strikesthe firstdynodeitmayproduce afew(1,2or3) secondaryelectronswhichareacceleratedtothe seconddynode thatisheldathigherpositive potentialtogeneratemoresecondaryelectrons andacascadeofsecondaryelectronsensues.

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vForoptimumperformance, theelectronmultiplier shouldoperateatsufficientlyhighvoltagesothat every ionarrivalproducesapulse andthenthis pulseisamplifiedandaslongasitisaboveaset threshold,itwillbepassedtothecountingcircuit. vWithfastpulse-processingcircuitry,thisisinthe orderof15to20nsandthemaximumcountrateof electronmultipliers isabout5x106c/s.

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Continuous -dynode electron multiplier v Theionsaredirectedtowardsacollectorwhoseentrance,in theformofahorn,ismadeofaleaddopedglass withwhich actsastheconversioncathode. v Thetubeismadeofglasswhichisimpregnated withleadto produceadesiredresistancebetweentheanodeandcathode. v Thusthereisacontinuousvoltagedropalongthetubeto accelerate theelectrons;thecurvatureensuresthatthe accelerated electrons will undergo secondary electron emission. v The ejected electrons are attracted towards a positive electrode andtheircollisionsagainsttheinternalwallsgive risetomultiplication,aswiththeseparateddynodes. toavoid ﺧﺎرج اﻟﻣﺣور v Theassemblyisusuallymountedoff-axis theimpactofneutralspeciesaswellasphotonsemittedbythe

.f١٥ilament,equallysusceptibletotheremovaloftheelectrons

Continuous dynode electronmultiplier

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Avantages: • Highsensitivities. • Eachionproducesnarrowpulseofelectronsatfinal dynode. – Bothanalogandioncountingpossible. – Numberofelectronsatfinaldynodeproportionalto efficiencyofelectronproductionatfirstdynode. • Longlifetime • Optimizedionandelectronicopticsformaximum performance. • Increasedsurfaceareaforenhancedsensitivityand extendedoperationallife. • Lownoise.

Disadvantagesandproblems: – Athighcount ratesresponsebecomesnon- linear. – Firstdynodecan bedamagedbyionsandtends todegradeovertime. – Massdependenceofelectronproductionatfirst dynode. – Spuriousions generatedbyelectrons. –Closelyspacedpulses cannotbeseparated(not aprobleminanalogmode). – Mustbeshieldedfrom externalmagneticand electricfields.

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Faraday Cups Collector • Technicallymoreanioncollectorthandetector. • Basicallywhatthenameimplies: acuptheionsenterand transfertheir chargetothecup. • Chargeisusuallytransferred toelectronicsoutsidethe vacuumsystem. • Typeofelectronicsdetermines whethermeasuredas charge,currentorvoltage

Faraday Cup

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Faraday Cups continued • Thefaradaycupseemssimplebutinpracticebecomes quitecomplicated. • Thefirstandmajorcomplicationisthattheionsentering haveenergiessignificantlyhigherthantheworkfunctionof thecupmaterial (stainlesssteel,carbon,graphite) • Thiscausesthegenerationoffreeelectrons(knowas secondaryelectrons). • Ifasecondaryelectronleavesthecupthismakesthe chargeonthecuplooklikeanadditional+ionhasentered.

Faraday Cup Sinceeachioncangenerate manysecondaryelectrons, eventhelossofasmallportion oftheseelectronscancausea largeerrorinmeasurement •+

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Whatdeterminesthenumberofsecondaryelectrons? – Massofions – Energyofions – Chargeonions – Angleofincidence – Materialofcup – Natureofion(monatomicvs.polyatomic) Howcanwereducetheeffectofsecondaryelectrons? – PassiveandActivetechniques • Passive – Makecupofmaterialthatgeneratesfewersecondary electrons – Makecupdeepandnarrow • Active – Magneticfieldtoconfineelectronstocup. – Slitplate(repellororplate)placedbeforecupwithnegative voltage,electronsthatleavecupareforcedbackintothecup

Faraday Cups

• Magnetic field plus repellor plate reduces secondaryelectronlosstoafewhundredpartsper millionorless. • Ionscanalsoenterthecupandbereflectedwithout givinguptheircharge, apositiveplatecanbe placedinfrontofthenegativerepellortoreflect backtheseions.

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Advantage • Moresensitiveandproducecurrentupto10-15 amp. • Signals from this collectors are stable and reproducible. • SimpleConstruction. Disadvantages • Theyhavelowsensitivity duetothecurrent inducedinthecircuitbyionbeam areverysmall. • Itdoesnotidentifybetween thetypeofionsorits energy.

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Daly Detector TheDalyDetectorsgetsaroundsomeoftheproblemsof traditionalmultipliers: Can be run in both analog and Ion Beam Ion counting mode

Vacuum Wall

Photomultiplier Daly Knob Tube -22KV Signal Out

Electrons Photons

Scintillator

Glass Plate Mount for Scintillator (part of vacuum wall)

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Daly Characteristic

Advantages: – KnobmorerobustthanfirstdynodeofEM – Smallermassdependence – Lowernoiseforsamecurrentgain. – Highcurrentgains,1iongives1to5electronsatknob, eachelectrongives5to10photonsatthescintillator,each photongives2to10electronsatthefirstdynodeofthePM – SignalPulseintensityusuallywellabovenoiseintensities – Linearwidedynamicrange(10-21 to10-13 A) Disadvantages: – Complicated – Longerdeadtime. – Scintillatordamagedbyhighelectroncurrentsandheat

Microchannel plate detectors(MCP)

v They consist of the union of a large number microchanneltronsarrangedlikehoneycombs. v Thisresemblesanelectronicversionofaphotographic plate. v Eachindividualdetectorisformedfrom aportionof microtube(25mmdiameter)whoseinterioriscoatedbya semiconductormaterialactingasacontinuousdynode. v Thissystempreservesthespatialresolutionoftheinput chargedions. v UsefulinTime-of-Flightmassspectrometerandwherever thebeamcoversalargearea

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Advantages: Microchannelplates haveimprovedtime resolutionoverother detectors

Disadvantages: However,theysuffer frommanyof the sameproblemsas othermultipliers

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Vacuum system Thepumpingsystemisanimportantpartofany massspectrometer. • Fortheworkingofmassspectrometer,theion source,themassanalyzer,andthemassdetector mustbekeptunderhighvacuumconditionof 10-6- 10-7 torr. • Thisreducesthechanceofionscollidingwithother moleculesinthemassanalyzer. • Anycollisioncancausetheionstoreact,neutralize, scatter,orfragment. • Alltheseprocesseswillinterferewiththemass spectrum.

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• Thishighvacuuminspectrometerrequirestwo pumpingstages. Ø Thefirststage isamechanicalpumpwhich providesroughvacuumdownto0.1Pa(10– 3 torr). Ø Thesecondstage usesturbomolecularpumps ordiffusionpumpstoprovidehighvacuum. • ICRinstruments haveevenhighervacuum requirementsandoftenincludeacryogenic pumpforathirdpumpingstage.

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DATA SYSTEM: • Thefinalcomponentofamassspectrometeristhedata system. • Thispartoftheinstrumenthasundergonerevolutionary changesinthepasttwentyyears. • Ithasevolvedfromphotographicplatesandstripchart recorderstodatasystemsthatcontroltheinstrument, acquirehundredsofspectrainaminuteandsearchtensof thousandsofreferencespectratoidentifyanunknown. • Thesesystemsareevolvingsorapidly. Twoessentialfeaturesofrecorderarethat: Ø Recordersmusthaveaveryfastresponse,andableto scanseveralhundredpeakspersecond. Ø Theymustbeabletorecordpeakintensitiesvaryingbya factorofmorethan103. ٣١

DataSystems: DatasystemelectronicallyinterfacedtoMSisvitally importantfor: 1. ControlofMS – modesofscanning,voltages, currents,etc.,tuningandmasscalibration 2. Dataacquisition – thedigitalvoltagesignal responseforeachionscannedtransmittedfrom thefastADC 3. Dataprocessing – toobtainmassspectra,%RA abundance lists, elemental composition, selectedmasschromatograms,librarysearch – NBS/Wiley, quantitation (total ion chromatogram,TIC).

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