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3秒自动关闭窗口游程理论与二维copula函数的耦合建模及应用
modeling of coupled theory of runs..
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游程理论与二维copula函数的耦合建模及应用
modeling of coupled theory of runs and 2-d copula functions and its application
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3秒自动关闭窗口Basic Theory and Use of GC-MS_色谱技术_中国百科网
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Basic Theory and Use of GC-MS
&#160;&#160;&#160;&#160;原Word&版本为15M文件，&带附图。BASIC&THEORY&AND&USE&OF&GC-MSby&Dr.&Eugenia&SobolevaContent1.&&Introduction.2.&&GC-MS&systems&and&components.3.&&Vacuum&system3.1.&&Rotary&pump3.2.&&Diffusion&pump3.3.&&Turbomolecular&pump4.&&Interfacing&GC&and&MS4.1.&&Packed&column&interfacing4.2.&&Capillary&column&interfaces4.3.&&Inlets&for&liquid&chromatography4.4.&&probe&techniques5.&Ionizers.5.1.&&Electron&Impact&Sources&(EI)5.2.&Chemical&Ionisation&(CI)5.3.&Field&ionization&(FI)5.4.&Field&desorption&(FD)5.5.&Discharged&Ion&Secondary&Ion&MS&(DISIMS)&or&Fast&Atom&Bombardment&(FAB) 6.&Principals&of&MS&analysers.6.1.&Magnetic&sector&mass&spectrometer6.2.&Double-Focusing&Mass&Spectrometer.6.3.Quadrupole&mass&Spectrometer6.4.&Ion&trap&analyser. 6.5.&Time&of&flight&analyser. 6.6.&Ion&cyclotron&resonance&MS&(Fourier-Transform&Instrument).7.&Signal&detectors7.1.&Discrete&dynode&electron&multiplier. 7.2.&Continues&dynode&electron&multiplier. 7.3.&Faraday&cap.8.&&Data&recording.8.1.&&Computer&control&of&mass&spectrometer&and&data&acquisition.8.2.&&Acquisition&principles&of&HP&quadruple&mass&spectrometer.9.&&Tuning.10.&&Qualitative&determination&by&GC-MS.10.1.&&Library&searching.10.2.&&User&created&library.11.&&Basic&rules&for&interpretation&of&mass&spectrum.11.1.&&Quasi-Equilibrium&theory&(QAT)11.2.&&Inductive&and&mesomeric&effects.11.3.&&Isotope&abundance11.4.&&Nitrogen&rule12.&Tandem&mass&spectrometry.1.&&Introduction.The&combination&of&the&gas&chromatography&&with&the&mass-spectrometer&has&resulted&in&an&instrument&of&considerable&importance&in&the&field&of&chemical&analysis&and&detection.&It&could&be&argued&that&the&mass&spectrometer&is&just&one&of&a&range&of&detectors&available&to&the&gas&chromatography&user.&On&the&other&hand,&some&would&say&that&the&gas&chromatograph&is&only&one&of&many&inlets&that&could&be&connected&to&the&mass&spectrometer.&In&truth,&the&synergy&of&this&arrangement&goes&beyond&the&possibility&&of&the&two&separate&&instruments&and&almost&warrants&a&new&definition&as&an&instrument&in&its&own&right&-&GC-MS.&Advantages&of&GC-MS&as&a&new&technique&in&chemical&analysis&are:*&Qualitative&analyses&of&unknown&*&Conformation&of&molecular&*&High&sensitivity&and&advanced&selectivity.Application&areas&of&GC-MS&include&Chemistry,&Biology,&Medical&science,&Toxicology,&Environmental&science,&Mineralogy,&Semi-conduct&technology,&Archaeology,&etc.&2.&& GC-MS&systems&and&components.The&principal&elements&of&GC/MS&system&are&shown&in&Figure&1.Figure&1.&Block&diagram&of&a&typical&GC/MS&system.The&Gas&Chromatograph&introduces&sample&,&solvent,&and&carrier&gas&to&the&MS.&The&interface&(Fig.&2)&links&the&GC&to&the&MS.&It&is&used&to&remove&excess&carrier&gas.&The&pressure&requirement&&vary&with&MS&instrument&type&and&application,&but&1.3*10-3&Pa&is&a&typical&maximum,&and&often&lower&pressure&are&necessary.&The&gas&chromatograph,&by&contrast,&basically&operates&at&or&above&atmospheric&pressure,&and&so&it&is&not&surpassing&that&some&form&of&interface&is&required.This&usually&acts&as&sample&enrichment&devise,&extracting&much&of&the&carrier&gas,&although&direct&connection&through&tubing&of&a&very&fine&bore&is&also&possible.&All&the&enrichment&types&reflect&the&need&to&compromise&between&the&amount&of&sample&lost&and&the&amount&of&carrier&gas&extracted.&In&general,&they&are&more&efficient&with&a&light&gas,&and&so&helium&is&favoured&over&nitrogen&as&the&GC-MS&carrier&gas&&&&&&&&&&&Fig.&2.&The&interfacing&between&GC&and&MSThe&Mass&Spectrometer&(Fig.&3)&consists&of&ion&source,&mass&analyser,&and&detector.The&ion&source&generates&charged&species&and&produces&a&collimated&ion&beam.&Electron&impact(EI),&chemical&ionisation(CI),&fast&atom&bombardment(FAB),&and&some&other&types&of&ionisation&techniques&can&be&used.&The&mass&analyser&separates&ions&according&to&their&mass&and&charge,&sequentially&passes&just&one&m/z&value.&It&can&be&&magnetic,&quadrupole,&time&of&flight&or&ion&cyclotron&resonance.&The&detector&is&usually&a&multistage&or&continuous&dynode&electron&multiplier.&It&measures&the&ion&current&that&is&passed&by&the&mass&analyser&producing&an&analog&signal&as&a&function&of&mass.&&&&&&&&&&Fig.&3.&The&schematic&diagram&from&MS&to&detector.The&functions&of&the&Data&System&(Fig.&4.)&are&to&acquire&and&save&data,&to&process&data&and&generate&reports,&to&monitor&and/or&control&GC/MS&hardware,&to&provide&communications&linkages.&&The&interface&operating&at&high&frequency&converts&analog&signal&to&digital&data&so&as&digital&information&to&analog&control&signals.&&The&computer&hardware&consists&of&central&processor,&memory,&digital&interfaces,&disk&and/or&tape&drive.&The&software&comprised&of&master&executive,&acquisition&and&control&programs,&data&processing&package,&and&system&diagnostics.&&&&&&&&&&&&Fig.&4.&The&function&of&database&system&in&MS.3.& Vacuum&System.A&very&high&vacuum&is&essential&element&in&the&mass&spectrometer&for&the&following&reasons:*&the&mean&free&path&of&molecules&in&the&system&must&be&longer&than&the&ion&source.&The&mean&free&path&is&an&average&distance&an&ion&travels&before&it&strikes&something.&In&a&mass&spectrometer,&the&mean&free&path&must&be&long&enough&so&that&sample&ions&can&travel&from&the&ion&source&to&the&detector&without&colliding&with&other&*&ion-molecular&reactions&start&to&occur&as&the&pressure&rises,&producing&change&in&fragmentation&*&as&a&pressure&rises,&regulation&of&the&electron&current&through&the&ion&source&becomes&more&*&high&voltage&breakdown&may&occur&in&the&multiplier,&source,&or&analyser&if&the&pressure&becomes&too&*&oxygen&from&residual&air&and&leaks&will&cause&filament&burnout&in&the&ion&*&contamination&of&ion&source&components,&slits&or&&rods,&and&multipliers&increases&with&higher&pressure.The&international&standard&of&measure&for&pressure&in&the&SI&system&of&units&is&the&Pascal&(Pa).&Traditionally&in&high&vacuum&work&the&torr&has&been&the&base&unit&for&measurement&and&it&is&still&widely&used.&One&torr&is&defined&as&the&pressure&required&to&support&a&column&of&mercury&1&mm&hight.&One&torr&equivalent&to&133.332&Pa,&or&1&Pa&is&equivalent&to&7.5*10-3&torr.&Atmospheric&pressure&varies&with&the&weather,&but&for&scientific&purposes&considered&to&be&at&1.013*105&Pa&(760&torr).&At&atmospheric&&pressure&there&are&1*1019&molecules&per&cubic&centimetre.&Each&second,&3*1023&molecules&hit&each&square&centimetre&of&a&container’s&surface.&The&mean&free&path&of&each&molecule&is&6.5*10-8&m.&&For&a&typical&mass&spectrometer&with&a&path&length&of&500&mm,&the&pressure&must&be&reduced&to&1.3*10-2&Pa&(10-4&torr)&or&less&throughout&mass&spectrometer&to&prevent&collisions.&In&practice,&this&means&that&the&pressure&must&be&about&10&times&lowering&the&main&pumping&region&to&allow&for&pumping&speed&restrictions&coursed&by&slits&and&lenses&and&to&accommodate&very&long&path&lengths&encountered&in&some&instrument.&Hence,&typical&analyser&pressure&limits&are&3*10-3&Pa&(2*10-5&torr)&for&quadrupole&instruments&and&3*10-4&Pa&(2*10-6&torr)&for&magnetic&instrument.3.1.&Rotary&pump.&Rough&pumping&of&the&mass&spectrometer&is&necessary&before&the&very&high&vacuum&pumping&system&is&applied,&whenever&the&instrument&has&been&vented&to&the&atmosphere.&The&vacuum&usually&provided&by&a&rotary&pump,&typically&in&the&range&1&to&10-3&torr.&Sometimes&it&might&be&called&as&backing&pump,&foreline&pump,&or&rough&pump.There&are&number&of&different&types&of&rotary&pumps.&All&use&the&same&principle&of&tacking&in&a&large&volume&of&low-pressure&gas&and&compressing&it&to&a&smaller&volume&by&the&action&of&rotating&device.&The&most&common&type&consists&of&a&cylindrical&rotor&mounted&eccentrically&in&a&cylindrical&chamber.&(Fig.5)Figure&5.&Cross&section&of&rotary&vacuum&pump.&Spring-loaded&rotor&blades&are&mounted&diametrically&through&the&rotor&sealing&against&the&inner&surfaces&of&the&chamber.The&rotor&is&turned&by&electrical&motor.&The&rotor&blades&are&free&to&move&in&and&out&of&the&rotor&and&will&sweep&out&a&varying&volume&as&the&pump&is&turned.&The&gas&inlet&port&is&connected&to&the&point&of&largest&enclosed&volume.&Gas&is&ejected&through&a&spring-loaded&flap&valve,&which&prevent&gas&from&re-entering&the&pump.&The&sealing&is&further&improved&by&immersing&the&pumping&assembly&in&an&oil&bath&and&allowing&some&of&the&oil&into&the&pump&chamber.&This&seals&the&small&end&gaps&between&the&rotor&blades&and&pumping&chamber.&The&oil&also&serves&as&a&lubricant.&3.2.&Diffusion&pump.The&diffusion&pump&moves&gas&from&one&area&to&another&by&establishing&conditions&in&which&the&mean&free&path&for&molecules&in&one&direction&is&greater&&than&in&the&opposite&direction.&The&diffusion&pump&consists&of&an&enclosed&chamber&on&which&a&stack&of&concentric&jets&are&mounted.&(Fig.6)Figure&6.&Cross&section&of&diffusion&pump.&A&small&charge&of&oil&covers&the&bottom&which&is&heated&strongly.&The&oil&boils&vigorously&producing&a&heavy&vapour&stream,&which&passes&up&the&inside&of&the&stack&and&out&through&the&jets.&These&are&angled&downwards&so&that&the&escaping&vapour&produces&a&cloud&of&molecules&moving&rapidly&towards&the&bottom&of&the&pump.&The&outside&of&the&pump&is&cooled&by&water&jacket&or&cooling&rings&but&sometimes&by&an&air&blower.&The&oil&vapour&is&condensed&and&runs&down&to&the&bottom&of&the&pump&to&repeat&the&cycle.Any&gas&molecules&that&happen&to&diffuse&into&the&pump&will&suddenly&encounter&a&strong&stream&of&particles&moving&predominantly&downwards.&Consequently,&the&gas&molecules&will&be&driven&down&to&the&lower&part&of&the&pump.&A&pressure&difference&up&to&five&orders&of&magnitude&can&be&developed&across&the&pump.&The&diffusion&pump&can&create&an&ultimate&vacuum&of&about&10-5&-&10-8&torr&with&an&outlet&pressure&of&10-3&torr.&It&is&necessary&for&the&diffusion&pump&to&be&backed&with&rotary&pump,&since&it&would&be&a&number&of&problems&without&it: the&vapour&jets&would&encounter&such&a&large&number&of&molecules&that&the&flow&would&become&turbulent&and&not& the&oil&would&have&to&be&heated&much&more&strongly&to&achieve&a&sufficiently&high&boiling& there&would&be&sufficient&oxidation&of&the&oil&and&pump&element.&&3.3.&Turbomolecular&pump.The&pumping&action&of&a&turbomolecular&pump&is&similar&in&some&respects&to&that&of&a&diffusion&pump,&in&that&the&mean&free&path&of&gas&molecules&is&greater&in&one&preferred&direction&because&of&the&gas&molecule&collisions&with&moving&rotor&blades.&The&gas&is&moved&through&the&rotor&assembly.&Blades&are&angled&to&deflect&gas&molecules&downwards&towards&the&next&set&of&blades&and&finally&to&the&pump&outlet&(Fig.7)&.&As&the&gas&pushed&through&the&pump&its&pressures&increased.&If&the&pressure&is&increased&to&a&point&where&the&mean&free&path&becomes&small&and&a&collision&with&another&gas&molecule&rather&then&the&rotor&is&more&likely,&then.&The&pumping&action&will&cease.&To&prevent&this&the&turbomolecular&pump&must&be&backed&be&rotary&mechanical&pump.Figure&7.&Cross&section&of&turbomolecular&pump.&The&optional&turbomolecular&pump&has&a&pumping&capacity&of&250&L/sec&that&limits&the&maximum&GC&column&flow&rate&to&<4&ml/min.&This&allows&the&use&of&GC&columns&up&to&0.32&mm&ID.&Rotor&speeds&vary&with&type,&but&a&typical&unit&will&operate&from&40,000&to&80,000&rpm.&Consequently,&the&assembly&must&be&very&carefully&balanced&to&prevent&vibration.4.&Interfacing&GC&and&MS.The&major&difficulty&in&interfacing&a&GC&and&a&MS&is&due&to&the&great&pressure&difference&between&the&system.&A&typical&carrier&gas&flow&rate&for&a&packed&column&gas&chromatography&is&30&ml/min,&and&0.5-2&ml/min&for&capillary&columns.&The&MS&operating&pressure&&must&be&about&10-5&&torr.&A&good&interface&allows&the&GC&and&the&MS&to&operate&at&or&near&the&optimum&condition&for&each,&yet&it&must&also&permit&compounds&from&the&GC&to&be&transmitted&to&the&MS&without&anomalous&behaviour.&For&example,&no&loss&in&sensitivity,&no&reactivity,&no&alteration&of&the&GC&peak&shape&(Fig.&8).Figure&8.&Interfacing&GC&and&MS.&4.1.&&Packed&column&interfaces.For&optimum&system&performance&it&is&important&that&the&excess&carrier&gas&be&removed&but&that&the&sample&remain,&so&a&gas-sample&separator&is&required.&The&choice&of&gas&is&very&important.&Nitrogen&gas&has&much&lower&pumping&speed&then&the&lighter&gases&such&as&helium&and&hydrogen.&It&is&not&removed&so&easily&from&the&vacuum&chamber&by&the&pumping&system&and&would&be&greater&constrain&on&the&separator&performance.&Helium&being&the&most&common&because&of&safety&consideration.&It&also&has&a&low&molecular&weight.&The&sample&molecules&leaving&the&GC&are&one&or&two&orders&of&magnitude&heavier&than&helium.&The&diffusion&rate&for&helium&through&porous&materials&is&consequently&much&greater&than&that&of&the&sample&molecules.The&Watson-Bieman&Effusion&Separator&(Fig.&9)&consists&of&a&sintered&glass&tube&of&ultrafine&porosity&through&which&the&carrier&gas&effuses&to&be&pumped&away&by&an&vacuum&system.&The&remaining&carrier,&greatly&enriched,&passes&through&the&separator&to&the&MS&ion&source.&A&number&of&other&materials&can&be&used&in&place&of&the&sintered&glass&tube,&silver,&stainless&steel,&and&ceramic.&&&&&Fig.&9.&The&diagram&of&the&Watson-Bieman&interface.&(Youl)The&membrane&separator&(Fig.&10)&relies&on&the&solubility&of&organic&materials&in&a&silicone&polymer.&The&inert&carrier&gas&has&a&very&low&solubility&&in&the&membrane&material,&and&so&the&amount&of&helium&reaching&the&ion&source&is&small.&The&&membrane&separator&consist&of&a&chamber&across&which&is&stretched&an&extremely&thin&(0.025&mm)&silicone&membrane,&which&would&be&easily&ruptured&by&the&pressure&differential&across&it&if&it&were&unsupported.&A&fine&metal&mesh&of&sintered&glass&screen&is&used&for&this&purpose.&The&outlet&for&the&excess&carrier&gas&can&be&at&atmospheric&pressure.&Sample&yields&to&the&MS&is&compound-dependent&and&may&be&as&high&as&60%.&&&&Fig.&10.&The&diagram&of&permeable&membrane&interface.&(Youl)The&Jet&Separator&(Fig.&11).&The&molecular&weight&difference&between&helium&and&the&sample&can&also&be&used&to&advantage&in&jet&separators,&where&momentum&and&diffusion&rate&are&used&to&enrich&the&gas&flow&to&the&MS.&The&column&effluent&is&forced&through&a&fine&jet&into&an&evacuated&enclosure.&Opposite&the&first&jet&and&only&a&very&small&distance&from&it&is&mounted&a&second&jet.&Under&normal&gas-chromatography&flow&conditions&the&gas&will&pass&through&the&jets&at&near&supersonic&speeds.&The&heavy&sample&ions&travel&almost&in&a&straight&line&from&one&jet&to&the&other,&but&the&lighter&helium&molecules,&under&the&influence&of&the&pressure&differential,&diffuse&outwards.&To&achieve&the&required&ion&source&pressures&it&is&not&uncommon&to&employ&two&stages&of&jets&operating&in&series.&&&&&&&&&&&&&&Fig.&11.&The&diagram&of&glass&jet&separator.4.2&&Capillary&column&&interfaces.In&most&respects,&a&Capillary&Direct&Interface&is&the&best&interface&to&use.&Because&compounds&elute&directly&from&the&column&into&the&ion&source,&there&are&no&parts,&gas&flows,&or&voids&to&introduce&artefacts&into&the&analysis.&In&particular,&the&excellent&peak&shape&resulting&from&the&use&of&capillary&columns&is&preserved&when&using&Capillary&Direct&Interface.&The&entire&sample&elutes&from&the&column&into&the&source&resulting&in&maximum&sensitivity&.The&disadvantages&arise&because&the&column&is&connected&directly&to&the&MS.&This&means&that&the&column&may&not&be&changed&without&venting&the&system.&Furthermore,&only&low-bleed&columns&with&bonded&cross-linked&stationary&phase&may&be&used.&If&not,&the&stationary&phase&will&bleed&directly&into&the&source&coursing&&fast&contamination,&high&background,&and&short&intervals&between&cleanings.&Flow&rates&in&the&range&0.5&to&2&ml/min&for&directly&coupled&capillary&columns&are&too&high&for&some&mass&spectrometer&with&electron&impact&sources&(Table&1).Table&1.&Interface&overview.ColumnID&(mm)Typical&Flow&(ml/min)InterfaceNarrow&Bore0.1,&0.2,&0.250.1-1.0Capillary&DirectWide&Bore0.321-3Splitters&Jet&SeparatorMega&Bore0.533-15Splitters&Jet&SeparatorSource&pressure&can&be&reduced&to&acceptable&levels&either&by&fitting&a&splitter&or&by&modifying&the&source&pumping&characteristics.&There&are&some&disadvantages&associated&with&&capillary&splitter:*&sample&utilisation&is&reduced&by&split&*&some&designs&are&mass-*&split&ration&may&change&with&increasing&molecular&weight&of&eluting&*&dead&volume&(space&within&tubes&and&fittings&where&the&gas&flow&is&such&that&may&be&swirling,&or&mixing,&or&a&delay&in&sample&transfer)&can&cause&significant&peak&broadening&and&tailing.4.4&Probe&techniquesThe&chemically&pure&compounds&or&purified&components&are&able&to&be&introduced&to&the&MS&with&direct&probe&technique.Solid&probe:&A&solid&sample&is&placed&in&a&small&glass,&quartz&or&gold&vial&held&in&the&end&of&the&probe&wand.&The&probe&enters&the&vacuum&system&through&a&series&of&vacuum&interlocks.&Heating&the&probe&causes&the&sample&to&evaporate&into&the&ionizer&ready&for&analyses.&Either&EI&or&CI&conditions&might&be&used.&Unfortunately,&volatility&under&vacuum&condition&if&often&too&great&for&adequate&sample&separation&and&the&technique&has&limited&use.Desorption&CI:&The&sample&is&flashed&off&a&thin&small&wire&loop&mounted&on&the&end&of&a&direct-insertion&probe&straight&into&an&ionising&CI&reagent&gas&within&the&ion&source.&Thermally&labile&compounds&give&strong&spectra&with&little&cracking&and&good&molecular&ions.&Technique&may&be&useful&for&non-volatile&materials.Pyrolysis:&rapid&heating&rates&up&to&1000&C&applied&in&this&technique&to&produce&thermal&fragmentation.&The&sample&not&only&evaporate&into&the&ion&source&but&also&it&is&decomposed&by&the&heat.&It&is&used&to&analysed&generally&complex&chemical&samples&or&mixtures&(polymers,&bio-organisms).5.Ionizers.Before&mass&spectrometer&can&analyse&a&sample&it&is&necessary&that&the&sample&molecule&be&ionised.&After&that&the&magnetic&and&electric&field&of&the&mass&spectrometer&interact&with&the&charged&molecules,&separating&ions&of&different&mass-to-charge&ratio.&A&number&of&techniques&have&been&used&to&produce&charged&particles:&electron&impact(EI),&chemical&ionisation(CI),&field&sources,&particle&or&high&energy&sources,&fast&atom&bombardment(FAB).&The&most&prominent&type&for&GC/MS&work&is&EI.5.1&&Electron&Impact&Sources&A&cross&section&of&a&typical&electron&impact&source&is&shown&in&Figure&13.&Electrons&produced&from&a&heated&filament&wire&are&accelerated&through&a&chamber&where&they&undergo&collision&with&the&molecules&of&the&gas&to&be&analysed.&The&gas&molecules&ionised&as&a&result&of&these&collisions&are&drawn&out&of&the&ionisation&chamber,&focused,&and&injected&into&the&mass&analyser&by&a&series&of&&electrically&charged&plates.&Figure&13.&Cross&section&of&an&electron&impact&ion&source&.The&filament&is&heated&to&red&by&passing&a&relatively&large&current&through&it.&Electrons&are&released&from&the&hot&surface&by&thermal&emission.&The&filament&material&is&chosen&to&maximise&the&efficiency&of&this&process.&Rhenium,&rhenium&alloys,&or&uranium&being&most&commonly&used.&The&electrons&are&accelerated&toward&the&ion&volume&by&applying&a&voltage&known&as&the&electron&energy&voltage&between&it&and&the&filament.Some&of&the&electrons&enter&the&ionisation&chamber,&where&they&interact&with&any&gas&molecules&coming&either&form&GC/MS&interface,&probe&inlet&or&other&sample&inlet.&Collision&take&place,&and&the&energy&is&transferred&from&the&electrons&to&a&gas&molecules.&If&the&electron&energy&is&sufficient&&there&is&a&high&probability&that&some&of&the&gas&molecules&will&become&excited&enough&to&be&ionised.&If&the&electron&energy&is&further&increased,&then&the&molecules&can&become&so&excited&that&they&break&apart,&yielding&a&number&of&different&types&of&fragments.&The&principal&reaction&is&summarised&is&figure&14&by&considering&the&effect&of&fast&electrons&on&a&hypothetical&molecule&ABC.Figure&14.&Electron&Impact&Ionisation&Mechanism.&The&effect&of&a&fast&electron&colliding&with&the&hypothetical&molecule&ABC.Due&to&electron-electron&interaction,&the&molecules&lose&a&bound&electron.&The&resulting&molecule&is&an&ion&and&has&a&charge&(usually&+1),&though&ions&with&multiple&charge&do&occur.&The&number&of&molecular&ions&initially&formed&depends&on&the&energy&of&the&incoming&electrons,&increasing&with&the&electron&energy.&Above&a&certain&value&(about&50&eV),&increasing&the&electron&energy&does&not&increase&the&amount&of&molecular&ions&formed.&Fig.&15&shows&a&typical&ionisation&efficiency&curve&for&positive&ion&formation,&in&which&the&energy&of&ionisation&electrons&(eV)&is&plotted&against&the&number&of&ions&produced&(ionisation&efficiency).&The&curve&shows&we&would&get&the&maximum&yield&of&ions&&at&approximately&50&eV.&However,&at&that&value&we&are&very&close&to&the&steeply&increasing&part&of&the&curve&and&thus&any&small&unintentional&experimental&decrease&in&beam&energy&would&lead&to&a&dramatic&reduction&in&ionisation&efficiency.&If&we&increase&the&operational&beam&energy&to&70&eV,&we&sacrifice&only&a&small&amount&in&ionisation&efficiency,&but&move&well&into&plateau&region,&where&small&changes&in&beam&energy&have&little&effect&on&ionisation.&For&this&reason&electron&impact&mass&spectra&are&usually&recorded&at&70&eV.&&&&&&&&&&&&&&&&Figure&15.&A&typical&Ionisation&Efficiency&Curve.A&spectrum&recorded&with&a&beam&energy&set&at&or&about&the&value&of&the&ionisation potential&will&show&the&presence&of&only&molecular&ions.&Fragmentation&only&becomes&important&at&higher&electron&beam&energies.&The&spectra&recorded&at&different&eV&are&shown&in&Fig.&16.&&&&&&&&&&&&&&&&Figure&16A.&Mass&spectra&at&various&electron&energies.&Electron&impact&ionisation&is&the&most&widely&used&method&of&ionisation&in&mass&spectrometry&because&for&most&molecules&it&produces&both&molecular&and&fragment&ions.&Thus,&it&allowed&determination&of&both&relative&molecular&mass&and&molecular&structure.Usually,&most&structural&information&is&gained&when&the&maximum&number&of&fragment&ions&are&produced.&&But&there&considerable&number&of&molecules&for&which&all&or&almost&all&the&molecular&ions&formed&fragment&before&they&leave&the&ion&source.&For&this&type&of&molecule&it&is&difficult&to&determine&their&molecular&mass.Most&organic&compound&give&more&positive&than&negative&ions&when&ionisation&occurs&under&EI&conditions.&In&the&traditional&GC/MS&system&which&examine&positive&ion&beam&it&is&necessary&to&remove&the&slow&electrons&and&excess&fast&electrons&from&the&ion&volume.&A&plate&with&a&small&positive&potential&relative&to&the&ion&volume&is&used&to&mop&up&these&electrons.&It&is&usually&called&electron&collector&or&trap.&It&is&mounted&just&outside&the&ion&volume&on&the&side&opposite&the&filament.The&electron&beam&is&focused&into&the&ion&volume&by&the&use&of&a&filament&reflector.&A&metal&reflector&plate&is&positioned&behind&the&filament&and&is&electrically&connected&to&it.&Electrons&leaving&the&filament&will&be&repelled&from&the&reflector.For&positive&ions&detection&the&ion&volume&is&normally&held&at&positive&voltage&with&respect&to&the&ground.&This&is&referred&as&a&ion&energy&voltage.&&Some&sources&have&a&small&repeller&plate&mounted&inside&the&volume.&This&normally&has&a&small&positive&voltage&with&respect&to&the&ion&volume&and&causes&the&positive&ions&to&be&pushed&out&from&it.&The&ions&formed&inside&the&&source&will&adopt&the&same&potential&as&in&the&ion&volume.&Passing&from&the&ion&volume&toward&grounded&exit&causes&the&ions&to&lose&potential&energy&and&to&gain&kinetic&energy.&A&series&of&charged&plates&between&the&ion&volume&and&the&exit&plate&improve&the&ion&extraction&and&limit&the&angular&spread&of&the&emerging&ion&beam,&so&they&act&as&a&lens,&focusing&the&ion&beam.&It&is&for&this&reason&they&called&lens&plate,&but&the&plate&nearest&to&the&ion&volume&called&the&extractor.&(Fig.&13)For&negative&ion&work&the&ion&energy,&lens,&extractor,&end&repeller&voltage&must&be&reversed&in&polarity.&Negative&ion&work&is&more&common&with&chemical&ionisation&sources.There&are&several&drawbacks&of&electron&impact&ionisation:*&It&may&be&difficult&to&measure&relative&molecular&masses&for&molecules&sensitive&for&*&it&is&difficult&to&distinguish&between&*&some&compounds&may&undergo&thermal&decomposition&prior&to&ionisation&or&be&very&inclined&to&fragmentation&after&ionisation&because&of&the&temperature&required&for&*&others&may&simply&be&too&involatile&to&give&a&spectrum.When&this&problems&are&occur,&recourse&must&be&made&to&alternative&methods&of&ionisation.5.2&&Chemical&ionisation&(CI)In&Chemical&Ionisation&the&sample&molecule&is&surrounded&by&reactive&reagent&ions&and&charge&is&passed&to&the&molecule&by&a&chemical&interaction&without&much&energy&transfer.&Reagent&gas&is&mixed&in&the&ion&volume&of&the&CI&source&with&sample&gas&stream.&The&reagent&gas&pressure&is&about&1&torr.&The&majority&of&ions&are&formed&from&the&reagent&gas,&which&is&ionised&initially&by&electron&impact.&This&ions&may&combine&with&themselves&or&neutral&reagent&gas&molecules&to&form&the&reactive&plasma.&This&can&be&illustrated&by&considering&the&reactions&that&occur&when&methane&is&used&as&a&reagent&gas.-&Primary&electron&impact&reaction:CH4&&&&&&&&&&&&&&e&&&&&&&&&&&&&&CH4+,CH3+,&CH2+,&CH+,&C+,&H2+,&H+-&In&the&relatively&high&pressure&of&the&CI&source,&typically&0.5&to&1&torr,&a&series&of&secondary&reactions&occur:CH4+&+&CH4&?&CH5+&+&CH3CH3+&+&CH4&?&C2H5+&+&H2CH2+&+&CH4&?&C2H3+&+&H2&+H-&Tertiary&ion-molecular&reactions&occur&when&some&of&the&reaction&products&will&further&react&with&uncharged&methane:C2H3+&&+&CH4&?&C3H5+&+&H2(C2H2+&+&CH4)n&?&polymersAll&these&reactions&and&others&not&listed&occur&at&the&same&time.&The&most&prominent&ions&formed,&accounting&for&approximately&95&%&of&&the&total&ionisation&are&marked&in&bold&(see&above).&They&are&extremely&reactive&and&may&attack&the&sample&molecules,&passing&charge&to&them.&Furthermore,&proton&or&even&the&whole&reagent&ion&may&become&attached&to&the&sample&molecule&giving&(M+1)+,&(M+29)+,&and&(M+41)+&ions.&Often&the&protonated&and&additional&ions&have&greater&stability&than&molecular&ions&formed&in&the&EI&mode,&so&that&positive&identification&of&molecular&weight&may&be&possible&with&CI&even&where&there&is&no&molecular&ion&under&EI&condition.Reagent&ion-molecular&reactions&are&given&bellow:*&Proton&transferXYZ&+&CH5+&?&(XYZ+H)+&+&CH4&&&&&&&&&(M+1)*&Hydride&transferXYZ&+&C2H5+&?&(XYZ-H)+&+&C2H6&&&&&&(M-1)*&Additional&reaction&XYZ&+&C2H5+&?&(XYZ+C2H5)+&&&&&&&&&&&&(M+29)XYZ&+&C3H5+&?&(XYZ+C3H5)+&&&&&&&&&&&&(M+41)*&Additional&and&dissociation&(neutral&loss)XYZ&+&CH5+&?&(XYZ+H)+&?&XY+&+&HZZ=OH,&SH,&etc.Reagent&gas&selection.A&number&of&different&substances&can&be&used&in&CI.&The&most&typical&compounds&and&their&common&ions&are&given&in&table&1.Table&1.&Predominant&ions&at&pressure&about&1&torr.Reagent&gasIonsMethaneCH5+,&C2H5+,&C3H5+PropaneC3H7+,&C3H8+Iso-butaneC4H9+HydrogenH3+AmmoniaNH4+,&(NH3)2H+,&(NH3)3H+WaterH3O+Tetramethylsilane(CH3)3Si+Dimethylamine(CH3)2NH2+,&[(CH3)3NH]2H+,&C3H8N+For&ionisation&to&take&place&at&all,&chemical&reaction&between&the&sample&and&the&reagent&gas&must&be&exothermic.&The&grater&the&heat&of&the&reaction,&the&more&fragmentation&will&occur.&For&a&given&sample&the&heat&of&the&reaction&depends&only&on&the&reagent&gas&used.&The&general&equation&is:Heat&of&reaction&=&Proton&affinity&of&reagent&gas&-&Proton&affinity&of&sample.&The&heat&of&reaction&must&be&negative&for&ionisation&to&take&place.&The&statement&shows&that&the&reagent&gases&with&high&proton&affinity&will&give&lower&heats&of&reaction.&If&ionisation&occur,&then&these&reagent&gases&will&give&strong&molecular&or&quasi-molecular&ions&with&little&fragmentation&(Fig.&16,17).&Consideration&of&proton&affinities&allows&the&analyst&to&control&fragmentation&by&selecting&different&reagent&gases&(table&2).Table&2.&Proton&affinities&of&CI&Reagent&Gases.GasProton&Affinity,&Kcal/molHelium42Methane127Water&vapour167Isobutane195Ammonia207Methane&has&a&relatively&low&proton&affinity&and&is&likely&to&give&some&ionisation&with&an&unknown&sample&than&either&isobutane&or&ammonia.&It&also&has&distinct&advantage&of&being&a&reasonable&carrier&gas&-&sample-enrichment&devise&is&not&needed&in&the&GC/MS&interface&and&direct&coupling&is&possible.&This&make&methane&the&first&choice&for&a&general&purpose&CI&reagent&gas.&Though&there&are&several&disadvantages&that&may&be&significant:-there&is&usually&significant&CI&reagent&background&spectrum,&since&the&degree&of&fragmentation&is&greater&than&with&some&other&reagent&-methane&gives&a&large&number&of&addition&ions,&quasi-molecular&ions,&and&fragments&with&the&system&oil&background.Ammonia&does&not&give&the&same&degree&of&background&interference,&but&it&higher&proton&affinity&may&prevent&fragmentation.&Ammonia&also&more&difficult&gas&to&handle&-&it&is&corrosive,&hygroscopic&and&very&reactive&to&all&fittings,&pipes,&and&valves.Isobutane&has&also&found&widespread&use&as&a&general-purpose&reagent&gas,&but&it&is&less&efficient&than&methane&because&of&its&greater&molecular&mass.CI&spectra&of&compounds&produced&with&different&reagent&gases&are&given&on&Figure&16,&17.Figure&16B.&CI&spectra&of&methyl&stearate&(MW&298)&with&(a)&methane,&(b)&isobutane,&and&(c&)&&ammonia&as&reagent&gases.Figure&17.&CI&spectra&of&norleucine&with&(a)&ammonia,&and&(b)&methane.Hence,&&there&are&several&advantages&of&CI&over&the&more&conventional&EI&technique:*&more&information&on&molecular&ion,&the&molecular&weight&can&be&positively&identified&for&many&&&*&can&be&more&sensitive&and&highly&selective,&compounds&can&be&selectively&ionised&in&a&complex&mixture&by&choosing&a&proper&reagent&*&chemical&isomers&and&stereoisomers&can&be&*&complements&EI&data&(Fig.&18),&*&there&is&less&fragmentation,&yielding&simpler,&more&easily&understood&spectra,&good&for&single&ion&*&soft&condition&for&labile&compounds,&*&mobile&phase&can&also&be&used&as&reagent&gas,&allowing&direct&coupling&to&the&MS.Figure&18.&Comparison&of&EI&and&CI&spectra.&(a)&Norleucine&at&70&eV&EI,&(b)&Methane&CI.Negative&Ion&CI.Traditionally,&mass&spectrometers&have&been&configurated&to&investigate&positive&ions.&For&EI&work&this&is&reasonable,&since&most&compounds&yield&a&far&greater&number&of&positive&ions&than&negative.&With&CI&sources&many&ion&source&reaction&yield&negative&ions,&and&in&some&cases&ions&of&both&polarity&are&formed.&Observing&both&positive-&and&negative-ion&spectra&will&give&valuable&information&to&the&analyst.&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&Negative&ion&CI&is&even&more&widely&used&than&Positive&ion&CI&since&Negative&Ion&&(NI)&mass&spectrums&are&usually&very&simple.&Very&often&only&molecular&ion&can&be&found&in&NI&CI&spectrum.Since&the&positive&and&negative&CI&data&are&of&significant&interest&together,&it&is&often&desirable&to&obtain&both&at&the&same&time.&To&change&from&the&positive&to&the&negative&mode&requires&that&the&polarity&of&the&ion&energy&and&lens&plate&voltage&be&reversed.&With&suitable&power&supply&design,&high-speed&switching&is&possible&on&quadrupole&instruments&either&during&a&scan,&or&on&a&scan-to-scan&basis.&The&additional&conversion&dynodes,&one&for&negative&ions&and&one&for&positive&ions,&allows&the&multiplier&to&be&used&for&both&polarities.&5.3&&Field&ionisation&(FI)A&schematic&diagram&of&an&FI&ion&source&is&shown&in&Fig.&19.Figure&19.&Schematic&diagram&of&a&field&ionisation&ion&source.&The&most&important&components&are&the&electrodes,&an&a&node&and&cathode,&which&are&held&at&a&potential&difference&of&up&to&10&kV&and&are&sufficiently&close&together&to&develop&a&potential&of&up&to&108&V&cm-1&with&a&distance&between&electrodes&10-4&cm.&When&a&sample&molecule&in&the&vapour&phase&impinges&on&the&anode&or&comes&very&near&to&it,&the&potential&gradient&it&experiences&leads&to&an&electron&being&transferred&to&the&metal&anode&giving&rise&to&the&formation&of&a&positive&ion.&It&is&attracted&towards&the&cathode&and&as&this&has&a&hole&in&it,&some&ions&pass&through.&Most&mass&spectrometers&are&designed&to&analyse&ions&travelling&at&velocities&attained&by&acceleration&through&potential&differences&of&&volts.&The&ions&leaving&the&cathode&of&the&FI&source&will&have&passed&through&a&potential&difference&of&about&10000&volts.&The&focusing&devise&beyond&the&cathode&has&two&*&it&must&retard&the&ions&until&they&reach&the&correct&velocity&for&analysis*&it&must&focus&the&diverging&beam&of&ions&coming&through&the&cathode&into&a&coherent&beam&for&mass&analysisThe&cathode&is&often&called&the&emitter,&because&molecules&impinge&upon&it&or&come&very&close&to&it&are&emitted&as&ions.&The&emitter&is&usually&a&sharp&blade,&a&sharp&tip&or&a&wire.There&is&a&little&energy&transfer&to&the&molecule&and&thus&very&little&fragmentation.&The&technique&is&usually&yields&abundant&molecular&ion.&Disadvantages&of&FI&technique&include:*&Uncertainty&&of&the&determination&of&relative&molecular&masses&of&unknown&substances&(Quasi-molecular&ions&are&produced&because&of&high&sample&concentration&near&anode);*&Emitter&surface&absorbs&contaminants,&which&can&be&ionised&and&bonded&to&the&sample&molecule&producing&additional&ions.Advantages:*&FI&usually&shows&some&fragment&ions&that&can&be&structurally&informative&(Fig.&20).&Less&fragmentation&than&in&EI.Figure&20.&Mass&spectrum&of&xanthsosine&(a)&using&EI&and&(b)&using&FI.5.4&Field&Desorption&(FD).If&vaporisation&of&the&sample&is&a&problem,&it&might&be&overcome&by&producing&ions&directly&from&the&solid&state.&This&is&what&FD&does.&It&operates&on&the&same&principal&as&FI&in&that&it&uses&an&emitter&held&at&a&high&temperature&with&respect&to&a&cathode.&But,&the&sample&is&placed&on&the&surface&of&the&emitter&in&the&solid&face&state&and&ions&are&desorbed&directly&from&the&solid&towards&the&cathode.FD&is&a&softest&of&the&ionisation&discussed&in&this&lecture.&Generally&it&only&produces&molecular&ions&with&virtually&no&fragmentation.5.5&Discharged&Ion&Secondary&Ion&MS&(DISIMS)&or&Fast&Atom&bombardment&(FAB).In&FAB&technique&a&beam&of&fast&moving&neutral&atoms&are&directed&onto&a&metal&plate&coated&with&a&sample,&then&much&of&the&high&kinetic&energy&of&the&atoms&is&transferred&to&the&sample&molecules&on&impact&coursing&ion&ionisation.&The&bombarding&atoms&are&usually&gases,&either&xenon&or&argon.&In&order&to&achieve&a&high&kinetic&energy,&atoms&of&the&gas&are&first&ionised&and&these&ions&are&then&passed&through&an&electric&field.&After&acceleration,&the&fast&moving&ions&pass&into&a&chamber&containing&further&gas&atoms&and&collision&of&ions&and&atoms&lead&to&charge&exchange:Xe+.(fast)&+&Xe(thermal)&?&Xe(fast)&+&Xe+.(thermal)The&fast&atoms&formed&retain&most&of&the&original&kinetic&energy&of&the&fast&ions&and&carry&on&in&the&original&direction.&Deflector&plate&with&negative&potential&is&employed&to&remove&fast&ions&and&the&ions&with&thermal&energies.When&the&fast&atoms&bombard&the&sample,&both&positive&and&negative&ions&are&formed.&If&the&plate&on&which&the&sample&is&placed&is&held&at&suitable&potential&with&respect&to&the&an&ion&exit&plate,&either&positive&or&negative&ions&can&be&directed&towards&the&analyser.&A&schematic&representation&of&the&FAB&source&is&shown&in&Fig.&21.Figure&21.&Schematic&representation&of&a&FAB&source.&FAB&spectra&usually&provide&relatively&abundant&molecular&or&quasi-molecular&ions&and&also&show&some&structural&important&fragment&ions.Two&examples&involving&both&positive&and&negative&ion&FAB&spectra&are&shown&in&Fig.&22a&and&22b.Figure&22.&(a)&Positive&ion&FAB&Spectrum&of&Vasopressin&(MW&1055);&(b)&Negative-Ion&FAB&Spectrum&of&Potassium&Salt&of&Phenethicillin&(MW&402).6.&Principles&of&MS&analysers.Once&ionised,&the&sample&molecules&and&their&fragments&can&be&separated&on&the&basis&of&their&different&mass-to-charge&ratios.The&most&common&forms&of&mass&analyser&used&in&GC/MS&instruments&are&magnetic&sector,&quadrupole,&and&ion&trap.&Single&focusing&magnetic&sector&mass&spectrometer&will&be&discussed&first.&Fig.23.6.1&&Magnetic&sector&Mass-spectrometer.Fig&23.&Single&focusing&magnetic&sector&mass&spectrometer.&Ions&are&accelerated&from&the&ion&source&to&the&magnetic&sector&by&the&accelerating&voltage&V,&that&is&usually&very&high&(about&8000volts).&The&spread&of&the&ion&beam&is&limited&by&the&source&and&collector&slits.&The&gain&in&kinetic&energy&is&proportional&to&the&potential&energy&lost&or&potential&difference&(V)&through&which&the&ions&have&passed&so&that1/2&mv2&=&zV&(1)where&z&is&the&charge&on&the&ion.After&acceleration&ions&enter&the&analyser&region.&If&current&flows&through&a&conductor&in&a&magnetic&field,&then&a&force&will&be&experienced&at&right&angles&to&both&the&field&and&the&direction&of&current.&Electric&current&in&our&case&can&be&defined&as&passage&of&positively&charged&particles.&As&it&is&not&a&conductor&but&particles&themselves&there&are&no&physical&restrain&on&the&ions,&so&they&will&respond&to&the&exerted&force,&which&will&always&be&at&right&angles&to&the&direction&of&travel:&F&=&Bzv&(2)where&B&is&a&magnetic&field&strength,&v&-&ion&velosity.&Under&this&condition&the&resultant&trajectory&will&be&circular.&If&particle&of&mass&m&travels&with&velocity&v&around&the&circumference&of&a&circle&of&radius&r,&then&it&experiences&a&centrifugal&force&F&=&mv2/r&(3)somv2/r&=&Bzv&(4)From&equation&(1)v2&=&2zV/m&From&equation&(4)Bz&=&mv/rv&=&Bzrt/mv2&=&B2z2r2/m2so&2zV/m&=&B2z2r2/m2&m/z&=&B2r2/2VTo&make&ions&that&have&different&m/z&ratios&follow&the&correct&trajectory&and&reach&the&detector&one&can&either&vary&V&keeping&B&constant&or&vary&B&keeping&V&constant.&In&first&case&it&will&be&a&voltage&scanning&spectrometer,&in&second&a&magnetic&scanning&spectrometer.&Magnetic&scanning&instrument&are&more&sensitive&and&are&therefore&&used&on&all&commercially&available&spectrometers.&The&instrument&can&not&distinguish&between&M+&and&2M2+&.Resolution&(or&resolving&power)&is&defined&as&the&ability&of&the&mass&spectrometer&to&separate&adjacent&masses&by&a&given&amount.&In&the&10&%&valley&definition&it&is&assumed&that&two&masses&1&amu&&apart&are&separated&with&valley&between&them&of&a&high&equal&to&10%&of&&the&peak&height.&(Fig.24)Figure&24.&Equivalence&of&10&%&valley&resolution&definition.&Resolving&power&is&defined&also&as&the&mass&to&be&measured&divided&by&the&difference&in&masses&to&be&identified:R=M/(MExample1:&Instrument&has&resolving&power&4000.Mass&region&of&interest&-&400.Accuracy&of&measurement&is&0.1.Example&2:&a)&The&mass&of&C8H16+(&is&112.12528&and&that&of&C7H12O+(&is&112.08888.&The&resolving&power&required&is112&/&0.03640&=&3077b)&&The&mass&of&C40H82+(&is&562.64206&and&that&of&C39&H78O+(&&is&562.60566.&The&resolving&power&required&is&562/0.03640&=&15440Therefore&only&pair&a)&can&be&differentiated&with&magnetic&analyser.6.2&Double&focusing&mass-spectrometer.The&resolving&power&of&magnetic&mass&spectrometer&is&limited&by&the&factor&that&not&all&ions&of&the&same&m/z&have&exactly&the&same&kinetic&energy&after&acceleration.&It&is&possible&to&reduce&a&spread&of&kinetic&energies&&for&each&ion&by&applying&an&extra&focusing&devise&between&the&ion&source&and&the&magnetic&analyser.&This&takes&a&form&&of&a&pair&of&curved&metal&plates,&called&the&electrostatic&analyser&(ESA)&or&electric&sector&with&an&electrical&potential&maintained&across&them.(Fig.&25)&Figure&25.&Schematic&diagram&of&mass&spectrometer&using&both&electrostatic&and&magnetic&focusing.&Ions&enter&the&plate&with&kinetic&energy&arising&from&accelerator:1/2mv2&=&zVThe&ions&are&deflected&towards&the&bottom&plate.&That&is,&ions&experience&a&force&acting&at&right&angles&to&the&direction&of&their&flight.&When&the&ions&enter&the&region&between&the&plates&they&experience&the&potential&difference&(E)&between&them.&The&force&(F)&acting&on&an&ion&of&charge&z,&is&given&by&eq.:F=zEThus&the&electric&force&will&tend&to&deflect&the&ions&from&their&original&flight&path&into&a&circular&path.&Provided&that&this&electric&force&is&balanced&by&the&centrifugal&force&the&ions&attain&as&a&consequence&of&their&velocity,&v,&they&follow&a&circular&path&which&is&the&circumference&of&a&circle&of&radius&R:zE&=&mv2/RFrom&previous&equations:mv2&=&2zV&=&zeRR&=&2V/EIons&passing&through&the&field&E&will&follow&curves&of&different&radius&depending&on&their&kinetic&energies.Ions&of&any&m/z&values,&which&have&been&accelerated&through&a&potential&V&and&passed&through&a&field&E,&all&follow&a&curve&of&radius&R,&provided&that&they&have&the&same&kinetic&energies.&&If&their&kinetic&energies&are&different&,&trajectories&of&different&radiuses&will&be&observed&(Fig.&26).&&Figure&26.&Flight&paths&of&ions&of&different&kinetic&energies&in&a&ESA.A&slit&placed&between&the&electrostatic&analyser&and&the&magnetic&analyser&only&ions&of&the&required&kinetic&energy,&those&that&follow&a&circle&of&radius&R,&allows&to&pass&through&(Fig.&25).&An&instruments&that&operate&on&this&principles&are&known&a&s&double-focusing&mass&spectrometer.&In&this&devise&an&electric&sector&acts&as&an&energy&analyser,&while&the&magnetic&sector&acts&as&a&mass&analyser.&With&such&an&instrument&the&resolving&power&105&can&be&obtained.Example3:&To&resolve&a&doublet&arising&from&a&difference&of&13CH&to&N&in&the&formulas&of&two&ions&requires&a&resolving&power&of&about&6000&at&mass&50&amu,&12,000&at&mass&100&amu,&and&over&60,000&at&500&amu.Only&high&resolution&MS&can&do&this&job.&6.3&Quadrupole&Mass&Analyser.Another&common&type&of&analyser&used&in&mass-spectrometry&is&the&quadrupole&mass&analyser,&shown&on&Figure&27.&Mechanically,&the&quadrupole&is&an&extremely&simple&devise,&but&with&a&very&complex&mathematical&theory&behind,&which&will&not&be&discussed&here.&As&its&name&implies,&a&Quadrupole&Mass&Filter&consists&of&four&poles,&or&rods.&In&the&cross&section&of&quadrupole,&the&four&rod&are&arranged&at&the&corners&of&a&square.&In&HP&&MSD,&these&"rods"&have&been&reduced&to&four&metallic&strips&mounted&on&the&interior&surface&of&a&glass&tube.&The&quadrupole&in&the&HP&5973&MSD&is&a&fused-silica&tube&coated&with&a&thin&layer&of&gold.&The&tubes&in&each&MSD&have&been&formed&so&that&the&mounting&areas&have&a&hyperbolic&cross-section.&The&dimension&of&tubes&are&accurate&to&within&a&few&millionths&of&cm.&The&use&of&very&accurate&hyperbolically&shaped&rods&theoretically&gives&the&best&mass&peak&shape&and&resolution&for&a&quadrupole&mass&filter.Figure&27.&Fore&rods&in&quadrupole&mass&analyser.Two&pairs&of&rods&located&between&an&ion&source&and&a&detector.&Ions&from&an&ion&source&enter&the&mass&analyser&region&under&very&small&accelerating&potential,&5-15&V.&A&voltage&made&up&of&two&components&is&applied&to&the&&the&first&component&is&a&standard&DC&potential,&and&the&second&an&radiofrequency&component&(RF):Vx&=&+&(Vdc&+&Vrfcos(t)Vy&=&-&(Vdc&+&Vrfcos(t)Opposite&rods&are&electrically&connected.&Considering&only&one&pare&of&rods&2&and&3&on&Figure&28&a,&where&rod&3&is&at&a&positive&potential&with&respect&to&rod&2&,&positive&ions&will&be&drawn&towards&the&negative&rod.&When&RF&voltage&is&applied&at&1MHz&the&ions&would&be&attracted&to&rod&2&when&polarity&is&the&same,&and&to&rod&3&when&polarity&is&reversed.&This&change&in&polarity&is&occurring&at&a&very&high&rate&and&if&we&now&sum&together&both&components,&the&ions&now&follow&an&erratic&path,&which&depend&on&their&kinetic&energy&(Fig.&28&b).a)b)c)Figure&28.&(A)&Deflection&of&positive&two&ions&between&two&rods&set&at&a&fixed&potential&&(B)&at&a&variable&(DC+RF)&potential&difference.&c)&Electrical&connection&between&the&rods&of&a&qudrupole&mass&analyser.&The&same&DC&voltage&and&RF&field&are&also&applied&to&rods&1&and&4.&Simultaneously&to&their&application&to&rods&2&and&3.&The&DC&connections&are&such&that&rod&4&is&at&a&negative&potential&with&respect&to&1,&but&the&RF&field&applied&to&rods&1&and&4&is&180(&out&o&phase&with&that&applied&to&rods&2&and&3.&At&any&one&pair&of&DC&and&RF&values,&only&those&ions&of&one&particular&kinetic&energy&pass&directly&between&the&rods&and&out&the&other&end.&All&other&ions&collide&with&the&rods.If&the&ratio&between&the&DC&voltage&and&the&RF&field&is&kept&the&same,&and&both&parameters&are&vary&together,&then&ions&of&different&m/z&values&can&be&analysed&&and&a&mass&scan&can&be&made.&As&demonstrated&in&the&Mathieu&Stability&Diagram&(Fig.&29),&more&than&one&combination&of&DC&and&RF&could&be&used&to&"control"&a&given&mass.&Therefore,&the&DC&and&RF&must&be&set&to&produce&a&stable&ion&trajectory&for&only&one&amu&at&a&time..&By&using&combination&of&DC&and&RF&that&occur&above&the&scan&line,&only&one&amu&will&be&stable&on&the&mass&filter&and&exit&to&be&detected&by&the&electron&multiplier.&To&define&a&line&we&need&two&points&known&as&slope&and&intercept.&ChemStation&(in&HP&equipment)&will&refer&to&slope&as&amu&Gain&and&intercept&as&amu&Offset..&Amu&gain&affect&ratio&of&DC&voltage&to&RF&frequency&on&the&mass&filter.&This&control&the&width&of&mass&peaks.&Increasing/decreasing&the&amu&Gain&will&have&an&effect&on&low&mass&but&a&much&greater&effect&of&high&mass.&Amu&offset&also&affects&the&ratio&of&DC&voltage&to&RF&frequency&on&the&mass&filter&and&also&controls&the&width&of&the&mass&peaks.&A&higher&offset&yields&narrower&peaks.&Increasing/decreasing&the&amu&Offset&will&have&an&equal&effect&across&the&entire&mass&range.Figure&29.&Mathieu&Stability&Diagram.The&resolving&power&of&quadrupole&mass&analyser&is&about&the&same&as&that&of&a&single-focusing&device,&i.e.&5000.The&quadrupole&mass&filter&has&certain&advantages&over&a&magnetic&sector&instrument.&This&are&as&follows:a)&&it&is&relatively&cheap&to&b)&&it&is&smaller&and&c)&&more&d)&&more&accuracy&is&achieved&in&computer&control&of&rod&voltages&rather&then&magnetic&e)&&the&mass&scale&is&linear,&i.e.&not&proportional&to&B2;f)&&scanning&is&very&fast.The&scan&rate&is&the&time&it&takes&record&the&spectrum.&Scan&is&usually&measured&in&number&of&seconds&per&decade&of&mass&(mass&changes&by&a&factor&of&10).&Most&scans&are&of&the&order&of&1&sec&per&decade&for&magnetic-sector&mass&analyser.&This&is&because&magnets&need&a&certain&amount&of&time&to&reset&before&next&scan.&The& quadrupole&mass&analyser&can&scan&its&full&mass&range&in&a&few&ms,&while&its&resetting&time&is&even&less.Selective&ion&monitoring&mode&(SIM).Due&to&quadrupole&ability&to&perform&scan&and&reset&in&a&few&ms&and&possibility&to&accurately&control&rod&voltage,&it&is&often&used&for&selected&ion&monitoring.&During&a&typical&scan&run,&each&mass&is&measured&for&&approximately&100&usec.&In&contrast,&each&mass&in&SIM&mode&is&typically&100&msec&(Fig.&30).&Since&signal/noise&is&proportional&to&the&square&root&of&the&measurement&time,&it&follows&that&SIM&mode&will&be&roughly&30&times&more&sensitive&than&scan&mode.&In&practice,&improvements&of&20-100&are&possible.Figure&30.&Scan&ans&SIM&modes&in&quadrupole&mass&analyser.&(Note&that&the&two&figures&on&the&opposite&page&are&not&to&scale:&the&figure&in&scan&mode&should&have&about&4000&steps,&which&would&make&it&unreadable).6.4&&Ion-trap&analyser.An&ion&trap&is&a&devise&that&can&store&ions&for&an&extended&period&of&time&by&the&use&of&electric&and/or&magnetic&fields.&Figure&31&&shows&a&cross-section&view&of&a&simple&ion&trap.&This&consists&of&a&central&doughnut-shaped&ring&electrode&and&pair&of&end-cap&electrodes.Figure&31.&Schematic&diagram&of&an&ion-trap&mass&analyser.&A&variable&RF&voltage&is&applied&to&the&ring&electrode,&while&the&two&end&caps&are&connected&to&earth.&A&burst&of&gaseous&ions&from&the&sample&under&investigation&is&introduced&through&a&grid&in&the&upper&end&cap.&The&RF&voltage&is&then&scanned.&Ions&with&an&appropriate&m/z&value&circulate&in&a&stable&orbit&within&the&ring&cavity.&As&the&voltage&is&increased,&the&orbits&of&the&heavier&ions&become&stabilised,&while&those&of&the&lighter&ions&become&destabilised,&resulting&in&the&latter&leaving&the&cavity&via&openings&in&the&lower&end&cap.&The&emitted&ions&then&pass&into&a&detector.&As&with&&the&quadrupole&mass&analyser,&the&ion&trap&analyser&is&very&simple&and&the&resolution&is&of&the&order&of&1&part&in&1000.&Characteristics&of&ion-trap&MS&are&as&follows:*&compact&*&lower&*&mass&range&limited&by&500&amu*&does&not&require&turning.6.5.&&Time&of&flight&analyser.The&principal&of&operation&is&based&on&the&periodic&production&of&positive&ions&by&bombardment&of&the&sample&with&brief&pulses&of&electrons.&The&frequency&of&these&pulses&is&typically&10-50&kHz,&with&lifetime&of&0,25&usec.&The&positive&ions&that&are&formed&are&then&accelerated&by&an&electric&field&pulse&of&103-104&V,&which&has&a&similar&frequency&but&slightly&lags&behind&the&ionisation&pulse.&The&accelerated&ions&pass&into&a&region&that&contains&no&external&field&called&drift&tube&ca.&1m&in&length.&If&all&of&the&ions&have&the&same&kinetic&energy,&then&liner&velocity&will&depend&on&their&masses-the&heavier&ions&will&arrive&at&the&detector&later&than&lighter.&Mathematically&this&can&be&expressed&by&the&following:mv2/2&=&zVv2&=&2zV/mv&=&((2zV/m)since&v&=&L/twhere&L&is&a&drift&tube&length,&and&t&is&a&flight&timet&=&L((m/2zV)The&difference&in&transit&time&for&two&ions&of&masses&m1&and&m2&can&be&expressed&as&follows:(t&=&L((m1&-&(m2)/(2zVFigure&32.&Schematic&diagram&of&a&time-of-flight&mass&analyser.Characteristics&of&TOF&mass&spectrometer:*&resolution&is&about&1&part&in&1000.*&simplicity,*&virtually&unlimited&mass&range,*&relatively&high&scan&rate&(900&amu&on&1sec).6.6&Ion&cyclotron&resonance&MS&(Fourier-Transform&Instrument,&FT-MS)FT&spectrometer&contains&an&ion&trap&(section&6.4)&within&which&ions&are&allowed&to&circulated&in&defined&orbits&over&extended&period&of&time.&Ion-cyclotron&resonance&phenomenon&describes&the&behaviour&of&these&ions.&When&a&gaseous&ions&moves&into&a&strong&magnetic&field&its&motion&become&circular&in&a&plane&that&is&perpendicular&to&the&direction&of&the&field.&The&cyclotron&frequency&is&defined&as&the&angular&frequency&of&this&motion.&Having&an&equation&where&centrifuge&force&is&equal&to&magnetic&field&force&we&can&write:(c&=&v/r&=&Bz/mCyclotron&frequency&depends&on&only&m/z&value.&When&ion’s&velocity&is&increased&then&ots&radius&of&rotation&will&also&increased.An&ions&trapped&in&a&circular&path&in&a&magnetic&is&also&capable&of&adsorbing&energy&from&an&AC&electric&field&if&the&frequency&of&the&field&matches&the&cyclotron&frequency.&When&adsorbed,&this&energy&can&increase&the&ion’s&velocity&and,&therefore&its&radius&of&travel&without&affecting&(c.&So&only&ions&with&similar&&cyclotron&resonance&frequency,&i.e.&similar&m/z&values&will&be&set&in&motion&in&phase&with&the&AC&field,&while&ions&with&different&m/z&values&will&be&unaffected.&If&such&a&group&of&ions&approaches&the&upper&plate,&then&induced&electrons&will&be&attracted&from&earth&to&this&plate,&thus&causing&a&momentary&current.&The&frequency&of&current&is&characteristic&of&the&m/z&value&of&the&ions.Characteristics&of&FT-ICR&mass&spectrometer:*&rapid&scan*&wide&mass&range&(&5.000&amu*&very&high&resolution&(&106&*&ions&generation&and&mass&analysis&occur&on&the&same&region*&it&is&very&easy&to&switch&between&positive-&and&negative-&ion&spectra*&very&expensive.7.&Signal&detectors.The&ion&current&flowing&through&the&mass&spectrometer&in&a&GC/MS&system&is&extremely&small,&but&it&may&change&rapidly&over&a&wide&range.&To&detect&this&current&requires&electronic&circuits&of&high&signal&gain,&wide&dynamic&range,&and&good&frequency&response.&A&current&of&10-15&A,&equivalent&detection&limit&for&normal&GC/MS&systems.&Even&above&this&limiting&level&the&signals&must&be&amplified&significantly&before&the&usual&signal-processing&and&data-handling&techniques&can&be&applied.&The&amplification&is&usually&achieved&with&an&electron&multiplier&mounted&within&the&mass&spectrometer&vacuum&envelope.&Multipliers&provide&a&signal&gain&in&the&range&104&to&106.&The&current&flowing&out&from&them&is&still&extremely&small&and&is&usually&further&amplified&by&high-input&impedance&amplifiers&called&electrometers.7.1.&Discrete&dynode&electron&multiplier.The&detector&consists&of&&a&series&of&electrodes&(dynodes)&arranged&close&to&each&other,&with&potential&is&about&200&V&between&plates&(each&stage&is&more&positive&than&previous&one).&The&dynodes&have&Cu/Be&surfaces&with&good&emissive&properties.&Ions&strike&the&first&plate,&causing&electron&emission.&The&electrons&skip&down&the&plate,&causing&a&cascade&of&further&emission&from&each&subsequent&stage.&Typically&each&stage&contributes&a&factor&of&2&or&3&to&the&gain,&and&so&about&16&stages&are&needed&(Fig.33).Figure&33.&Schematic&diagram&of&the&Discrete&Dynode&Electron&Multiplier.&Characteristics:*&Gain&about&106&can&be&achieved&for&multiplier&in&a&good&conditions*&The&efficiency&falls&off&as&the&surfaces&become&dirty&due&to&contamination&by&water&vapour,&and&the&gain&falls&dramatically&after&a&short&exposure&to&the&atmosphere.*&Procedure&to&rejuvenate&the&surfaces&is&straightforward.7.2.&Continues&dynode&electron&multiplier.A&high&negative&voltage,&1800&V&typically.&Os&applied&across&a&glass&trumpet,&whose&inner&surface&is&coated&with&tin&oxide.&The&coating&serves&both&as&the&emitting&surface&and&as&the&potential&dropper.&It&is&equivalent&to&a&continuous&thin-film&metal&oxide&resistor,&and&a&potential&gradient&is&developed&down&the&envelope.&The&incoming&ion&hits&the&inside&surface&of&the&electron&multiplier’s&horn,&where&it&liberating&electrons&from&the&surface.&These&electrons&cascade&down&the&horn,&ejecting&more&electrons&with&every&impact&(Fig.&34)Figure&34.&Continues&dynode&electron&multiplier&detector.&Characteristics:&*&A&gain&of&105&to&106&is&typically&achieved&with&1800&to&2000&V.*&finite&supply&of&electrons.&One&may&operate&the&electron&multiplier&at&high&sensitivity&for&a&relatively&short&&alternatively,&one&may&use&the&same&electron&multiplier&at&a&lower&voltage&value&fore&a&much&longer&time.*&extremely&susceptible&to&surface&contamination.*&not&sensitive&to&water&vapour,&air,&or&the&most&common&CI&reagent&gases*&can&be&removed&for&cleaning&or&exposed&to&the&atmosphere&during&system&shutdown&without&sudden&loss&of&sensitivity.7.3&Faraday&cap.Ions&strike&the&collector&electrode&which&is&inclined&with&respect&to&the&path&of&the&ions&so&that&particles&striking&or&leaving&the&electrode&are&reflected&away&from&the&entrance&to&the&cup.&Collector&electrode&and&surrounding&cage&are&connected&to&ground&potential.&This&act&to&neutralise&the&positive&ions&striking&the&plate&by&drawing&electrons&from&ground&through&a&resistor.&The&resulting&potential&drop&across&the&resistor&is&amplified&via&an&amplifier&(fig.&35).Figure&35.&Schematic&representation&of&the&Faraday-sup&detector.&Characteristics:*&no&internal&amplification,&generates&very&low&signal,&poor&for&trace&analysis.,&less&sensitive&than&electron&multiplier.*&simple&and&inexpensive.7.4.&Other&Types&of&Detectors.*&Photographic&plates&coated&with&silver&bromide&emulsion.&Can&be&used&when&simultaneous&observation&of&a&wide&range&of&m/z&values&is&necessary.*&Scintillation-type&detector&consists&of&a&crystalline&phosphor&dispersed&on&a&thin&aluminium&sheet&which&is&mounted&on&the&widow&of&a&photomultiplier&tube.&The&electrons&produced&when&the&positive&ions&strike&a&cathode,&impinge&upon&the&phosphor.&Thus&producing&a&scintillation&which&is&detected&by&the&photomultiplier&tube.8.&Data&recording.Computers&have&become&almost&essential&for&full&productivity&from&mass&spectrometers,&particularly&GC/MS&systems.&A&typical&on-line&mass&spectrometer-computer&system&includes&interface&electronics,&disk&storage&of&data,&and&a&visual&display&unit&for&presentation&of&mass&spectra&and&chromatograms,&in&addition&to&facilities&for&making&permanent&copies&of&the&graphs.&The&main&features&are&illustrated&in&Fig.&36.Figure&36.&The&possible&communications&of&data&system&to&other&functions.There&are&number&of&instrument&variables&that&the&computer&controls:*&source&*&accelerating&*&scan&*&magnet&field&strength&or&quadrupole&voltage&;*&integration&of&chromatographic&*&background&subtraction&in&GC/MS&(for&column&bleed&and&instrumental&contamination);*&control&of&the&voltages&to&focus&only&in&a&limited,&selected&number&of&ions,&instead&of&scanning&complete&mass&spectra&(quadrupole);*&store&spectra&and&information&on&a&*&perform&quick&search&of&files&from&the&library&((200&000&entries)&that&closely&match&the&spectrum&being&investigated.8.1&Computer&control&of&mass&spectrometer&and&data&acquisition.The&electron&multiplier&sends&out&a&continually&varying&electrical&current&or&voltage&called&analogue&signal&.&If&it&is&to&be&handled&by&the&computer&it&needs&to&broken&up&into&segments,&or&digitised&(Fig.&37)Figure&37.&Analogue&and&digital&forms&of&a&mass-spectrometer.&This&is&achieved&by&the&use&of&an&analogue-to&digital&converter&(ADC)&which&samples&the&analogue&voltage&at&very&precise&regular&time&intervals.&A&real&digitised&spectrum&might&include&&thousands&of&such&voltage&readings&that&will&have&some&bearing&on&the&processing&time&of&the&computer.The&amount&of&information&&being&fed&into&the&computer&might&be&reduced&by&either&reducing&the&number&of&sampling&points&(Fig.&38)&or&rejecting&voltage&reading&below&a&certain&threshold&value&by&assuming&that&these&points&arise&from&the&baseline&noise&in&the&signal&(Fig.&39).&Figure&38.&Peaks&from&a&mass&spectrum&in&analogue&&and&digital&forms,&obtained&by&using&different&sampling&rate.&Figure&39.&Digital&signal&from&a&mass-spectrometer&output,&showing&a&threshold&setting.The&singlet&peak&appears&to&be&distorted&and&the&doublets&appears&to&be&a&very&distorted&singlet.&Therefore&too&low&sampling&rate&must&be&avoided.&Rates&of&10-20&point&per&amu&ensure&adequate&mass&measurement.8.2.&&Scan&Acquisition&principles&of&HP&quadrupole&mass&spectrometer.There&are&two&modes&of&acquisition&available&on&the&Chemstation:&Scan&and&Selected&Ion&Monitoring&(SIM).Scan&mode&scans&all&the&masses&(within&the&range&specified)&generated&during&the&acquisition&process&rather&than&only&a&selected&number&of&masses.&It&is&the&mode&to&use&when&analysingunknown&compounds.&It&is&also&the&mode&to&use&if&library&searches&to&be&applied.Mass&spectral&data&has&three&dimensions:&retention&time,&mass,&and&abundance(Fig.&40).&The&mass&axis&goes&into&the&page,&the&retention&time&axis&goes&across&the&page,&and&the&abundance&axis&goes&up&the&page.Figure&40.&Mass&Spectral&Data&in&Three&Dimensions.If&we&add&the&abundance’s&at&each&instant&in&time&and&repeat&for&every&scan,&we&will&get&the&picture&shown&on&Figure&41&a,&b.a)&b)Figure&41.&a)&Spectrum&vs.&Retention&time&b)&Total&Ion&Chromatogram&-abundance&versus&time.&Figure&41&b&is&one&way&of&looking&at&the&data&from&the&Chemstation.&This&is&a&plot&of&abundance&versus&time&from&a&single&injection.&There&is&no&information&about&what&masses&were&detected.&At&each&point&in&the&chromatogram,&the&abundance’s&of&all&the&ions&detected&were&added&together&to&create&the&"total&ion&abundance"&at&that&point&in&time.&Thus,&this&is&a&"total&ion&chromatogram".Mass&peak&detection.At&the&beginning&of&a&scan,&the&quadrupole&mass&filter&is&ready&and&waiting&at&the&top&of&the&specified&scan&range&.&To&obtain&a&mass&spectrum,&the&mass&filter&moves&in&regular,&discrete&step&of&0.1&amu&from&the&top&of&the&scan&range&to&the&bottom.&The&number&of&times&the&abundance&of&each&mass&is&measured&or&sampled&during&a&scan&is&the&sampling&rate.&The&total&time&needed&for&one&scan&acquisition&includes&the&time&the&quadrupole&mass&filter&steps&from&high&to&low&mass.&The&scan&range&and&sampling&rate&affect&this&time.&When&the&low&end&of&the&scan&range&is&reached,&the&quadrupole&mass&filter&is&reset&to&the&upper&limit&mass&in&preparation&for&the&next&scan.&The&reset&time&is&less&than&the&time&required&performing&other&operation,&so&the&total&cycle&time&is&not&affected&by&this&reset&time&(Fig.&42).Figure&42.&The&digital&scanning&process.&At&the&same&time&the&data&are&transferred&to&the&Chemstation.&The&Chemstation&then&stores&the&data&file&on&the&disk,&updates&the&screen&display&with&any&real&time&chromatograms&that&are&to&be&drawn,&and&other&information.Scan&cycle&time&and&scan&times&are&determined&when&one&chooses&the&mass&spectrometer&parameters&in&data&acquisition.&It&is&important&to&select&these&skilfully,&remembering&about&trading&chromatographic&quality&for&mass&spectral&quality.&The&scan&time&depends&on&three&factors:1)&The&number&of&samples&taken&at&each&step&(sampling&rate).2)&&The&mass&range.3)&&The&number&of&m/z&values&recorded&during&the&scan.User&selected&mass&spectrometer&parameters&determine&the&magnitude&of&each&of&these.&The&system&gives&an&"estimate"&of&the&scan/second&that&will&be&achieved.&For&good&qualitative&data,&a&goal&should&be&10&scans&across&a&peak.&For&good&quantitative&data,&a&goal&-&10&to&20&scans&across&a&peak.In&choosing&the&mass&spectrometer&parameters&in&acquisition,&there&is&a&trade&off&between&mass&spectral&quality&and&chromatographic&quality.&If&you&scan&quickly,&each&chromatographic&peak&will&have&many&spectra&taken&as&it&elutes.&This&makes&reconstruction&of&the&chromatogram&possible,&but&the&quality&of&the&spectra&may&be&poor&due&to&the&speed&at&which&they&were&taken.&If&you&scan&slowly,&the&quality&of&the&mass&spectra&data&may&be&good,&but&the&chromatographic&peak&shape&is&poorly&defined.The&second&mode&of&acquisition&available&on&quadrupole&Chemstation&calls&Selected&Ion&Monitoring&(SIM).&SIM&allows&the&mass&spectrometer&to&detect&specific&compounds&with&very&high&sensitivity.&In&SIM&mode,&the&instrument&is&set&to&acquire&data&at&masses&of&interest&instead&of&stepping&the&mass&filter&over&a&wide&range&of&masses.&Because&the&mass&spectrometer&collects&data&at&only&the&masses&of&interest,&it&responds&only&to&those&compounds&that&possess&the&selected&mass&fragments.&In&essence,&the&instrument&is&focused&on&only&the&compounds&of&interest.&Also,&because&only&a&few&masses&are&monitored,&much&more&time&may&be&spent&looking&at&these&masses,&with&the&attendant&increase&in&sensitivity,&accuracy,&and&precision.During&a&typical&scan&run,&each&mass&is&measured&for&approximately&100&usec.&In&contrast,&each&mass&in&SIM&mode&is&typically&100&msec.&Since&signal/noise&is&proportional&to&the&square&root&of&the&measurement&time,&it&follows&that&SIM&mode&will&be&roughly&30&times&more&sensitive&than&scan&&mode.&In&practice,&improvements&of&20-100&are&possible,&depending&on&instrument,&background,&sample&matrix,&etc.&(Fig.&30).9.&&Tuning.Before&any&data&are&required&it&is&necessary&that&the&mass&spectrometer&be&tuned&satisfactorily.&The&mass&spectrometer&should&be&tuned&for&maximum&sensitivity&consistent&with&good&mass&spectra&peak&shapes&high-to-low&mass&ratio,&required&mass&separation&(resolution).Tuning&involves&adjusting&a&number&of&mass&spectrometer&parameters.&Some&parameters&are&purely&electronic&and&affect&only&the&way&the&electronics&process&the&signal.&Other&parameters&affect&voltage&settings&or&currents&to&parts&in&the&MSD’s&ion&source,&mass&filter,&and&detector.Reference&compounds&for&mass&calibration&should&give&spectra&with&ions&spaced&evenly&across&the&mass&range&of&interest.&The&range&of&ion&intensities&should&be&small&enough&to&be&used&satisfactorily&in&a&single&setting&of&the&mass&spectrometer&single&gain.&The&compound&should&be&easily&introduced&onto&the&ion&source.&Nowadays&reference&inlet&&usually&take&the&form&of&a
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