仪器分析英文原版教案(16)

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1、MassSpectrometry(Chapter20):BasedonionizationofgasphasemoleculefollowedbyanalysisofthemassesoftheionsproducedTheMassSpectrum:Graphofionintensityversusmass-to-chargeratio(m/z)(unitsdaltons,Da)Fig20-1molecularionpeak(M+)m/zcorrespondstoMWofsingly-chargedmoleculefragmentpeakm/zlessthanMWofsingly-c

2、hargedmoleculebasepeakmostintensem/zCEM333page18.1InstrumentComponents:Fig20-10¥sampleintroductionsystem-vaporizesample¥ionsource-ionizesanalytegasmolecules¥massanalyzer-separatesionsaccordingtom/z¥detector-countsions¥vacuumsystem-reducescollisionsbetweenionsandgasmoleculesCEM333page18.2Ionsour

3、ces:CEM333page18.3Hardionsourcesleaveexcessenergyinmolecule-extensivefragmentationSoftionsourceslittleexcessenergyinmolecule-reducedfragmentationFig20-2CEM333page18.4GasPhaseIonSources:(A)ElectronImpact(EI)IonSource:Electronbombardmentofgas/vapormolecules-+--4M+e(~70eV)®M+2e(about10%ionized)Fig

4、20-3Electronenergy~70eV1eVº1.6x10-19C´1V(1V=1J×C-1)-19=1.6x10J-1=96.486kJ×molCEM333page18.5EISpectra:¥hardsource(incidentenergy70eVÈthanchemicalbond)¥moleculeselectronically,vibrationallyandrotationallyexcited¥extensivefragmentationÞfragmentions¥basepeakm/zÇM+¥complexspectra-helpsidentification

5、-poorformeasuringMWofcompoundCEM333page18.6Fragmentationpatterns(Fig20-4):CEM333page18.7Whataboutpeaksatgreaterm/zthanM+?Twosources:¥IsotopePeaks-samechemicalformulabutdifferentmasses12C1H35Clm=842213C1H35Clm=852212C1H35Cl37Clm=862heightsvarywithabundance13C1H35Cl37Clm=87212137CH2Cl2m=8813Cis1.

6、1%12C,37Clis32.5%35Cl¥CollisionProductPeaks-onlycommonpeakisprotontransfertogive(M+1)+peak(increaseswithincreasingpressure)AdvantagesofEI:¥highioncurrents-sensitive¥fragmentationaidsidentificationDisadvantagesofEI:¥weakorabsentM+peakinhibitsdeterminationofMW¥moleculesmustbevaporized(MW<103Da)¥m

7、oleculesmustbethermallystableduringvaporizationCEM333page18.8(B)ChemicalIonization:¥ManymodernMSinstrumentscanperformchemicalionizationinadditiontoEIEIionizationinexcess(analyte10-100ppm)ofreactantgasMostcommonreactantgasismethane