《仪器分析 Instrumental Analysis》课程教学资源（教材讲义）18_Mass Spectrometry

Mass Spectrometry (Chapter 20): Based on ionization of gas phase molecule followed by analysis of the masses of the ions produced The Mass Spectrum: Graph of ion intensity versus mass-to-charge ratio (m/z)(units daltons,Da) Fig 20-1 91 100 Base peak 80 Mw=106 60H 106 Molecular 40 ion peak YCH2CH, 20 04 0 102030405060708090100110 m/ molecular ion peak (M+)m/z corresponds to MW of singly- charged molecule fragment peak m/z less than MW of singly-charged molecule base peak most intense m/z CEM 333 page 18.1

Mass Spectrometry (Chapter 20): Based on ionization of gas phase molecule followed by analysis of the masses of the ions produced The Mass Spectrum: Graph of ion intensity versus mass-to-charge ratio (m/z) (units daltons, Da) Fig 20-1 molecular ion peak (M+) m/z corresponds to MW of singly￾charged molecule fragment peak m/z less than MW of singly-charged molecule base peak most intense m/z CEM 333 page 18.1

Instrument Components: 10-510-81 Readout Fig20-10 sample introduction system-vaporize sample ion source -ionizes analyte gas molecules mass analyzer-separates ions according to m/z detector counts ions vacuum system-reduces collisions between ions and gas molecules CEM 333 page 18.2

Instrument Components: Fig 20-10 • sample introduction system - vaporize sample • ion source - ionizes analyte gas molecules • mass analyzer - separates ions according to m/z • detector - counts ions • vacuum system - reduces collisions between ions and gas molecules CEM 333 page 18.2

Ion sources: TABLE 20-1 Ion Sources for Molecular Mass Spectrometry Basic Type Name and Acronym lonizing Agent Gas phase Electron impact(ED) Energetic electrons Chemical ionization(CI Reagent gaseous ions Field ionization(Fl） High-potentialelectrode Desorption Field desorption (FD) High-potentiaeeoe Electrospray ionization(ESI) High electrical field Matrix-assisted desorption/ionization (MALDI) Laser beam Plasma desorption(PD) Fission fragments from252Cf Fast atom bombardment(FAB) Energetic atomic beam Secondary ion mass spectrometry(SIMS) Energetic beam of ions Thermospray ionization(TS) High temperature CEM333 page 18.3

Ion sources: CEM 333 page 18.3

Hard ion sources leave excess energy in molecule-extensive fragmentation Soft ion sources little excess energy in molecule-reduced fragmentation Fig 20-2 CH:(CH2)sCH2OH Hard Ionization 112 20 120 T140 160 141 CHj(CH2)gCH2OH (M-OH)* Soft CHCH,CH时 69 Ionization (M) 84 100 140 160 6 CEM 333 page 18.4

Hard ion sources leave excess energy in molecule - extensive fragmentation Soft ion sources little excess energy in molecule - reduced fragmentation Fig 20-2 CEM 333 page 18.4

Gas Phase Ion Sources: (A)Electron Impact (ED)Ion Source: Electron bombardment of gas/vapor molecules M+e(~70eV)→Mt+2e (about 10%ionized) Fig 20-3 Shield Heater Electron slit Pirst acce Molecular Filamen I slit leak 三( To mass analyzer Repelle onizing region Electron lon accelerating beam Anode Electron energy ~70 eV 1eV=1.6x10-19Cx1V (1V=1J.C-1 =1.6x10-19J =96.486 kJ.moli-1 CEM 333 page 18.5

Gas Phase Ion Sources: (A) Electron Impact (EI) Ion Source: Electron bombardment of gas/vapor molecules M + e - (~ 70 eV) ® M+ + 2e- (about 10-4 % ionized) Fig 20-3 Electron energy ~70 eV 1eV º1.6x10-19 C´1V (1V = 1 J×C -1 ) =1.6x10-19 J = 96.486 kJ×mol-1 CEM 333 page 18.5

EI Spectra: hard source (incident energy 70 ev>>than chemical bond) molecules electronically,vibrationally and rotationally excited ·extensive fragmentation→fragment ions TABLE 20-2 Some Typical Reactions in an Electron-Impact Source Fragmentation ABCD+e→ABCD++2e ABCD+→A++BCD A+BCD*→BC++D →CD+AB*一CR+B AW +CD Rearrangement followed by fragmentation Colis followed by ABCD++ABCD→(ABCD)2+→BCD'+ABCDA+ ·base peak m/zMt ·complex spectra -helps identification poor for measuring MW of compound CEM 333 page 18.6

EI Spectra: • hard source (incident energy 70 eV » than chemical bond) • molecules electronically, vibrationally and rotationally excited • extensive fragmentation Þ fragment ions • base peak m/z « M+ • complex spectra - helps identification - poor for measuring MW of compound CEM 333 page 18.6

Fragmentation patterns(Fig 20-4) methylene chloride 100 49 mw=84 84 80人 60 40H 204 20 30 40 5060 90100110 (a) I-pentanol mw=88 |M-(H2O and CH2=CH2) (CH2=OH) Base peak M-(H2O and CH3) M-(H20) Molecula ion peak 40 50 60 80 9010011 (b) CEM 333 page 18.7

Fragmentation patterns (Fig 20-4): CEM 333 page 18.7

What about peaks at greater m/z than M+? Two sources: Isotope Peaks-same chemical formula but different masses 12clH235Cl2m=84 13clH235Cl2m=85 12CH235C13CIm heights vary with abundance 13CH235c37c1m=87 12CH237CL2m=88 13Cis1.1%12C,37C1is32.5%35C1 Collision Product Peaks -only common peak is proton transfer to give (M+1)+peak (increases with increasing pressure) Advantages of EI high ion currents-sensitive fragmentation aids identification Disadvantages of EI: weak or absent M+peak inhibits determination of MW molecules must be vaporized (MW 103 Da) molecules must be thermally stable during vaporization CEM 333 page 18.8

What about peaks at greater m/z than M+? Two sources: • Isotope Peaks - same chemical formula but different masses 12C 1H2 35Cl2 m = 84 13C 1H2 35Cl2 m = 85 12C 1H2 35Cl37Cl m = 86 13C 1H2 35Cl37Cl m = 87 12C 1H2 37Cl2 m = 88 heights vary with abundance 13C is 1.1 % 12C, 37Cl is 32.5 % 35Cl • Collision Product Peaks - only common peak is proton transfer to give (M+1)+ peak (increases with increasing pressure) Advantages of EI: • high ion currents - sensitive • fragmentation aids identification Disadvantages of EI: • weak or absent M+ peak inhibits determination of MW • molecules must be vaporized (MW < 103 Da) • molecules must be thermally stable during vaporization CEM 333 page 18.8

(B)Chemical Ionization: Many modern MS instruments can perform chemical ionization in addition to El EI ionization in excess (analyte 10-100 ppm)of reactant gas Most common reactant gas is methane EI ionization of methane produces CH4++CH4→CH5++CH3 CH3++CH4→C2H5++H2 These ions react with analyte: CHs++A>CH+AH+proton transfer C2Hs+A>C2H4+AH+proton transfer C2Hs+A>C2H6+(A-H)*hydride elimination analyte most common ions (M+1)+and (M-1)+ sometimes (M+17)+(addition of CHs+)or (M+29)+(addition of C2H5+) CEM 333 page 18.9

(B) Chemical Ionization: • Many modern MS instruments can perform chemical ionization in addition to EI EI ionization in excess (analyte 10-100 ppm) of reactant gas Most common reactant gas is methane EI ionization of methane produces CH4 + + CH4 ® CH5 + + CH3 CH3 + + CH4 ® C2H5 + + H2 These ions react with analyte: CH5 + + A ® CH4 + AH+ proton transfer C2H5 + + A ® C2H4 + AH+ proton transfer C2H5 + + A ® C2H6 + (A - H)+ hydride elimination analyte • most common ions (M+1)+ and (M-1)+ • sometimes (M+17)+ (addition of CH5 +) or (M+29)+ (addition of C2H5 +) CEM 333 page 18.9

Desorption/Ionization Sources: Applicable to non-volatile (>105 Da)or non-stable analytes Energy applied to analyte causing desorption and ionization Exact mechanisms still under investigation (A)Electrospray Ionization(ESI): Explosion of charged droplets containing analytes solution analyte pumped through charged (1-5 kV) capillary small droplets become charged solvent evaporates,drop shrinks,surface charge density increases charge density reduced by expulsion of charged analyte molecules ("Coulomb explosion") Soft ionization-little fragmentation Easily adapted to FIA,capillary EP and HPLC Quadrupole Cylindrical mass electrode spectrometer Capillary ample Skimme First Second pumping pumping stag Fig 20-8 CEM 333 page 18.10

Desorption/Ionization Sources: Applicable to non-volatile (>105 Da) or non-stable analytes Energy applied to analyte causing desorption and ionization Exact mechanisms still under investigation (A) Electrospray Ionization (ESI): • Explosion of charged droplets containing analytes - solution analyte pumped through charged (1-5 kV) capillary - small droplets become charged - solvent evaporates, drop shrinks, surface charge density increases - charge density reduced by expulsion of charged analyte molecules ("Coulomb explosion") Soft ionization - little fragmentation Easily adapted to FIA, capillary EP and HPLC Fig 20-8 CEM 333 page 18.10