Transcript Molecular Mass Spectroscopy Surface Characterization
Molecular Mass Spectroscopy
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Molecular structure Composition of mixtures
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Molecular mass spectra Ion Source Mass Spectrometers Applications
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Molecular Mass Spectra
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Removal of electron by electron bombardment
In vapor phase
Charged species is the molecular ion Electron causes excitation and fragmentation
Major product is base peak
Assigned 100% relative abundance Smaller fragments also form
12-2
Ion Sources
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Ion source has profound effect on spectra
Gas phase source
Vaporized then ionized
Desorption source
Conversion of liquid or solid to gas Hard source
Ion in excited state
Fragments produced Soft source
Little fragmentation
Mainly ion of molecule
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Electron-Impact Source Sample as vapor Ionized by beam of electrons
W or Re filament
70 V potential 1E-6 effective
M+e ->M + +2e High potentials in accelerating region
1E3 to 1E4 volts KE of ion in 1000 V
KE=qV=zeV
KE=1*1.6E-19 C*1000 V
1.6E-16 J
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KE independent of mass
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Velocity varies with mass KE=0.5mv
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Electron Impact Spectra
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Energy from e accelerated by 70V
Find in J/mol to compare bond energy KE=eV
1.6E-19 C *70 V=1.12E-17 CV/e -
For a mole
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1.12E-17 J*6.02E23 =6.7E6 J/mol Bond energy 200 to 600 kJ/mol
12-7
Electron Impact Spectra
12-8
Electron Impact Spectra
12-9
Electron Impact Spectra
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Sensitive method Fragments useful in identification Lack of molecular ion peak
M + , difficult to identify specie Molecules must be in vapor phase
Stability issues in vapor phase MW<1000 dalton
12-10
Isotopics
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Isotopic variation can impact spectra
12-11
Chemical Ionization
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Sample ionized by secondary ionization
Reagent gas ionized by electrons, then ionized reagent gas reacts with sample gas
Reagent to sample ratio
1E3 to 1E4 Methane most common reagent gas
CH 4 + and CH 3 + (about 90%), CH 2 +
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Chemical Ionization
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Produces ions that are 1 proton more or 1 proton less than molecule
Transfer of C 2 H 5 + give M+29 peak
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Field Ionization
Large electric field
1E8 V/cm
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Mainly produces M and M+1 peaks
12-13
Comparison of spectra
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a-electron impact b-field ionization c-desorption
12-14
Large molecule desorption
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Solid or liquid samples directly energized into gas phase
Molecular or protonated ion
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Matrix Assisted Laser Desorption/Ionization (MALDI)
Soft Ionization
Sample dissolved in solution containing UV absorber and solvent
Solution evaporated and precipitate formed Pulsed laser used to excite precipitate Molecular ion desorbed from surface of precipitate
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Electrospray Ionization
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Solution pumped through a needle
Needle is at kV potential compared to surrounding electrode
Droplets become charged
Solvent evaporates, droplets shrink and charge density increases Can be combined with a number of methods Useful for large molecules M + , M 2+
12-17
Electrospray MS spectra
12-18
Fast Atom Bombardment
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Samples in glycerol Bombarded by Xe or Ar atoms
Several keV Atoms and ions sputtered from surface Production of fragments
12-19
Mass Spectrometers
12-20
Magnetic sector
12-21
Ion Trap Analyzer
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Variable radio frequency voltage applied to the ring electrode Ions of appropriate m/z circulate in stable orbit scan radio frequency
heavier particles stable
lighter particles collide with ring electrode ejected ions detected by transducer as an ion current
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Ion Trap
12-23
Applications
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Identification of Pure Compounds:
Nominal M+ peak (one m/z resolution) (or (M+1)+ or (M 1)+) gives MW (not EI)
Exact m/z (fractional m/z resolution) can give stoichiometry but not structure (double-focusing instrument)
Fragment peaks give evidence for functional groups (M-15)+ peak methyl
(M-18)+ OH or water (M-45)+ CO 2 H series (M-14)+, (M-28)+, (M-42)+..sequential CH2 loss in alkanes
Isotopic peaks can indicate presence of certain atoms
Cl, Br, S, Si
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Isotopic ratios can suggest plausible molecules from M+, Comparison with library spectra
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