Molecular mass spectroscopy
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Molecular Mass Spectroscopy. Molecular structure Composition of mixtures Molecular mass spectra Ion Source Mass Spectrometers Applications. Molecular Mass Spectra. Removal of electron by electron bombardment In vapor phase Charged species is the molecular ion

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Molecular Mass Spectroscopy

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Molecular mass spectroscopy

Molecular Mass Spectroscopy

  • Molecular structure

  • Composition of mixtures

  • Molecular mass spectra

  • Ion Source

  • Mass Spectrometers

  • Applications


Molecular mass spectra

Molecular Mass Spectra

  • 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


Ion sources

Ion Sources

  • 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


Electron impact source

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

        • KE independent of mass

        • Velocity varies with mass

        • KE=0.5mv2


Electron impact spectra

Electron Impact Spectra

  • 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

        • 1.12E-17 J*6.02E23 =6.7E6 J/mol

    • Bond energy 200 to 600 kJ/mol


Electron impact spectra1

Electron Impact Spectra


Electron impact spectra2

Electron Impact Spectra


Electron impact spectra3

Electron Impact Spectra

  • 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


Isotopics

Isotopics

  • Isotopic variation can impact spectra


Chemical ionization

Chemical Ionization

  • 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

    • CH4+ and CH3+ (about 90%), CH2+


Chemical ionization1

Chemical Ionization

  • Produces ions that are 1 proton more or 1 proton less than molecule

    • Transfer of C2H5+ give M+29 peak

  • Field Ionization

    • Large electric field

      • 1E8 V/cm

        • Mainly produces M and M+1 peaks


Comparison of spectra

Comparison of spectra

  • a-electron impact

  • b-field ionization

  • c-desorption


Large molecule desorption

Large molecule desorption

  • Solid or liquid samples directly energized into gas phase

    • Molecular or protonated ion

  • 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


Electrospray ionization

Electrospray Ionization

  • 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+, M2+


Electrospray ms spectra

Electrospray MS spectra


Fast atom bombardment

Fast Atom Bombardment

  • Samples in glycerol

  • Bombarded by Xe or Ar atoms

    • Several keV

  • Atoms and ions sputtered from surface

  • Production of fragments


Mass spectrometers

Mass Spectrometers


Magnetic sector

Magnetic sector


Ion trap analyzer

Ion Trap Analyzer

  • 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


Ion trap

Ion Trap


Applications

Applications

  • 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)+ CO2H

    • series (M-14)+, (M-28)+, (M-42)+..sequential CH2

  • loss in alkanes

    • Isotopic peaks can indicate presence of certain atoms

      • Cl, Br, S, Si

        • Isotopic ratios can suggest plausible molecules from M+,

  • Comparison with library spectra


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