Ionization Sources

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Ionization Sources
2013-02-25 23:11:22

Mass Spec
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  1. EI - Electron Impact Ionization?
    uses el- to bombard analyte molecules to produce molecular ions and fragments
  2. MI - Molecular Ionization?
    analyte molecule interacts w electron, the energy absorbed will remove a non-bonding electron.
  3. Hard Ionization Technique?
    • Removal of an electron from the molecule.
    • Lots of fragmentations
  4. Soft Ionization Technique?
    • transfer of charge from a matrix or solvent.
    • less fragmentation, more parent or molecular ion.
  5. 3 ionization methods
    Gas-phase, Desorption, and spray
  6. Gas-Phase Ioniz Methods
  7. CI - Chemical Ionization?
    ionization source s filled w reagent gas (methane, isobutane, ammonia). reag gas is converted to ions in rxn w energetuc el- therefore reacts w analytye to produce protonated molecules.
  8. Desorption Ioniz Method
    • MALDI
    • FAB
  9. MALDI?
    Matrix-assisted Laser Desorption Methods. Analyte suspended in solvent, solvent is evap creating solid solution of matrix and analyte. Laser causes sublimation of matrix molecules.
  10. FAB?
    Fast Atom Bombardment - energy stream of atoms of noble gases focused onto analyte molecules. charge transfer occurs. (for proteins and peptides, thermally unstable (labile) and high MW)
  11. Spray Ioniz Methods?
  12. ESI?
    Electrospray Ionization. Soft. solvent & analyte pass thru metallic capillary - create electrically charged spray, reduce droplet size, liberate desolvated ion.
  13. APCI?
    Atmospheric Pressure Chem Ioniz - sample introduced to nebulizer, hot N2 gas forces sample through, thin fog is formed, sample molecules are desorbed and ionized with corona discharge.
  14. ADV/DIS of CI
    • ADV - parent ion present, selective & sensitive
    • DIS - sample must be volatile, not good for high MW or therm unstable, dirty, needs high pump, no frag, not universal
  15. ADV/DIS of EI?
    • ADV - frag, use w insoluble samples, interface w GC, non-polar samples
    • DIS - less stable molecules = no parent identification, requires volatile sample, not goo for high MW or thrm unstbl
  16. mass chromatogram vs mass spec?
    • mass chrom - time of x-axis
    • mass spec - m/z on x-axis
  17. why are FAB & MALDI not ideal for quant analysis?
    poor reproducibility (depending on sample prep)
  18. Mass Accuracy?
    difference between theoretical and observed mass. (milliamu)