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Lecture 11

Lecture 11. GC-MS Ionization. GC-MS. GC. MS. Computer. What is MS?. GC. MS. Computer. GC. What is MS?. MS. Gas phase. Ions. GC-MS Ionization Sources. Electron Impact Ionization (EI). Positive Chemical Ionization (PCI). Negative Chemical Ionization (NCI).

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Lecture 11

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  1. Lecture 11 GC-MS Ionization

  2. GC-MS GC MS Computer

  3. What is MS? GC MS Computer

  4. GC What is MS? MS Gas phase Ions

  5. GC-MS Ionization Sources Electron Impact Ionization (EI) Positive Chemical Ionization (PCI) Negative Chemical Ionization (NCI)

  6. Electron Impact Ionization(EI) ‘Hard’ ionization technique 70 eV AB + e- AB.+ + 2e- Ions formed here

  7. Electron Impact Ionization(EI) ‘Hard’ ionization technique 70 eV AB + e- AB.+ + 2e- Extensive Fragmentation Limited molecular ion - molecular weight Structural information High Vacuum (10-5 torr) To limit reactions between molecules Very reproducible Libraries for comparison

  8. Chemical Ionization ‘Soft’ ionization technique Introduce reagent gas (102 torr) 70 eV NH3 + e- NH3.+ + 2e- NH3.+ + NH3 NH4+ + NH2.

  9. Positive Chemical Ionization(PCI) ‘Soft’ ionization technique Introduce reagent gas (102 torr) 70 eV NH3 + e- NH3.+ + 2e- NH3.+ + NH3 NH4+ + NH2. Proton transfer reaction NH4+ + analyte [analyte-H]+ + NH3 Strong molecular ion

  10. Positive Chemical Ionization(PCI) Compounds must be able to accept a proton Amines, alcohols Proton transfer reaction NH4+ + analyte [analyte-H]+ + NH3 Strong molecular ion

  11. Positive Chemical Ionization(PCI) Jurgen H. Gross, Mass Spectrometry – A Textbook, Chapter 7

  12. Negative Chemical Ionization(NCI) ‘Soft’ ionization technique Introduce reagent gas (102 torr) 70 eV NH3 + e- NH3.+ + 2e- NH3.+ + NH3 NH4+ + NH2.

  13. Negative Chemical Ionization(NCI) ‘Soft’ ionization technique Introduce reagent gas (102 torr) 70 eV Thermal Electrons ~ 0.1 eV NH3 + e- NH3.+ + 2e- NH3.+ + NH3 NH4+ + NH2. Electron capture reaction e- + analyte analyte.- Strong molecular ion

  14. Negative Chemical Ionization(NCI) Compounds must be able to accept an electron Electronegative atoms – halogens (F, Cl, Br, I) Conjugated systems – phenyl rings Electron capture reaction e- + analyte analyte.- Strong molecular ion

  15. Negative Chemical Ionization(NCI) Jurgen H. Gross, Mass Spectrometry – A Textbook, Chapter 7

  16. Compare/Contrast? Selectivity NCI > PCI > EI v. selective universal Sensitivity Game of S:N NCI > PCI > EI Selectivity Sensitivity a Structural Information EI > PCI = NCI Difficult to interpret fragmentation

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