1. Lecture 11 GC-MS Ionization

2. GC-MS GC Computer MS

3. What is MS? GC Computer MS

4. What is MS? MS Gas phase Ions GC

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 AB + e - AB.+ + 2e - 70 eV Ions formed here

7. Electron Impact Ionization (EI) ‘Hard’ ionization technique AB + e - AB.+ + 2e - 70 eV 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 NH 3 + e - NH 3.+ + 2e - 70 eV Introduce reagent gas (10 2 torr) NH 3.+ + NH 3 NH 4 + + NH 2.

9. Positive Chemical Ionization (PCI) ‘Soft’ ionization technique NH 3 + e - NH 3.+ + 2e - 70 eV Introduce reagent gas (10 2 torr) NH 3.+ + NH 3 NH 4 + + NH 2. NH 4 + + analyte [analyte-H] + + NH 3 Proton transfer reaction Strong molecular ion

10. Compounds must be able to accept a proton Amines, alcohols NH 4 + + analyte [analyte-H] + + NH 3 Proton transfer reaction Strong molecular ion Positive Chemical Ionization (PCI)

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

12. ‘Soft’ ionization technique NH 3 + e - NH 3.+ + 2e - 70 eV NH 3.+ + NH 3 NH 4 + + NH 2. Introduce reagent gas (10 2 torr) Negative Chemical Ionization (NCI)

13. ‘Soft’ ionization technique NH 3 + e - NH 3.+ + 2e - 70 eV Introduce reagent gas (10 2 torr) Thermal Electrons ~ 0.1 eV e - + analyte analyte.- Electron capture reaction NH 3.+ + NH 3 NH 4 + + NH 2. Strong molecular ion

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

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

17. Compare/Contrast? Selectivity NCI > PCI > EI universalv. selective Sensitivity NCI > PCI > EI Game of S:N Sensitivity  Selectivity Structural Information EI > PCI = NCI Difficult to interpret fragmentation