Lecture 11 GC-MS Ionization. GC-MS GC Computer MS.

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Presentation transcript:

Lecture 11 GC-MS Ionization

GC-MS GC Computer MS

What is MS? GC Computer MS

What is MS? MS Gas phase Ions GC

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

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

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

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

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

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

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

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

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

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)

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

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