1. Fragmentation in EI-MS – A Summary  -Cleavage  -Bonds adjacent to heteroatoms (such as N, O, S) are cleaved preferentially, because the ensuing charge is stabilized by the heteroatom. Primary fragmentation reaction; Occurs only once in sequential fragmentation reactions (no homolytic bond cleavage in the generated cation); Analogous to Norrish Type 1 reaction in photochemistry;

2. Onium Reaction Neutral alkenes are formed by an XH elimination-like reaction (X could be N, O, S, P). Secondary fragmentation reaction of an cation (onium- ion); McLafferty Rearrangement Can be described as a  -cleavage with an H-atom migration (usually via a 6-membered transitions state). Primary and secondary fragmentation reaction; Analogous to Norrish Type 2 reaction in photochemistry;

3. Benzyl and Allyl Cleavages and Fragmentation of Aromatic Hydrocarbons Double bond systems stabilize charges on a benzylic or allylic carbon in a similar way as heteroatom. Benzyl and Allyl Cleavages are Primary Fragmentation Reactions; m/z = 91/65 and 77/51 are typical fragment pairs of monosubstituted alkyl aromatics; the formation of 77 is less favoured; tropylium ion

4. Fragmentation Reactions in Hydrocarbons The most abundant fragments have 3-4 carbon atoms (m/z between 40 and 60) and the abundance of ions of higher mass decreases in an asymptotic like decay; fragment peaks are often 14 Da apart revealing a successive loss of CH 2.

5. Organic Structural Spectroscopy by Lambert, Shurvell, Lightner EI-MS

6. Ionization The deciding criteria are often the following: 1.Physical state of the sample 2.Volatility and thermal stability of the sample 3.Type of information sought Comparison EI, CI, and DI are suitable for high resolution MS; EI works well only for thermally stable and volatile samples; CI, SI, and DI cause much less fragmentation (normally no radicals are formed); DI and SI must be combined with tandem mass detection (MS-MS) to extract more structural information from fragmentation; Organic Structural Spectroscopy by Lambert, Shurvell, Lightner

7. Advantages & Disadvantages of EI-MS

8. CI-MS

10. reactive gas isobutane reactive gas methane

11. DI & MALDI

12. Desorption mechanism in fast atom bombardment ionization (FAB). A- represents a negatively-charged ion and C+ a positively-charged ion (sample for example quaternary ammonium salt). translational excitation by momentum transfer

15. SI-MS

17. Atmopheric Pressure Chemical Ionization (APCI) is a variant on CI that is closely related to electrospray ionization. In a typical APCI source, a plasma is generated by a DC discharge and ion-molecule or electron-molecule reactions occure at atmospheric pressure.

18. Summary of Ionization Methods EI pro: very good reproducibility, high resolution MS contra: only volatile samples, high fragmentation, no negative ions CIpro: selective and soft ionization, positive & negative ions, high resolution MS contra: only volatile samples DIpro: soft ionization, positive & negative ions, high resolution MS contra: tandem mass spectroscopy (MS-MS) is necessary when structural information is desired SIpro: soft ionization, positive & negative ions, contra: no high resolution MS

19. Different Methods for Ion Separation