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Fragmentation in EI-MS – A Summary

Fragmentation in EI-MS – A Summary. a -Cleavage a -Bonds adjacent to heteroatoms (such as N, O, S) are cleaved preferentially, because the ensuing charge is stabilized by the heteroatom. Primary fragmentation reaction;

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Fragmentation in EI-MS – A Summary

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  1. Fragmentation in EI-MS – A Summary • a-Cleavage • a-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 b-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 CH2.

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

  6. Ionization • The deciding criteria are often the following: • Physical state of the sample • Volatility and thermal stability of the sample • 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 Organic Structural Spectroscopy by Lambert, Shurvell, Lightner

  8. CI-MS

  9. reactive gasisobutane reactive gasmethane

  10. DI & MALDI

  11. translational excitation bymomentum transfer 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).

  12. SI-MS

  13. 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.

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

  15. Different Methods for Ion Separation

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