Transcript – A Summary Fragmentation in EI-MS

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;
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 (oniumion);
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;
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
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.
EI-MS
Organic Structural Spectroscopy by Lambert, Shurvell, Lightner
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
Advantages & Disadvantages of EI-MS
Organic Structural Spectroscopy by Lambert, Shurvell, Lightner
CI-MS
reactive gas
isobutane
reactive gas
methane
DI & MALDI
translational excitation by
momentum 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).
SI-MS
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.
Summary of Ionization Methods
EI
pro: very good reproducibility, high resolution MS
contra: 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
Different Methods for Ion Separation