In mass spectrometry (MS), fragmentation is a process where molecules break into smaller pieces (ions) inside the mass spectrometer. This helps in identifying the structure of the molecule because the pattern of fragments can be matched to parts of the molecule. Here’s how it happens, explained:
- Ionization of the molecule:
- First, the molecule is ionized to form a molecular ion (usually by losing or gaining an electron).
- Example: In electron ionization (EI), the molecule loses an electron and becomes a positively charged ion (M⁺•).
- Fragmentation of the molecular ion:
- The molecular ion is often unstable because of the energy added during ionization.
- This instability causes it to break at specific bonds, forming smaller charged fragments.
- The way it breaks depends on the bond strengths and the stability of resulting ions.
- Formation of characteristic fragment ions:
- The fragments themselves are ions, which are detected by the mass spectrometer.
- The mass-to-charge ratios (m/z) of these fragments create a fragmentation pattern, which acts like a fingerprint for that molecule.
Example in simple terms:
Suppose we have ethanol (CH₃CH₂OH). During mass spectrometry:
- Ionization produces CH₃CH₂OH⁺•
- Fragmentation can produce ions like CH₃⁺, CH₂OH⁺, etc.
- The mass spectrometer detects these ions and shows a spectrum.
Key points about fragmentation:
- Occurs mainly in electron ionization (EI), but other methods can cause it too.
- Helps determine the structure of unknown molecules.
- The pattern of fragments is often reproducible and characteristic of a molecule.