- Created by: Kyra Mullarney
- Created on: 15-04-19 14:38
Molecular ions and fragment ions
- When an organic compound is placed in the mass spectrometer, it loses an electron and forms a positive ion, the molecular ion.
- The mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound.
- The molecular ion M+ is the positive ion formed when a molecule loses an electron.
1 of 5
Molecular mass from a mass spectrum
- To find the molecular mass, the molecular ion peak (M+) has to be located.
- The molecular ion peak is the peak on the far right with the highest m/z value.
- The small peak after the M+ peak is the M+1 peak. It exists because 1.1% of the carbon atoms present is the carbon-13 isotope.
2 of 5
- Some molecular ions break down into smaller pieces known as fragments in a process fragmentation.
- The other peaks are caused by fragment ions, formed from the breakdown of the molecular ion.
- The simplest fgragmentation breaks a molecular ion down into two species - a positively charged fragment ion and a radical.
- Any positive ions formed will be detected by the mass spectrometer but the uncharged radicals are not detected.
3 of 5
Using fragmentation peaks to identify a molecule
- The mass spectrum of each compound is unique, as molecules will all fragment in slightly different ways depending on their structures.
- Mass spectra can therefore be used to help identify molecules.
- Although two mat have the same molecular mass and the same molecular ion peak, the fragment ions may be different.
4 of 5
Common fragment ions
- CH3+ = 15
- C2H5+ = 29
- C3H7+ = 43
- C4H9+ = 57
5 of 5