Mass spectrometry is a powerful analytical technique used to determine molecular mass and structural information for organic compounds. In IB Chemistry (Topic 11 and Topic 21), the molecular ion peak is one of the most important features students must recognize. This article explains what the molecular ion peak is, how it forms, and how to use it to identify molecules quickly and accurately.
What Is a Molecular Ion?
When a molecule enters a mass spectrometer, it is bombarded with high-energy electrons.
This often knocks out one electron from the molecule without breaking any bonds.
This process forms the molecular ion, also called the parent ion:
M → M⁺· + e⁻
- M⁺· is the molecular ion
- It is positively charged
- It has the same mass as the original molecule (minus one electron, which is negligible)
Because the molecular ion retains the full structure and mass of the original compound, it provides crucial information about molecular identity.
What Is the Molecular Ion Peak?
The molecular ion peak (M⁺) is the peak on the mass spectrum that corresponds to the molecular mass of the original molecule.
It is usually the highest m/z peak that still represents the entire molecule rather than fragments.
The molecular ion peak allows you to:
- Determine the relative molecular mass (Mr)
- Identify the molecular formula
- Narrow down possible structures
For many molecules, it is the most valuable peak in the entire spectrum.
How the Molecular Ion Peak Forms
During electron-impact ionization:
- Molecules are vaporized
- High-energy electrons bombard the vapor
