17.3 Determination of Molecular Mass by Molecular Ion Peak

What is a Molecular Ion?

In a mass spectrometer, a sample is bombarded by a beam of high-energy electrons. When one of these electrons strikes a molecule with sufficient energy, it knocks out one electron from the molecule, forming a radical cation called the molecular ion ($M^{\bullet +}$):

$M+e^{-}\to M^{\bullet +}+2e^{-}$

The molecular ion retains the same mass as the original molecule (loss of one electron has negligible mass). Its mass-to-charge ratio ($m/z$) is therefore equal to the molecular mass of the compound (assuming charge $z=1$).

The Molecular Ion Peak ($M^{+}$ Peak)

In a mass spectrum, the molecular ion peak appears at the highest $m/z$ value (excluding small isotope peaks). Reading the $m/z$ value of this peak directly gives the molecular mass of the compound.

This could correspond to:

• Propanone ($C_3{H}_{6}O$): $(3\times 12)+(6\times 1)+16=58$
• Butane ($C_4{H}_{10}$): $(4\times 12)+(10\times 1)=58$

The $M+1$ Peak

A small peak appears one mass unit above the molecular ion peak. This is the $M+1$ peak, caused by the natural abundance of the ${}^{13}C$ isotope (~1.1%).

• A molecule with $n$ carbon atoms has an $M+1$ peak whose height relative to the $M$ peak is approximately $1.1n\%$.
• This allows estimation of the number of carbon atoms in the molecule:

$n\approx \frac{\text{height of }(M+1)\text{ peak}}{\text{height of }M\text{ peak}}\times \frac{100}{1.1}$

When the Molecular Ion Peak is Absent or Weak

Some compounds produce a very weak or absent $M^{+}$ peak because the molecular ion is highly unstable and fragments rapidly into smaller daughter ions before reaching the detector. This is common in:

• Branched-chain alkanes
• Tertiary alcohols
• Compounds with easily broken bonds

In such cases, the highest observable fragment peak must be used with caution — it may not represent the true molecular mass.

The Base Peak

The base peak is the tallest peak in the mass spectrum (highest relative abundance = 100%). It represents the most stable fragment ion produced during fragmentation. The base peak is used as the reference standard for measuring relative intensities of all other peaks.

Summary Table