Mass spectrometry

AS Level Chemistry · Analytical techniques. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A mass spectrum shows the positive ions made from a sample. Each peak has an m/e value, which for a 1+ ion equals its mass, and a height that shows its relative abundance.

For an element, each peak is one isotope, so the spectrum gives the relative atomic mass. For an organic compound, the molecular ion peak, M, is the whole molecule with one electron removed. Its m/e value is the relative molecular mass. Peaks at lower m/e are fragments, formed when bonds in the molecular ion break.

Small peaks just above M come from heavier isotopes. The [M + 1] peak is caused by 13C and gives the number of carbon atoms. An [M + 2] peak shows chlorine or bromine.

Rules to remember

Common mistake

Students take the tallest peak as the molecular ion. The tallest peak is often a fragment; the molecular ion is the peak of highest m/e, apart from the small isotope peaks. Always write fragments with a positive charge.

Worked example

A compound containing only carbon, hydrogen and one type of halogen has molecular ion peaks at m/e 78 and 80 with heights in the ratio 3 : 1. The peak at 78 has abundance 60.0 and the peak at 79 has abundance 2.0. Find the molecular formula.

  1. M and [M + 2] in the ratio 3 : 1 show one chlorine atom, and the peak at 78 contains 35Cl.
  2. n = (100 × 2.0) ÷ (1.1 × 60.0) = 200 ÷ 66 = 3.03, so there are 3 carbon atoms.
  3. Mass left for hydrogen = 78 − 35 − (3 × 12) = 7, so there are 7 hydrogen atoms.

Answer: C3H7Cl.

Practice questions

Try each one, then open the answer.

1. A mass spectrum shows peaks at M and [M + 2] in the ratio 3:1. What does this show about the compound?

  1. A
    it contains one bromine atom
  2. B
    it contains two chlorine atoms
  3. C
    it contains one chlorine atom
  4. D
    it contains one 13C atom
Show answer

Answer: C. Chlorine is 75% 35Cl and 25% 37Cl, so a molecule with one Cl gives M and M + 2 peaks in a 3:1 ratio. One bromine (79Br and 81Br roughly 50:50) gives peaks of about equal height.

2. The molecular ion peak of an organic compound is at m/e = 60. Which compound could it be?

  1. A
    propanal
  2. B
    propan-1-ol
  3. C
    butan-1-ol
  4. D
    ethanal
Show answer

Answer: B. The molecular ion peak gives Mr. Propan-1-ol, C3H7OH: 3 × 12.0 + 8 × 1.0 + 16.0 = 60.0. Propanal is 58, butan-1-ol 74 and ethanal 44.

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