Mass Spectrometry

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Analyse mass spectra in terms of m/e values and isotopic abundances (you don't need to know how the mass spectrometer works)
  2. Calculate the relative atomic mass of an element given the relative abundances of its isotopes, or its mass spectrum
  3. Deduce the molecular mass of an organic molecule from the molecular ion peak in a mass spectrum
  4. Suggest the identity of molecules formed by simple fragmentation in a given mass spectrum
  5. Deduce the number of carbon atoms, n, in a compound using the [M + 1]⁺ peak and the formula: n = (100 × abundance of [M + 1]⁺ ion) / (1.1 × abundance of M⁺ ion)
  6. Deduce the presence of bromine and chlorine atoms in a compound using the [M + 2]⁺ peak

What is Mass Spectrometry?

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Mass spectrometry is an analytical technique used to identify unknown compounds and determine the masses of atoms and molecules. It works by creating charged particles (ions) from a sample and then separating these ions based on their mass-to-charge ratio.

The instrument produces a mass spectrum – a graph that shows the different ions detected. You don't need to know how the machine works inside, but you do need to understand how to read and interpret the mass spectrum it produces.

Understanding Mass Spectra: m/e Values and Isotopic Abundances

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What is m/e?

The m/e ratio (also written as m/z) stands for mass-to-charge ratio. It's the mass of an ion divided by its charge.

Formula: m/e = mass of ion / charge of ion

Example: An ion with a mass of 24 and a charge of 2+ has an m/e value of 24 ÷ 2 = 12.

Most ions in a mass spectrum have a charge of +1, so their m/e value is simply equal to their mass.

Reading a Mass Spectrum

A mass spectrum is a graph with:

  • x-axis: m/e values (which tells you the mass of the ions)
  • y-axis: relative abundance (which tells you how common each ion is, shown as a percentage or relative height)

Each peak on the graph represents a different ion. The height of each peak shows how abundant (common) that particular ion is.

Isotopes in Mass Spectra

Remember that isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This means they have different mass numbers.

When an element has isotopes, its mass spectrum will show separate peaks for each isotope.

Example: Boron Boron has two isotopes:

  • Boron-10 (¹⁰B) with a relative abundance of 19.9%
  • Boron-11 (¹¹B) with a relative abundance of 80.1%

The mass spectrum of boron shows two peaks:

  • A peak at m/e = 10 with height 19.9
  • A peak at m/e = 11 with height 80.1

This tells us that in nature, about 20% of boron atoms are boron-10 and about 80% are boron-11.

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