Weighing molecules

Section 3 of 4

A mass spectrometer weighs molecules. It tells a chemist how heavy a whole molecule is, and which pieces it breaks into.

Chemists weigh atoms on a scale where a carbon-12 atom weighs exactly 12, a hydrogen atom about 1 and an oxygen atom about 16. Add up the atoms and you have the molecule's mass. Ethanol, C₂H₆O, weighs 2 × 12 + 6 × 1 + 16 = 46.

Inside the machine

  1. A little of the sample is turned into a gas, and a beam of fast electrons crosses it. An electron that hits a molecule knocks one of the molecule's own electrons out, and the molecule is left with a positive charge: it is now an ion. An electron is about 1,800 times lighter than a hydrogen atom, so the ion weighs the same as the molecule.
  2. An electric push sends every ion down a long tube with the same energy.
  3. Light ions fly fastest and reach the detector first, and heavy ions arrive later. The machine times each one, works out its mass, and counts how many ions arrive with each mass.
How a mass spectrometer sorts molecules by mass A long tube seen from the side. At the left, a beam of electrons crosses the gas and knocks an electron off each molecule, making positive ions. Two dashed grids push every ion along the tube with the same energy. In the tube, a small, light ion has flown further than a big, heavy ion, with longer speed lines behind it. At the right end, a detector counts the ions as they arrive: light ions arrive first. electrons + + + heavy ion + light ion make ions push detector light ions arrive first
A time-of-flight mass spectrometer. Every ion gets the same push, so light ions fly fastest and reach the detector first. Other mass spectrometers steer the ions with a magnet, which bends the paths of light ions most.

Reading a mass spectrum

The result is a mass spectrum: a bar for each mass, as tall as the number of ions that arrived with it. The heaviest bar is usually the whole molecule with one electron missing, called the molecular ion. For ethanol it stands at 46, the molecule's mass.

The electron hit leaves an ion with a lot of spare energy, and many ions shake themselves apart. Each piece that keeps the charge reaches the detector too, and its mass is a clue to how the molecule is built. Ethanol's tallest bar is at 31, and 46 − 15 = 31, so that ion has lost a piece of mass 15: a CH₃ group. The C–⁠C bond broke and left CH₂OH⁺.

The mass spectrum of ethanol A bar chart of how many ions of each mass reach the detector, for masses from 10 to 50. The tallest bar is at 31, the molecule after it lost a CH3 group. A bar about half as tall is at 45, the molecule after it lost one hydrogen atom. A bar about a fifth as tall is at 46, the whole molecule, CH3CH2OH plus. Shorter bars stand at 15, 26, 27, 29, 30, 42 and 43. 46 the whole molecule, CH₃CH₂OH⁺ 45 it lost one hydrogen atom 31 it lost a CH₃ group 31 45 46 10 20 30 40 50 mass of the ion ions counted
The mass spectrum of ethanol, from real measurements. The whole molecule, CH₃CH₂OH⁺, stands at 46. The tallest bar, at 31, is CH₂OH⁺, left when the ion lost a CH₃ group.

Quick check

Propanone's molecular ion has a mass of 58, and its tallest bar is at 43. Which piece broke off?