AS chemistry

• A mass spectrometer is an analytical tool used to measure the mass-to-charge ratio (m/z) of ions
• Time-of-flight (TOF) mass spectrometer


  • Determines the m/z of ions based on how long they take to travel from the ion source to the detector
• How a TOF mass spectrometer works

  1. Ionisation - Turns the sample into positive gaseous ions

  2. Acceleration - Positively charged ions are accelerated towards a negatively charged plate by an electric field

  3. Ion drift - Ions enter a flight tube where they drift at a steady speed related to their mass

  4. Detection - Ions hit the detector and gain an electron which creates an electric current
• Electron impact (EI) ionisation

  Bombarding the vaporised sample with high-energy electrons to remove an electron from each molecule, resulting in +1 ions
• Electrospray ionisation (ESI)


  Dissolving the sample in a volatile solvent and spraying it through a charged needle, which adds an H+ ion from the solvent to each molecule
• Lighter ions, having lower m/z ratios, move through the tube quicker and reach the detector sooner than heavier ions
• Mass spectrum

  Plots the relative abundance of ions against their mass-to-charge ratio (m/z)
• For electron impact, the m/z of each peak equals the relative mass of the ion
• For elemental samples, each peak represents a different isotope. The relative height of each peak shows how abundant the isotope is
• For molecular samples, the peak at the highest m/z value is the molecular ion (M+). The m/z of the molecular ion peak matches the molecule's relative molecular mass (Mr)
• Calculating relative atomic mass (Ar) from mass spectra

  1. Multiply the relative isotopic mass by its relative abundance for each isotope

  2. Add these products together

  3. Divide the total by the sum of the relative abundances (100 if using percentages)