Mass spectometry

• What are the four stages of a mass spectrometer?

  Ionisation, acceleration, ion drift, and detection
• What are the main techniques of ionisation in mass spectrometry?

  • Electrospray ionisation
  • Electron impact ionisation
• Why is electrospray ionisation considered a gentler technique?

  It prevents fragmentation of the sample
• What types of materials is electrospray ionisation typically used for?

  Polymers and biological materials like DNA
• How does electrospray ionisation create positively charged ions?

  A high voltage rips a proton off the solvent and attaches it to the sample molecules
• What happens during electron impact ionisation?

  The sample is vaporised and hit with electrons, knocking off electrons from the molecule
• What is a consequence of using electron impact ionisation?

  This method often causes the sample to fragment
• What is the purpose of the acceleration stage in a mass spectrometer?

  Molecules are accelerated to have the same kinetic energy
• What is the relationship between mass and speed in the acceleration stage?

  Lighter particles move faster and are detected before heavier particles
• How is the time of flight calculated in the ion drift stage?

  Time of flight = distance/velocity
• What does the equation for time travelled in the ion drift stage depend on?

  Time = distance ÷ √(2KE/M)
• How does the mass of ions affect their time of flight?

  Lighter ions take less time as the time is dependent on the square root of the mass
• What happens during the detection stage of a mass spectrometer?

  The ions hit a negatively charged plate, causing a current
• What does the size of the current in the detection stage indicate?

  The size of the current gives a measure of the number of molecules hitting the plate
• What information is provided by the spectrum produced by the mass spectrometer?


  • X-axis: mass/charge ratio
  • Y-axis: % abundance
  • Main peak: molecular ion peak (greatest mass/charge ratio)
• What does the molecular ion peak represent in the mass spectrum?

  It represents the mass/charge value of the molecule being analyzed
• What do smaller peaks around the molecular ion peak indicate?

  They are from the same molecules but with different isotopes
• Why do isotopic molecules have different mass/charge ratio values?

  Because they have different masses
• What causes smaller and significantly lighter peaks in the spectrum?

  They are due to fragmentation of the molecule
• How is relative atomic mass defined?

  • Average weighted mass of an atom relative to carbon-12
  • Calculated taking into account all isotopes and their relative abundances
• What is the significance of the term "average" in relative atomic mass?


  It indicates that the value considers all isotopes and their relative abundances
• How can the mass spectrum be used to calculate relative atomic mass?

  • View different isotopes and their relative abundance
  • Isotopic mass is along the x-axis
  • Isotopic abundance is along the y-axis
• How do you calculate the relative atomic mass using isotopic abundances?

  Relative atomic mass = (80% × 11) + (20% × 10) = 10.8
• What are the isotopes of boron mentioned in the example?

  10B and 11B
• What is a mass spectrometer used for?

  To determine the relative atomic mass of a sample
• What is the fundamental principle behind all types of mass spectrometry?

  Forming ions and separating them based on their charge:mass ratio
• What does time-of-flight (TOF) spectrometry record?

  The time taken for ions to reach a detector
• What are the stages of TOF spectrometry?

  1. Ionisation
  2. Acceleration
  3. Ion drift
  4. Detection
  5. Analysis
• What happens during the ionisation stage of TOF spectrometry?

  The sample is vaporised and injected into the chamber
• What are the two types of ionisation in TOF spectrometry?

  Electron bombardment and electrospray
• How do ions behave during the electron bombardment ionisation process?

  Ions lose an electron
• How do ions behave during the electrospray ionisation process?

  Ions gain a proton
• What occurs during the acceleration stage of TOF spectrometry?

  Molecules are given energy to have the same kinetic energy and are accelerated towards a negative plate
• What happens during the ion drift stage of TOF spectrometry?

  Ions are deflected by a magnetic field and travel in a curved path
• What is the role of the negative plate during the detection stage of TOF spectrometry?

  Positive ions hit the negative plate, gain an electron, and cause a current to flow
• How does the abundance of ions affect the current during detection in TOF spectrometry?

  Greater abundance results in greater current
• What is produced during the analysis stage of TOF spectrometry?

  A mass spectrum using current values and flight times
• What do mass spectra show?

  The abundance of different ions separated by their mass/charge ratio
• What is the first step in mass spectrometry?

  Ionisation
• What occurs during ionisation in mass spectrometry?

  The sample is ionised to form positive ions