The Mass Spectrometer

• What does a mass spectrometer calculate?

  The relative atomic masses
• Why is the apparatus of a mass spectrometer kept under a vacuum?

  To prevent the ions produced colliding with molecules from the air.
• Describe electron impact ionisation

  - The sample is vaporised and high energy electrons are fired at it from an electron gun

  - This knocks off one electron forming a 1+ ion
• Describe electrospray ionisation

  • Sample is dissolved in a volatile solvent and forced through a hypodermic needle connected to the positive terminal of a power supply
  • This produced a positively charged droplets that have gained a proton
• Describe the acceleration stage

  • The positive ions are attracted towards a negatively charged plate and accelerate towards it
  • Lighter ions are faster
• Describe the ion drift stage

  • The ions pass through a hole in the negatively charged plate
  • Forming a beam and travel along the flight tube to a detector
• Describe the detection stage

  • The ions with the same charge arrive at the detector
  • The lighter, faster ones are first
  • The flight time is recorded
  • The positive ions pick up an electron from the detector causing a current it flow.
• Which type of ionisation causes fragmentation?

  Electron impact
• Why are there peaks with higher m/z ratios?

  Dues to to some molecules containing 2H or 13C
• Why are there peaks with much lower m/z ratios?

  Due to fragmentation
• Show how these expressions can be used to show the time of flight as:

  𝒕 = 𝒅√ 𝒎 / 𝟐𝑲𝑬

  -