Time Of Flight Spectrometer

• The mass Spectrometer can be used to determine all the isotopes present in a sample of an element and to therefore identify elements
• The mass Spectrometer needs to be under a vacuum otherwise air particles would ionise and register on the detector
• There are 4 essential steps in a mass Spectrometer

  • Ionisation
  • Acceleration
  • Flight tube / Ion drift area
  • Detection area
• Ionisation

  The sample can be ionised in a number if ways. Two of these techniques are electron impact and electrospray ionisation
• What are the two ionisation techniques?

  Electron impact and electrospray ionisation
• Electron impact

  A vapourised sample is injected at low pressure

  An electronic gun fires high energy electrons at the sample

  This knocks out an outer electron

  Forming positive ions with different charges e.g Ti -> Ti+ + e-
• Electo spray ionisation

  • The sample is dissolved in a volatile, polar solvent
  • Injected through a fine hypodermic needle giving a fine mist or aerosol
  • The tip of needle has high voltage
  • At the tip of the needle the sample molecule, M gains a proton H+ from the solvent forming MH+
  • M(g) + H+ -> MH+(g)
  • The solvent evaporates away while the MH+ ions move towards a negative plate
• Electron impact is used for elements and substances with low formula mass electron impact can cause larger organic molecules to fragment
• Electrosoray ionisation is used preferably for larger organic molecules. The softer conditions of this technique mean fragmentation does not occur
• Acceleration

  • Positive ions are accelerated by an electric field to a constant kinetic energy
  • KE(J) = ½ × m(Kg) × v²(ms-1)
  • This equation rearranged gives
  • V = square root of 2KE / m
  • (you don't need to learn this equation but will get asked to use them in a calculation)
• Acceleration

  Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass. Lighter particles have a faster velocity, and heavier particle have a slower velocity
• Flight tube / Ion drift area

  • The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move faster
  • The heavier particles take longer to mover through the drift area
  • The ions are distinguished by different flight times
  • t= d/v. t = time if flight (s) d = length of flight tube (m) v = velocity of the particle (ms-1)
  • Combining the two equations gives you
  • t= d × square root of m/2KE
• Detection

  • The ions reach the detector and generate a small current, which is fed to a computer for analysis.
  • The current is produced by electrons transferring from the detector to the positive ions.
  • The size of the current is proportional to the abundance of the species
  • For each isotope the mass Spectrometer can measure a m/z (mass/charge ratio) and an abundance
  • Sometimes two electrons may be removed from a particle forming a 2+ ion. ²⁴Mg²+ with a 2+ charge would have a m/z of 12