ToF mass spectroscopy

• What is the equation for ionisation of a molecule (X) using electrospray ionisation?


  X(g) + H+ —> XH+(g)
• Describe how a time of flight mass spectrometer works

  1) Acceleration - positive ions attracted towards a negatively charged plate.

  
  

  2) Ion Drift - ions pass through hole in plate, form a beam with constant kinetic energy, travel along tube to detector. Therefore Tof is directly proportional to the square root of mass

  
  

  3) Detection - positive ions pick up electrons, current flows, m/z value and time of flight recorded. Largest current from most abundant ions
• what can a mass spectrometer be used for?


  to determine all the isotopes present in a sample of an element and to therefore identify elements.
• why does mass spectroscopy need to be under a vacuum?


  otherwise air particles would ionise and register on the detector
• state the essential 4 steps in a mass spectrometer


  1. Ionisation
  2. Acceleration
  3. Ion Drift
  4. Detection
• what are the two ways a sample can be ionised?

  Electron impact

  Electro Spray Ionisation
• when is electron impact used?


  when ionising elements and substances with low formula mass.
• why isn't electron impact used on larger organic molecules?


  can cause larger organic molecules to fragment
• why is electro spray ionisation preferred for larger organic molecules, rather than electron impact?


  the softer conditions of this technique mean fragmentation does not occur
• describe electron impact


  • A Vaporised sample is injected at low pressure
  • An electron gun fires high energy electrons at the sample
  • This knocks off an outer electron
  • Forming positive ions with different charges
• describe electro 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
• what does the velocity of each particle depend on and why?

  mass because all the particles have the same kinetic energy
• what happens in the acceleration stage of mass spectroscopy?


  • Positive ions are accelerated by an electric field
  • To a constant kinetic energy
• what happens in the detection stage of mass spectroscopy?


  • 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
• what can the mass spectrometer measure for each isotope?


  a m/z (mass/charge ratio) and an abundance
• diagram of time of flight mass spectrometer

  diagram

  
• if asked to give the species for a peak in a mass spectrum...

  give charge and mass number e.g. 24Mg+
• explain ion drift

  • 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 move through the drift area.
  • The ions are distinguished by different flight times
• Cl has two isotopes Cl-35 (75%) and Cl-37(25%)

  what will the mass spectra look like?


  3 peaks

  
• Br has two isotopes Br-79 (50%) and Br-81(50%)

  what will the mass spectra look like?


  3 peaks

  
• what can a molecular ion further fragment to form?

  The molecular ion can further fragment to form new ions, molecules, and radicals
• what does mass spectroscopy give accurate information about?


  • relative isotopic mass
  • the relative abundance of isotopes
• what can mass spectrometry be used for?


  • to identify elements
  • to determine relative molecular mass
• what is the equation associated with electro spray ionisation?


  X (g) + H+ →  XH+ (g)
• what is the equation associated with electron impact ionisation?


  X (g) → X+ (g) + e-
• what happens when a 2+ charged ion is produced in ionisation?


  ion will be more affected by the magnetic field producing a curve path of a smaller radius so m/z is halved