Mass Spectrometry

A mass spectrometer machine can be used to analyse elements or compounds.

Time Of Flight (TOF) mass spectrometry

  • There are 4 things that occur when a sample is squirted into a TOF mass spectrometer:
  • Electronspray ionisation:
  • The sample is dissolved in a polar solvent and is pushed through a small nozzle at a high pressure.
  • A high voltage is applied which results in the particles in the sample to lose an electron.
  • These ionised particles are then separated from the solvent, leaving a gas of positive ions
  • Acceleration:
  • The positive ions are accelerated by an electric field
  • The particles need to be positively charged to accelerated by the electric field
  • The electric field gives the same kinetic energy to all of the ions regardless of size
  • The ions with a lower m/z (mass/charge) ratio experience greater accelerations
  • Ion Drift:
  • The ions leave the electric field at a constant speed with equal amounts of kinetic energy
  • They enter a region with no electric field and drift through it at the speed they left the electric field
  • Ions with lower mass/charge rations will be drifting at a higher speed
  • Detection:
  • As ions with a lower mass/charge ration travel through the drift region at a higher speed, they will reach the detector in a quicker time relative to the heavier ions
  • The detector detects the change in current created when the ions (cations as they have had an electron removed) collides with the detector plate. It then records how long they took tp pass through the spectrometer. The data produced can then be used to determine the mass/charge value need to produce a mass spectrum
  • Time of Flight uses alcohol groups (ethanol) which has a space hydrogen bond. This hydrogen bonds with X (x being the substance in question) causing it to break apart

X + H+ -> XH+

  • The various atoms are accelerated towards a negative plate which then measures which atoms get there at what time to calculate the masses.
  • In older methods of spectrometry the electron ionisation. An electron gun is fired at an atom, causing it to lose 1 electron as it is knocked out.

X(g) -> X+(g) + e

  • The test however is not greatly accurate as often X+ would be further broken down.

Calculating Relative Atomic Mass

  • Averages are used to find the RAM of an element
  • Take the value of the x axis and multiply it to the Y axis.
  • Add the value of step 1 for all major peaks on the graph
  • Divide by the total number of the Y axis

e.g.

polarity

(15 x 18) + (29 x 62) + (31 x 100) + (32 x 63)

243

7184

243

= 29.56

Identifying Elements

  • Mass spectrometry can be used to identify elements
  • Used in determining different isotopes of an element as it produced more than one line in a mass spectrum as the isotopes all have different masses
  • The patterns produced can be used as a ‘fingerprint’ and can identify specific elements that are contained