Mass spectrometry

• Electron impact

  Used for elements and low Mr compounds

  It knocks off one electron from each atom to form +1 ions
• Equation for electron impact

  X(g) = X+(g) + e-
• Electrospray ionisation

  Used for high Mr compounds

  Sample is dissolved in solvent (methanol) and injected through a fine hypodermic needle as a fine spray into a vacuum in the ionisation chamber

  Very high voltage is applied to end of needle where spray emerges

  particles gain a proton and becomes a fine mist

  Solvent evaporates leaving +1 ions and mass increases by 1
• Equation of electrospray

  X(g) + H+(g) = XH+(g)
• Stage 1 - Ionisation

  Always a gas
• Acceleration of ions - the ions are accelerated using an electric field so that all the ions have the same kinetic energy
• Separation of charged ions - ion drift occurs where the ions enter the flight tube and ions with different masses have a different time of flight.

  The lighter ions travel faster and take less time to reach the detector
• Detection

  The detector is a negatively charged plate- a current is produced when the ions hit the plate ( the more ions that hit the detector the bigger the current)

  mass spectrum shows the number of particles (abundance) of each mass hit the detector