time of flight mass spectrometer

• what is TOF used for?


  finding the abundance and mass of isotopes

  determining Ar and Mr
• what is TOF?


  particles are ionised, accelerated, time taken for ions to travel fixed distance is used to find the mass
• what do all atoms of the same charge have?


  same kinetic energy
• what is the mass spectrum?

  shows mass to charge ratio (m/z) and relative abundance
• why are particles ionised?


  only positive ions are accelerated and detected
• what is electron impact ionisation?


  high speed electrons fired at sample

  electron knocked off

  positive ion formed
• what is the electron impact ionisation equation?


  X (g) --> X+ (g) + e-
• what is electron impact ionisation used for?


  elements

  small molecules
• what are the problems and solutions of electron impact ionisation?


  atoms may lose more than one electron- use minimum energy

  molecules can break into fragments- use electrospray ionisation
• what is electrospray ionisation?


  sample dissolved in high volatile solvent

  passed through high voltage needle

  sample gains a proton

  positive ion formed
• what is electrospray ionisation used for?


  large molecules
• what ions accelerate fastest?


  smallest m/z

  lighter particles
• what is acceleration?


  positive ions attracting negative field plates (electric field)

  gains energy
• what is the ion drift region?


  positive ions move at a constant velocity
• which ions reach the detector first?

  smallest m/z

  lighter ions
• why is TOF done in a vacuum?


  prevent collisions with air molecules
• why are collisions with air molecules prevented?


  they change energy which knocks ions off course
• what is the detector?


  positive ions gain electrons

  movement of electrons

  current produced
• what is the electrospray ionisation equation?


  X(g) + H+ --> XH+(g)
• what equation represents the detector?


  X+(g) + e- --> X(g)
• how is the abundance calculated?


  proportional to the current produced
• what is the molecular ion peak?


  represents greatest m/z value
• what is the kinetic energy calculation?


  Ek=0.5mv^2
• what is the velocity calculation?

  V=d/t