MASS SPECTROMETRY

• WHAT IS MASS SPECTROMETRY?

  technique used to measure mass to charge ratio of ions
• HOW DOES A MASS SPECTROMETER WORK?

  • determines mass of separate atoms/ molecules
  • forms ions from sample
  • separates ions according to ratio of charge to mass
• KEY POINTS IN MASS SPECTROMETRY?

  1. Ionisation
  2. Acceleration
  3. Ion Drift
  4. Detection
• WHY DO ISOTOPES HAVE SIMILAR CHEMICAL PROPERTIES?

  same number of electrons in outer shell
• HOW IS ABUNDANCE OF ISOTOPES MEASURED IN A MASS SPECTROMETER?

  current produced is proportional to abundance
• HOW ARE IONS DETECTED IN A MASS SPECTROMETER?

  hit detector which generates a current
• WHAT PROPERTY SEPARATES IONS DURING THE ION DRIFT STAGE OF MASS SPECTROMETRY?

  different masses (all accelerated to same KE -> different masses = different velocities)
• IONISATION?

  • sample vapourised into gas form
  • put into mass spectrometer and immediately ionised (electrospray or electron impact)
• WHAT DOES IT MEAN TO IONISE SOMETHING?

  give it a charge
• ELECTROSPRAY IONISATION?

  • voltage applied
  • each particle gains H+ ion
  • converts sample
  • into gas of positive ions
• ELECTRON IMPACT IONISATION?

  • electron gun used
  • to fire high energy electrons
  • at particles
  • electrons repel a single electron out of each particle
  • forming +1 ions
• ACCELERATION?

  • ions accelerated by an electric field
  • with acceleration plate
  • which is negatively-charged on front (attracts ions)
  • positively-charged on back (repels ions further down mass spectrometer)
  • ions now all have same KE (lighter particles move faster)
• ION DRIFT?

  • electric field is absent
  • ions not deflected
  • pass through
  • lighter ions drift faster
• DETECTION?

  • ions are detected
  • lighter ions detected first
  • so ions are separated based on mass
  • mass spectrum generated
  • using an electric field
• WHAT IS ELECTROSPRAY IONISATION USED FOR?

  high Mr compounds/ large molecules/ proteins
• WHAT IS ELECTRON IMPACT IONISATION USED FOR?

  elements/ low Mr compounds/ small molecules
• ANOTHER NAME FOR AN ELECTRON GUN?

  hot wire filament
• WHAT IS CURRENT PRODUCED PROPORTIONAL TO?

  abundance/ number of ions
• WHERE DOES MASS SPECTROMETRY TAKE PLACE?

  in a vaccum
• WHAT DOES KE EQUAL?

  0.5 * m * v^2
• WHAT DOES v EQUAL?

  square root (2 * KE/ m)
• WHAT DOES t EQUAL?

  d / v
• WHAT DOES t EQUAL (EXPANDED)?

  d * square root (m/ 2 * KE)
• Br ISOTOPES?

  50% 79 Br, 50% 81 Br
• Cl ISOTOPES?

  75% 35 Cl, 25% 37 Cl
• WHY MUST A SAMPLE BE IONISED?

  • to accelerate
  • to deflect/ bend beam
• WHAT CAN BE ADJUSTED IN THE SPECTROMETER TO ENABLE IONS TO BE DIRECTED ONTO THE DETECTOR?


  electromagnet
• HOW IS THE RELATIVE MOLECULAR MASS OF A COVALENT COMPOUND OBTAINED FROM ITS MASS SPECTRUM?

  highest m/z value
• WHAT IS THE REACTION WHEN AN ION HITS THE DETECTOR?

  X+ + e- -> X
• HOW TO TELL WHICH m/z VALUE PRODUCES THE BIGGEST CURRENT?

  most abundant ion
• HOW IS THE TOF MASS SPECTROMETER ABLE TO SEPARATE TWO SPECIES TO GIVE TWO PEAKS?

  • positive ions accelerated
  • to constant KE
  • by electric field
  • faster ions arrive at detector first
• WHY CAN PARTICLES WITH THE SAME MASS AND VELOCITY BE DEFLECTED BY DIFFERENT AMOUNTS IN SAME MAGNETIC FIELD?

  different charges/ m/z
• WHY IS THE VALUE OF ISOTOPES DIFFERENT IN THE PERIODIC TABLE?

  Ar in periodic table takes account of other isotopes
• HOW IS KRYPTON IONISED IN THE MASS SPECTROMETER?

  • beam of electrons from electron gun
  • knocks out electrons
  • form positive ion
• HOW TO TELL WHICH ISOTOPE IS DEFLECTED THE MOST?

  lowest m/z
• HOW IS THE CURRENT GENERATED?

  electrons flow at detector to positive ion