Time of flight mass spectometry

• Mass spectrometry

  Is an instrumental technique that finds the molecular mass of atoms and molecules
• With elements, what does mass spectrometry determine?

  -the relative isotopic mass

  -the relative abundance for each isotope
• 5 steps of time of flight mass spectrometry

  1)vaporisation

  2)ionisation

  3) acceleration

  4) ion drift

  5) detection
• relative intensity

  Abundance of each isotope
• Time of flight mass spectrometry (1.vaporisation)

  Sample must be heated to become a vapour or dissolved in a solvent so it can travel through the time of flight mass spectrometer
• Time of flight mass spectrometry (2.ionisation-electrospray)

  sample dissolved in a volatile solvent, injected through a fine needle with a high voltage and dispersed as an aerosol and the sample gains a proton

  M + H+ ——>MH+
• Time of flight mass spectrometry- what are the 2 types of ionisation

  Electrospray ionisation

  electron impact
• Time of flight mass spectrometry (2.ionisation-electron impact)

  -high energy electrons fired at the sample from an electron gun and it leads to sample to lose electrons

  -used for small molecules and elements because it causes fragmentation of the molecule so it is not good for larger molecules

  -M(g) ——> M+(g) + e-
• Time of flight mass spectrometry (3.acceleration)

  -positive ions are accelerated. They are attracted to a negatively charge plate and sped up to a constant kinetic energy

  -particles with a lower mass/charge (m/z) ratio will accelerate quicker
• Time of flight mass spectrometry (4.ion drift)

  -ions separate based on their mass

  -high mass ions have a low velocity

  -low mass ions have a high velocity
• Time of flight mass spectrometry (5. Detection)

  -ions hit the detector plate and they gain an electron, which produces a current so ions are detected as electrical current

  -the size of current ∝ abundance of each isotope

  -particles with lower m/z reach the detector first as they travel fastest
• Molecular ion peak

  Gives us the Mr of the molecule
• What percentage of chlorine isotopes have a mass of 35?

  75%
• What percentage of chlorine isotopes have a mass of 37?

  25%
• Ratio of cl2 70,72,74

  9:6:1
• How many isotopes does bromine have ?

  2
• What percentage of bromine isotopes have a mass of 79?

  50%
• What percentage of bromine isotopes have a mass of 81?

  50%
• Ratio of Br2 158,160,162

  1:2:1
• M/z

  Mass of an isotope divided by charge (as most have +1 charge,mass=charge)
• How to calculate mr from TOF mass spectrometry using electrospray

  Mr=mass of MH+ - 1
• Why do we subtract one from the mass in TOF mass spectrometry using electrospray

  Because hydrogen (H) has a mass of 1
• What is the m+1 peak caused by in TOF mass spectrometry using electron impact

  Due to carbon-13 or hydrogen-2 isotope
• relative abundance

  A measure of how much of an isotope is naturally present in the environment
• What is the Mr if it is electron impact (TOF mass spectrometry)

  Mr<200
• How to calculate isotope peaks (TOF mass spectrometry)

  Isotope peaks= n+1
• Why is it necessary to ionise molecules when measuring their mass in a TOF mass spectrometer

  -so the ions are accelerated as they are attracted to a negatively charged plate

  -only ions will create a current when hitting the detector
• Time of flight mass spectrometry causes ions to separate based on what?

  Their mass-and hence velocity
• Kinetic energy equation

  KE=1/2mv^2
• Velocity equation

  v=d/t
• distance equation using kinetic energy

  d = t√(2KE/m)
• Time equation

  t= d√(m/2KE)
• time of flight spectrometry equation

  t1/√(Ar1) = t2/√(Ar2)
• How to calculate mass of one ion in kg

  (Ar or Mr/Avagadros constant ) ÷ 1000