Atomic Structure

• An atom consists of a nucleus containing protons and neutrons surrounded by electrons
• How molecules are ionised using electrospray ionisation

  Sample is dissolved in volatile solvent then injected through a needle at high voltage which makes each particle gain a proton/H+
• How TOF mass spec is able to separate 2 species to give 2 peaks:

  Positive ions are accelerated by an electron field to a constant KE so the lighter ion will reach the detector first
• Why it is necessary to ionise molecules when measuring their mass in TOF mass spec:

  Ions (not molecules) will interact and be accelerated by an electric field and only ions create a current when hitting the detector
• Describe how ions form in TOF mass spec:

  A high voltage is applied to a sample in a polar solvent and the sample molecule gains a proton forming XH+
• Explain the trend in 1st ionisation energies of Group 2 elements from magnesium to barium:

  Decreases. Ions get bigger so there is weaker attraction of ion to lost electron
• Ionic equation of calcium with excess water:

  Role of water:

  Ca (s) + 2H2O (l) --> Ca2+ (aq) + 2OH- (aq) + H2 (g)

  Water is oxidising agent
• Does magnesium or aluminium have lower first IE and why?

  Aluminium. Its outer electron is in the 3p orbital which is further away from the nucleus so easier to remove
• State 2 differences between the plum pudding model and the current model

  Current: Nucleus contains protons and neutrons

  Electrons now arranged in orbitals
• Explain the pattern in the 1st IE of elements from lithium to neon

  1. 1st IE increases, increased nuclear charge, similar shielding and stronger attraction between nucleus and outer electron
  2. B has lower IE than Be, its outer electron is in 2p and higher in energy than 2s
  3. O has lower IE than N, 2 electrons in 2p need to pair, pairing causes repulsion
• Give the meaning of the term relative atomic mass:

  Average mass of 1 atom of an element relative to 1/12 mass of 1 atom of carbon 12
• Define the mass number of an atom:

  Number of protons and neutrons
• Electron Configuration Order:

  1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s


  S holds 2

  P holds 6

  D holds 10
• Explain why the 2nd IE of calcium is lower than the 2nd IE of potassium

  Removing an electron from a Ca+ ion involves removing an electron from the 4s orbital but for K+ it would be from 3p orbital.

  This is closer to the nucleus.

  The Ca+ also has more shielding
• Write an equation to represent the first IE of Sodium


  Na(g) ---> Na+(g) + e-
• Explain why the first ionisation energy is different from that of phosphorus

  Sulfur is the first of the period 3 elements where a pair of electrons occupies an orbital. This pair of electrons in the 3p orbital repel each other making it easier to remove
• Define the term 'relative molecular mass'


  The weighted average mass of one molecule relative to 1/12 of the mass of a carbon-12 atom
• Which statement about the molecules in a sample of a gas is correct?

  At a given temperature they all move at the same speed
• What are the steps for TOF?

  1. Ionisation (Electrospray or Electron Impact)
  2. Acceleration
  3. Ion Drift
  4. Data Analysis
• What are the conditions for TOF MASS SPEC?

  Must be kept under a high vacuum - to prevent the ions that are produced from colliding with molecules present in the air
• Electrospray Ionisation Steps

  Used for large, organic molecules; no fragmentation occurs

  1. Sample is dissolved in a volatile and polar solvent
  2. Injected at high pressure through a fine needle connected to the positive terminal of a high voltage supply (causes tip to have high voltage)
  3. Causes the sample molecule (X) to gain a proton (H+) from the solvent, forming XH+. Equation is: X(g) + H+ ---> XH+(g)
  4. The solvent evaporates into the vacuum while the XH+ ions do not
• Electron Impact Ionisation


  Used for elements/substances with low formula mass, cannot be used with larger molecules as fragmentation will occur

  1. The sample is vapourised and injected at low pressure
  2. An electron gun fires high energy electrons at the injected sample
  3. This causes an outer electron to be knocked out from each particle, forming a 1+ ion. Equation: X(g) ---> X+(g) + e-
• Acceleration Steps - TOF


  The positive ions formed from either ionisation method are accelerated to a constant KE by an electric field towards a negatively charged plate

  • Lighter ions travel fastest (velocity is dependent on mass)
• Ion Drift & Detection

  The positive ions pass through a hole in the negatively charged plate, forming a beam

  The ions travel along the flight tube, where they then reach a detector (also a negatively charged plate)

  The positive ions reach the negatively charged detection plate - they gain an electron and produce a current (current is the flow of electrons from the plate to ions)

  • The amount of current produced is proportional to abundance of species
• Data Analysis - TOF

  Two pieces of info are analysed in combination:

  • The current that is produced
  • The flight tube times

  This produces a spectra with the relative abundance produced by ions with varying mass/charge (m/z) ratios
• TOF Equation

  KE = 1/2mv^2 - KE in J, mass in KG, velocity in ms-1

  t=d/v - time in s, distance/length in m, velocity in ms-1

  Rearranged:

  v=√2KE/m

  t=d√m/2KE
• Give two reasons why the sample must be ionised

  To accelerate (the sample)

  To detect the beam
• State what can be adjusted in the mass spectrometer to enable ions formed by the different isotopes to be directed onto the detector

  The electric field
• Explain how ions are detected and relative abundance is measured in a TOF spectrometer


  Ions hit the detector

  Relative abundance is proportional to the current
• Describe how the molecules are ionised using electrospray ionisation

  Sample is dissolved in a volatile solvent

  Injected through needle at a high voltage

  Each molecule gains a proton