1.2 Mass number and isotopes

• Where do you find the mass number of an element?

  Top number on the periodic table
• What is an isotope?

  An isotope is a variant of an element that has the same number of protons but a different number of neutrons in its nucleus.
• What is the relative mass of a proton?

  1
• Where are the electrons?

  In the shells
• Relative atomic mass - the average mass of an atom compared to the mass of carbon-12, which is 12.
• Relative isotopic mass - the average mass of an isotope of an atom compared to the mass of carbon-12, which is 12.
• Relative molecular mass - the average mass of a molecule compared to the mass of carbon-12, which is 12.
• The mass spectrum is essentially a fingerprint for a molecule that can be identified by computers using a spectral database.
• 4 stages of mass spectroscopy:

  1. ionisation
  2. acceleration
  3. deflection
  4. detection
• Ionisation - gaseous atoms are bombarded by electrons from an electron gun and are IONISED. Sufficient energy is given to forms ions of 1+ charge.
• Acceleration - ions are charged so can be ACCELERATED by an electric field
• Deflection - charged particles will be DEFLECTED by a magnetic or electric field
• Detection - by electric or photographic methods
• How does deflection work?

  • the radius of the path depends on the values of the mass/charge ratio
  • ions of heavier isotopes have larger m/z values so follow a larger radius curve
  • as most ions are 1+ charged the amount of separation depends on their mass
  • if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved
• Doubling the charge halves the m/z value. Abundance stays the same.
• Time of flight mass spectrometer:

  1. ionisation
  2. acceleration
  3. ion drift
  4. detection
• Electrospray ionisation - the sample is dissolved and pushed through a small nozzle at high pressure and a voltage is applied to it.

  M + H+ -> MH+ (ion weighs +1 Mr)
• Electron impact ionisation - the sample is vaporised and an electron gun is used to fire high energy electrons to knock off an electron from each molecule.

  M + e- -> M+ + 2e- (ions weighs same as Mr)
• Acceleration

  • The positively charged ions are accelerated by an electric field so they all have the same kinetic energy.
  • Lighter ions move faster than the larger ones.
• Ion drift

  • Ions enter a region with no electric field
  • Ions drift through
  • Lighter ions will drift faster than the heavier ones
• All ions are accelerated by an electric field into 'field-free' drift region with the same kinetic energy.
• KE = 1/2 x mass x velocity squared
• velocity = distance / time
• Detection

  • Lighter ions move quicker, therefore reach the detector in less time than heavier ones.
  • The detector detects charged particles by ions gaining electrons here
  • Abundance determined by size of current flowing in detector (current is proportional to abundance)
• Uses of mass spectrometry

  • Monitoring breath of patients under anaesthesia
  • Analysing molecules in space
  • Detecting traces of toxic chemicals in contaminated marine life
  • Identifying unknown compounds
  • Detecting banned substances such as steroids in athletes
  • Determine the abundance of each isotope of an element
  • Gain further information about the structure and chemical properties of molecules
• Explain how ions are accelerated, detected and have their abundance determined in a time of flight (TOF) mass spectrometer.

  Ions are accelerated by attraction to negatively charged plate and are detected by gaining electrons. Abundance determined by current flowing in the detector.
• Outline how the TOF mass spectrometer is able to separate these two species to give two peaks.

  Positive ions are accelerated by an electric field to a constant kinetic energy. The positive ions with m/z of 104 have the same kinetic energy as those with m/z of 118 and move faster. Therefore, ions with m/z of 104 arrive at the detector first.
• Calculating Relative Atomic Mass

  1. Read off the relative isotopic abundance and the relative isotopic mass and multiply them.
  2. Add these totals up'
  3. Divide by the sum of all relative isotopic abundances (100 if it's a percentage)
• Molecular ion, M+ is the positive ion formed in mass spectrometry when spectrometry when a molecule loses an electron.
• Fragmentation is the process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion.
• The high energy electrons can produce the molecular ion and cause the molecules to fragment in the process named fragmentation.

  C2H5OH+ -> CH3 + CH2OH+

  This results in a positive fragment ion and a neutral species.
• What does mass spectrometry?

  We can determine the molecular mass of a molecule by using mass spectrometry by locating the M+ ion peak. This produces the peak with the highest m/z value in the mass spectrum.
• We can use fragments to determine the structure of an unkown compound to (in most cases) give its precise identity. This is because

  • Although the molecular ion peak will have m/z value, the fragmentation patterns will be different.
  • Organic compounds produce a unique mass spectrum, which is used as a fingerprint.