1.3 - Comparing Masses of Atoms - Mass Spectometry

• atomic number

  number of protons in the nucleus of an atom
• mass number


  sum of number of protons and neutrons in the nucleus of an atom
• nuclear charge

  total charge of all protons in the nucleus

  same value as the atomic number

  increases as you go across the periodic table
• molecular ion peak

  the peak with the highest mass to charge ration (m/z) in the mass spectrum, the M peak
• mass spectrometry

  comparing the masses of atoms
• mass spectometer

  separates atoms and molecules according to their mass and shows the relative numbers of the different atoms and molecules present
• how a mass spectometer works(summarised)

  beam of high energy electrons bombards sample so it's converted into positive ions in gaseous form

  charged particles are separated to produce a mass spectrum - distinguishes ions based on their mass-to-charge ratio
• ways of separating ions in a mass spectometer

  . using electric field which accelerates ions into an magnetic field which then deflects in onto a detector

  . by accelerating ions then separating them by their flight time through a field free region

  . transmission quadrupole instrument: varies fields in it subtly so ions with a particular mass to charge ratio can pass through detector
• why there is a high vacuum inside a mass spectometer

  to allow sample being tested to be studied with no interference from atoms or molecules in air (so ions don't collide with molecules)
• how a mass spectometer works

  . vaporised sample is put in mass spectometer where it's ionised by a coil firing e- and charged plates

  . ions are accelerated and pass through electromagnets which generate a magnetic field causing ion to deflect (how much it deflects depends on its mass and charge)

  . if ion hits wall it's lost

  . ions of different masses will hit different parts of detector which is connected to an amplifier and a chart recorder

  
• why ions need to be accelerated in a mass spectometer

  so that particles move at the same velocity so are all affected by electromagent similarly
• why particles need to be ionised in a mass spectometer

  to allow them to be accelerated, deflected, and detected

  charged particles generate current in detector
• how particles can be ionised in a mass spectometer

  electrospray ionisation

  electron impact ionisation
• electrospray ionisation

  sample is dissolved in volatile, polar solvent and injected through a fine hypothermic needle creating fine aerosol; high voltage applied to tip of needle so sample becomes ionised

  soft ionisation (less energy applied)

  used for substance with a higher molecular mass, e.g.: biological molecules
• electron impact ionisation

  gaseous sample is bombarded by a beam of energetic electrons producing ions

  hard ionisation technique (large number of lower mass fragments)

  used for organic/inorganic molecules with a low formula mass
• electromagnet in mass spectometer

  its strength will alter how strongly/weakly ions are ionised as they pass through electromagnet based on mass and size of ion
• use of mass spectometer

  identify individual masses of ions of atoms and molecules
• interpreting mass spectrum - e.g.: chlorine

  diatomic molecule with 2 isotopes so molecules can be made of:

  Cl-35 + Cl-35 (most abundant as Cl-35 is the most abundant)

  Cl-35 + Cl-37

  Cl-37 + Cl-37 (least abundant as Cl-37 is the least abundant)

  A) chlorine molecules (diatomic) - m/z is double Cl ions

  B) Cl molecules made of Cl35, Cl37 or both

  C) 2 isotopes: Cl-35 and Cl-37