Syllabus notes  

These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

 

Stages of operation:

  1. injection
  2. vaporisation
  3. ionisation
  4. acceleration
  5. deflection
  6. detection
injection: the sample is injected into the vaporisation chamber
vaporisation it is vaporised and the gas streams into the ionisation chamber
ionisation the electron beam knocks an electron off the vaporised particles makong positive ions
acceleration the positive ions are attracted towards the accelerating plates
deflection the magnetic field deflects the lighter ions more than the heavy ions
detection as the magnetic field is varied by the controller, ions with different masses are detected - these are recorded on the mass spectrum.
click on the shockwave demonstration to see all these stages in operation

The angle of deflection of each fragment is proportional to it's mass (actually the mass:charge ratio, but as the charge is always the same and equal to the charge on an electron, but positive, then we can talk about the mass alone), and so it is possible to find the relative atomic mass of each 'spike' the height of the spike represents the frequency, therefore, the abundance can be calculated.

The relative atomic mass is the weighted average of the isotope masses.

Example:

Rubidium has two isotopes Rubidium-85 and Rubidium 87 which have relative abundancies of 72% and 28% respectively.

In 100 atoms there are 72 Rb atoms with a mass of 85 and 28 Rb atoms with a mass of 87

Total mass of the rubidium atoms is:- (72 x 85) + (28 x 87) =8556

Therefore the average mass = 85,56

Rubidium has a relative atomic mass of 85,56