Stages of operation:
|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.
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