IB syllabus > atomic theory > 12.1 

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

12.1 - Mass Spectrometer

12.1.1: State the principles of a mass spectrometer and outline the main stages in its operation.

The mass spectrometer is an instrument used for two main purposes:

1. Measuring the exact masses of atoms.

2. Measuring the masses of the breakdown products from molecules when they are smashed to pieces by high energy electrons. This is also called the fragmentation pattern and may be useful in elucidation of the structure of a molecule.

The operating principles are the same in both cases. A sample is injected into the instrument and it is vapourised before meeting a stream of high energy electrons that turn the atoms into ions (by dislodging electrons) or, if we are dealing with molecules, causes the molecules to break apart (fragment). The ions that are produced in each case are separated by magnetic fields and detected with a high degree of accuracy.

The final read-out may be graphical or digital and gives information about the relative abundance of all of the particles produced by the stream of electrons as well as their exact masses. A typical graphical read-out for the analysis of an element looks like this:

Here we can see that there are two peaks in the mass spectrum, one at m/e (this means mass to charge ratio) = 20 and the other at m/e = 22. T

hese peaks correspond to the ions produced from the diferent isotopes of Neon. As neon has two common isotopes Ne20 and Ne22, any naturally occurring sample of neon will contain these two isotopes. The mass spectrum shows that the peaks are in the ratio 10:1 and so there is 10 times as much Ne20 as Ne22 in the sample. From this data the relative atomic mass of neon can be calculated.

RAM = [(10 x 20) + (1 x 22)]/11

Spectra of molecules are rather more complex due to the breakup (fragmentation) of the molecule in the electron beam.

Here we can see that there is a fragmentation pattern caused by the molecule breaking apart in the electron bombardment.

The molecule is shown on the spectrum and the most important peak is the one at m/e = 116 which gives the relative molecular mass of the molecule. This peak is said to be due to the "molecular ion" and is caused by the molecule itself losing only one electron before going to the detector.

The m/e value of the molecular ion can be measured to such a degree of accuracy (many decimal places) that it can be used to determine the exact number of each type of atom within the molecule.

A full treatment of the fragmentation pattern is possible to give information regarding how the molecule is bonded together but this is not needed for this section. Fragmentation patterns are dealt with in option H Further analytical chemistry

 

Mass spectrometer 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.


12.1.2: Describe how the mass spectrometer may be used to determine relative isotopic, atomic and molecular masses using the 12C scale.


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


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