12.1 - Mass Spectrometer
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 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:
|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.
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