Mass Spectrometer.

GCSE Science

Mass Spectrometer

Used to determine

the relative isotopic masses and abundance of isotopes
the relative molecular mass (Mr) and abundance of the organic compound

Principles of the mass spectrometer

Apparatus enclosed in total vacuum, so that there are no colisions between the sample being investigated and the atmospheric air or the residue from previous samples.

Vacuum pump is to reduce pressure so less thermal energy is needed to vaporise the sample. The pump is to remove any traces of the previous sample traces of the air.

Sample under analysis must be vaparised i.e. converted to gaseous state. This is achieved by heating it in the furnace.

Sample then enters the ionisation chamber; here the sample is bombarded by a beam of high energy electrons. A beam of these high energy electrons bombard the atoms causing them to loose an electron. A removal of an electron from the atom is known as ionisation. This results in the formation of positively charged ions (cations), mainly single charged ions.

If the sample is simple the molecules are ionised by being bomdarded by high energy ...

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Sample under analysis must be vaparised i.e. converted to gaseous state. This is achieved by heating it in the furnace.

Sample then enters the ionisation chamber; here the sample is bombarded by a beam of high energy electrons. A beam of these high energy electrons bombard the atoms causing them to loose an electron. A removal of an electron from the atom is known as ionisation. This results in the formation of positively charged ions (cations), mainly single charged ions.

If the sample is simple the molecules are ionised by being bomdarded by high energy electrons, causing bonds to vibrate and weaken, some bonds between molecules to produce small pieces of the original molecule known as smaller fragments and/or free radicals.

Smaller pieces of the original lecule are known as moecular ions (M+) and are positively charged. Molecular can further fragment to form even smaller pieces, some of which may have a positive charge or no charge at all. Fragments with no charge are known as free radicals.

Ions are deflected (bend) along the circular path by the variable magnetic field. The lighter ions are deflected more then the heavier ones. The speed at which the ions pass via the circular path depends on their m/e. The strength of the magnetic field is varied to ensure ions of different m/e (mass to charge ratio) reach the detector.

The intensity or the ion beam is picked up by the detector. Only gaseous ions with particular mass: charge ratio will hit the detector (fragments with no charge will not be detected). The detector counts the number of ions striking it . The signal produced by the detector is proportional to the number/or the abundance of ions striking it.