Mass spectroscopy key points

This article lists the important points on mass spectroscopy that AQA ask again and again …

• Mass spectroscopy is a process which is used to identify the masses and abundances of isotopes of an element in a sample.
• It can also be used to find the relative molecular mass of an organic compound.
•  There are four stages: ionisation à acceleration à deflection à detection
• Ionisation: Vaporised sample is placed in an ionisation chamber and bombarded with high energy electrons fired from an electron gun. These knock off an outer shell electron forming a M⁺ ion.
•  Acceleration: The ions are attracted towards a negative charged plate which accelerates them through the vacuum.
• Deflection: The beam of ions is deflected by a magnetic field which deflects ions with a high mass/charge ratio less than those with a low mass/charge ratio.
• Detection: The strength of the magnetic field is steadily increased until the beam is focussed on the detector. As the ions strike the charged plate they set up a current whose magnitude is proportional to the number of ions that strike.
• In the mass spectrum of an element below: The number of peaks = the number of isotopes in the element, the relative abundance (sometimes expressed as % abundance) is the relative amount of each isotope in the sample and the m/z ratio is the mass to charge ratio of each isotope. As the charge is usually +1 in a mass spectrometer, it is the same as the relative isotopic mass.
• Ions with a charge of +2 would give a peak with low abundance at a value of ½ the m/z ratio of the parent ion. E.g. ⁶³Cu⁺ has an m/z of 63, but ⁶³Cu²⁺ has an m/z of 31.5. The abundance is low because the amount of energy needed to form 2+ ions is normally too large for the mass spectrometer.