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Used to measure the molecular mass of a substance.
In a mass spectrometer, the sample is ionised firstly by firing high-speed electrons at it, and then
accelerated. The time taken for the sample to travel across a drift region and be picked up by a
detector is measured. This can then be used to calculate the relative atomic mass, from which it can
be estimated what the sample is.
Interpreting mass spectra:
1. Identify the molecular ion peak (usually the furthest right). It may be very small if the mass
spectrum is mainly composed of fragments.
2. List masses of the other major peaks, and work out the difference in mass between them
and the molecular ion peak. These mean you can work out the fragment responsible for the
3. Identify any isotope peaks. Eg. For any molecule containing Carbon atoms, there will always
be a small (M+1) peak created, caused by 13C. If it contains Cl or Br, it will have an (M+2) peak
as well. With Br, it should be similar in height to the M peak.
Common fragments to learn:
Fragment Ch3+ C2H5+ C3H7+ OH+ C6H5+ (Phenyl ion)
Molecular Mass 15 29 43 17 77
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1. A beam of Infrared radiation is fired through the substance.
2. IR energy is absorbed by bonds in molecules, increasing vibrational energy.
3. Different bonds will absorb different wavelengths. Also, the location of a bond within a
molecule will impact upon the wavelengths of IR it absorbs.
4. If you look at a table, you can work out what the bonds in a molecule are, and therefore the
functional groups likely to be present.
Troughs = radiation that has been absorbed.…read more
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Used to separate and identify molecules.
1. Stationary phase is a thin layer of silica or alumina fixed to a glass or metal plate. (Silica plate)
2. Pencil line is drawn near the bottom of the plate, and a small drop of each sub stance is
placed on the line.
3. Plate is put in a beaker with a small volume of solvent below the level of the baseline. The
solvent is the mobile phase.