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Chemistry Unit 2 Analytical Techniques
Mass spectrometry - Forming a molecular ion
The electron gun knocks an electron from the molecule, forming a molecular
ion. The molecular ion will fragment to produce smaller cations and free
radicals and make up the spectra.
The peak with the highest m/z value will be caused by the molecular ion. If
the molecular ion is unipositive then the m/z value equals the Mr of the
Other peaks are caused by stable fragments and are shown as a
percentage of the most stable fragment (the base peak).
In any spectra there will be a tiny peak one unit along from the molecular ion
peak; this is due to the 13C isotope.
A pair of atoms joined by a chemical bond is always vibrating. When you
shine a beam of infra-red radiation though a sample the bonds absorb the
energy and vibrate more. Bonds can only absorb radiation with the same
frequency as the natural frequency of the bond.
Radiation that emerges from the sample will be missing the frequencies that
correspond to the bonds in the sample. The instrument plots a graph of the
intensity of the radiation emerging from the sample (transmittance) against
frequency of radiation. It's expressed as a wavenumber (cm-1).
The dips in the graph (peaks) represent particular bonds. The area below
1500cm-1 usually has many peaks caused by complex vibrations; it is unique
for any particular substance and is called the fingerprint region.