Analytical Techniques

This is not mine, it is from this lovely website here:

They are from the AQA section but are useful to anyone that does analytical techniques in their A-level

 These notes go into more detail than is necessary, and I am hoping to upload condensed versions soon :)

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  • Created on: 31-05-13 20:12
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Topic 2.11
High Resolution Mass Spectrometry
Infrared Spectroscopy

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The technique of mass spectrometry was used in Unit 1 to:
a) determine the relative abundances of different isotopes in a sample of an element and
hence to deduce relative atomic masses
b) identify the peak with the largest m/z ratio in a spectrum of a molecule and hence
determine the rmm of the molecule
On closer inspection of the molecular ion peak, it is actually possible to reveal more information
than just the relative molecular mass.…read more

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a) Introduction
All molecules contain covalent bonds and these bonds are constantly rotating, bending and
stretching with characteristic frequencies. When these bonds are subjected to infrared radiation
with the same frequency, they will bend or stretch more and the radiation of that frequency will
be absorbed. Each bond absorbs one or two distinct frequencies only, so when infrared
radiation containing a full range of frequencies is passed through a sample of an organic
compound, some frequencies are absorbed and others pass through unaffected.…read more

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Any other bonds present, however, will give more distinctive peaks:
Bond Frequencies
absorbed/ cm1
CO 1120 1310 (sharp)
C=C 1620 1690 (sharp)
C=O 1630 1815 (sharp)
OH (alcohols) 3230 3350 (broad)
OH (acids) 2500 3000 (broad)
CO 1000 1300 (sharp)
It is easy to recognize the peaks due to the presence of the C=O and OH bonds in particular,
as the absorbance is very high (the transmittance is low). Note that whenever a bond is also
involved in intermolecular hydrogen bonding, the peak becomes broad.…read more

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CO and OH):
Note the strong broad absorption at 3100 ­ 3500 cm1, characteristic of an OH bond in an
alcohol. Note also the sharp peak at 1050 cm1 resulting from the CO bond.
iii) carboxylic acids (C=O, CO and OH):
Note the very broad absorption at 2500 ­ 3000 cm1, characteristic of an OH bond in a
carboxylic acid (it tends to mask the CH peak completely). Note also the sharp peaks at 1710
cm1 (C=O) and 1300 cm1 (CO).…read more

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Identifying impurities
Often organic compounds contain impurities. The extent and nature of these impurities can be
determined using infrared spectroscopy. The bonds present in the impurity but not in the
desired product will absorb radiation, giving peaks that would not be expected given the
structure of the molecule.
Eg a sample of ethanal is believed to be impure and an infrared spectrum of the sample is
obtained.…read more

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Global warming
The ability of covalent bonds in molecules to absorb infrared radiation is very useful for
identifying molecules, functional groups and impurities. It is also, however, the cause of global
Radiation from the sun (including visible and ultraviolet light) is absorbed by the earth and
emitted at a lower frequency (usually in the infrared region).…read more


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