Analytical Techniques for AQA AS Level Chemistry Unit 2

Notes on analytical techniques: high-resolution mass spectrometer and IR spectra

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Analytical Techniques
Analytical techniques are used to determine the molecular formula and structures (of which there may be
multiple, due to positional isomerism) of compounds. Different techniques are often used in combination.
Mass spectrometry
This is used to determine the m/z value of a molecular ion and their relative abundance and it can be
used to determine the molecular formula of a compound
This works by ionisation of gaseous compounds with a high-energy electron gun and is accelerated,
then deflected (where different masses of compounds are deflected differently), then detected by a
detector by generating an electric current
Usually, the m/z value for an ion is equal to the molecular mass (Mr) of a compound
If the molecule has been fragmented, there will be more than one peak which represents the different
components of the compound, but the m/z value for the compound is found by finding the parent ion
peak ­ the far right peak
Some molecular ions have the same Mr value to the nearest integer. For example, C6H12, C5H8O, C4H4O2
and C4H8N2 all have an Mr of 84, but if more precise values taken by a high-resolution mass
spectrometer were used, then these could be distinguished
The precise m/z value can then be compared against a database of other compounds and a computer
can then show the molecular formula of the compound
This can show the presence of different isotopes, as these will have different m/z values due to the
different masses, e.g. Cl-35 and Cl-37. As a result, two peaks will show up in compounds with chlorine
such as in chloroalkanes, in which there will be two peaks two mass units apart
Disadvantages: It does not distinguish between different isomers because these have the same
molecular formula but different structural formula, e.g. C3H6O has 9 structural isomers
Infrared Spectroscopy
This is used to determine the types of bonds in a compound and therefore it is used for the
identification of functional groups in organic compounds
It can be used to distinguish between functional group isomers such as aldehydes and ketones which
have different functional groups and therefore different types of bonds in their structure
Each type of compound has its own unique molecular fingerprint which can be used, compared against
a database of compounds to identify a compound
This works by shining polychromatic (more than one wavelength) infrared light onto a gaseous
compound which absorbs the infrared light. Different bonds absorb different wavelengths either by
bond-stretching or by bond-bending
Light that goes through the gaseous compound is analysed for the transmittance (the amount that
goes through) of the various wavelengths in infrared light (shown in graphs as the wavenumbers in cm-1
which are reciprocal wavelengths: 1/) and these can be interpreted to show different bonds and
therefore the functional groups
It can also show the presence of impurities in a mixture as these will show up as extra peaks where
there should be no peaks (for example, a mixture of alcohols and ketones would show peaks at the
regions where there is absorption for O-H bonds and C=O bonds)
The region below 1500cm-1 wavenumbers is known as the fingerprint region, which does not show
individual peaks and is complex due to many different bonds absorbing, and it this region is unique and
can be compared using a computer or database


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