4.9 Spectroscopy and Chromatography
Methods of analysis
 Explain the effect of different types of radiation on molecules
and how the effects can be used in chemical analysis and
.following reactions
 Infra Red Spectroscopy
 Mass Spectroscopy
 NMR Spectroscopy
 Gas Chromatography and HPLC
How radiation affects molecules
Many analytical tools rely on equipment using different parts of the
electromagnetic spectrum. The light in our eyes detect just a small
part of the electromagnetic spectrum .The spectrum is made up of all
the electromagnetic radiations, including x rays used by dentists,
infrared radiation of microwaves used to heat food, radiowaves
used to bring us radio and television signals.
The diagrams above show a continuous spectrum of electromagnetic
radiation-called continuous because all the wavelengths are
represented.
Electromagnetic radiation is made up of two components: electrical
and magnetic.
This radiation is one of the ways energy is transmitted through
space. It is why we feel the warmth of the Sun on Earth and why we
get sunburned if we have too much ultraviolet radiation.
EMR is usually thought of as waves. Transverse waves produced by
oscillating electric and magnetic fields at right angles to each other.
See fig 1.7.1 p130 A2 Ed copy the diagram
In a vacuum emr travels at the speed of light, 3x108ms-1. Like all
waves emr can be described by their wavelength(λ), frequency(ƒ) and
speed(c).
c= ƒ x λ
c in ms-1, λ in m, ƒ in s-1 or more commonly as hertz (Hz)
Recap: frequency is the number of cycles in 1 second and
wavelength is the distance between two peaks/troughs !
When the frequency is higher the wavelength is smaller.
Electromagnetic waves can cover a wide range of wavelengths or
frequencies these can be split into 7 overlapping regions. Within
each region the waves have different properties.
Eg
The wavelength of a red traffic light is centred on 700nm. What is the
frequency of this radiation?
 Convert λ to m
700nm = 700 x 10-9m and c= 3x108 m/s
 c= ƒ x λ
3x108= ƒ x 700 x 10-9
ƒ = 4.29 x 1014Hz
(this is the frequency of red light)
Q. Barium nitrate is added to fireworks to colour them. When
heated the compound emits light of frequencies of around 5.45 x
1014 Hz. Calculate the wavelength and use the diagram above to work
out what colour the fireworks will be.
How the different radiations can be used to follow reactions and to
analyse chemicals.
Infrared radiation
Atoms in molecules can vibrate. These vibrations have definite
quantised energy levels. To increase the vibrations from one level to
the next a definite quantised amount of energy has to be absorbed.
These amounts of energy correspond to the radiation in the IR part
of the em spectrum.
When molecules absorb IR radiation they vibrate by stretching or
bending.
Only molecules that change their polarity as they vibrate can absorb
IR radiation. H-H, Cl-Cl will not absorb IR but H-Cl will.
We can use IR spectra to follow the progress of a reaction eg the
conversion of an alcohol to a carboxylic acid.....
Ethanol to Ethanoic acid (see fig 1.7.4 p 131 A2 ed)
Wine contains about 15% ethanol. Oxidation can occur through
contact with oxygen. Partial oxidation produces ethanol which helps
wine mature (useful) however further oxidation produces ethanoic
acid giving wine an unpleasant taste (vinegar!) and odd colour. Wine
makers need to monitor the composition of their wine as well as
tasting it!!
Describe how the spectra (fig 1.7.4) shows the extent of oxidation
Microwaves
Microwave energy is used to heat up our food in ovens. Microwave
energy creates an electric field that causes water molecules in food
to line upwith the field. The electric field then switches to the
opposite direction and that cause the molecules to rotate and line up
again. This causes the energy in microwaves to be changed into
thermal energy which heats up the food. This only happens to polar
molecules present in the food not non polar ones.
Microwaves can speed up chemical reactions significantly.
If water is being used to extract an organic compound dissolved in
tetrachloromethane (CCl4 ). If the organic compound becomes more
soluble in water as the increases then microwaves could be used to
heat the system. The temperature of the aqueous layer would rise
(water layer...water is also a polar molecule) but the temperature of
the non aqueous layer would stay the same (CCl4 is non polar).
Therefore the organic compound would dissolve in the aqueous layer
and there would be better separation.
Separating funnel
aqueous layer at top, non aqueous layer at the bottom ....once the
organic compound is dissolved in the water layer the bottom layer
can be drained off. The top layer contains the organic compound and
water. The water can be evaporated/distilled off depending on
whether the the product is solid or liquid at room temperature.
Radiowaves
These are used in nuclear magnetic resonance spectroscopy. They
help determine the number and environment of hydrogen atoms in
an organic molecule.
Ultraviolet
UV radiation can initiate reactions in the Earths upper atmosphere.
eg O2 dissociates into O atom radicals by homolytic fission. These
radicals can react with other O2 molecules to produce ozone.
Electrons in a molecule are usually at their lowest energy level. If the
molecule absorbs energy the electrons get excited and move up to a
higher energy level. This is called electronic transition. UV and visible
spectroscopy is possible because the outer electrons of
atoms/ions/mc can absorb in the UV and visible region. Compounds
that absorb only in the UV region appear colourless. If they absorb in
the visible region appear coloured.
Chromophores absorb UV or Visible Light. Chromophores are
structural features in a molecule that absorb UV/Vis light such as
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double/triple bonds
lone pairs of electrons
a delocalised electron system
conjugated electron system
If you have to identify whether a molecule will absorb UV/Vis
radiation look for the above.
Draw examples of each of the following so that you could recognise
them within a molecule.
double beam UV-visible absorption spectrometer
http://www.chemguide.co.uk/analysis/uvvisible/spectrometer.html
 the above link explains how the instrument works, summarise
the information in your own words
The shape of the absorption peak is characteristic of a particular
compound so can help identify compounds.
It is more commonly used to measure concentrations. Useful in the
steel industry to monitor the amount of trace metals ie manganese,
in the meat industry it can be used to determine the amount of
nitrates in meat.
Explain what Photolysis is. Draw the diagram showing how the
instrument works. Why is this technique useful?