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What are instrumental techniques?
Modern chemists use a range of instruments to analyse and
identify substances. Most produce quantitative data, which
requires expert interpretation.
There are many different types of machine used for analysis,
each producing a different type of information, such as:
 whether a substance is pure or a mixture
 the molecular mass of a compound
 the types of bonds in a molecule
 the arrangement of atoms in a molecule
 the isotopes of different atoms in a substance.
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Chemical identification
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Instrumental techniques and Nobel prizes
Many scientists have been rewarded with Nobel prizes for
their work uncovering the structures of complex molecules.
In 1964, Dorothy Hodgkin won a Nobel
prize for discovering the structures of
vitamin B12 and penicillin.
She did this by developing an
instrumental technique called x-ray
crystallography.
She later used the same technique to
investigate other biological molecules,
including cholesterol and insulin.
Without instrumental techniques, none of Hodgkin’s, or her
fellow scientists’, work would have been possible.
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Using instrumental techniques
A small amount of the substance
under investigation is placed
inside a machine, which then
analyses its chemical contents.
The scientist is then able to
interpret and evaluate the results,
and identify the elements and
compounds in the substance.
This could be for forensic, health
or environmental purposes.
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Advantages of instrumental techniques
Instrumental techniques are very powerful. They have many
advantages over more old-fashioned chemical methods of
analysis. This is because they:
 do not usually damage the substance during testing
 are much more sensitive than chemical techniques
 can identify all the substances in a mixture
 require only tiny amounts of a substance
 are reliable and accurate
 are quick.
However the test results often
require expert interpretation.
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Instrumental techniques: pros and cons
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Paper chromatography
Paper chromatography is
used to separate mixtures,
especially dyes or pigments.
chromatogram
Dots of single dyes are placed
alongside a dot of the
unknown mixture.
The solvent is drawn up the
paper by capillary action.
As the solvent moves up the
paper, the pattern of the single
dyes can be compared to that
of the mixture.
Which dyes does the mixture contain?
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Which ink?
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Rf values
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Calculating Rf values
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Thin layer chromatography
All chromatography involves a stationary phase and a
mobile phase.
In thin layer chromatography
(TLC) the stationary phase is a
layer of silica gel fixed onto a
glass plate.
glass plate
The mobile phase is a solvent
which travels up the plate,
silica gel
carrying the substances.
In paper chromatography,
what are the stationary and
mobile phases?
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How does TLC work?
TLC uses the same principals as paper chromatography.
Capillary action still draws the solvent up the matrix; however
while the molecules in paper chromatography are separated
based on mass, in TLC, separation often depends upon
solubility or charge, due to the interaction of solute and matrix.
A dry sample is placed in the silica gel matrix. As the solvent
front moves up the gel, it dissolves the sample and carries it
up the matrix with it.
Some of the particles in the sample stick more strongly to the
silica gel than others, so they lag behind the solvent.
Eventually the different substances in the sample separate
out, with similar molecules travelling a similar distance.
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TLC or paper chromatography
Thin layer chromatography has a number of advantages over
paper chromatography.
1. The glass plate is rigid, not flexible like paper, so it is easy
to control.
2. After separation, the substances in the mixture can be
recovered. The silica gel holding the separated substance
is scraped off the glass plate and added to a solvent.
The substance will dissolve and the silica gel can easily
be removed by filtration.
The glass plates can be re-coated with
silica gel and used over and over again.
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UV and locating agents
Many substances are white or colourless,
and so aren’t visible on a TLC plate.
One way of making
colourless substances
show up is to use UV light.
This usually works well for
organic compounds.
An alternative method is to use a chemical locating agent –
a chemical that reacts with the substance to form a coloured
compound.
For example, when ninhydrin is exposed to an organic
compound it stains it purple-brown.
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Chromatography: true or false
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Gas chromatography
Gas chromatography (GC)
is used widely in many
analytical laboratories,
including forensic police
labs, synthetic chemical
labs, and drugs testing labs.
In paper and thin layer
chromatography, the
mobile phase is a liquid.
However, as the name implies, the mobile phase in GC is an
inert gas. The stationary phase is usually a long thin tube of
silica gel.
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How does gas chromatography work?
Like all forms of chromatography, GC uses a stationary
phase to impede the movement of a mobile test substance.
Different substances are attracted to the matrix by different
amounts, and therefore journey along it at different speeds.
In GC, the sample is injected into the machine, where it is
vaporized. It is then washed over the matrix by an inert gas.
Some substances will be more attracted to the matrix than
others. These will take much longer to reach the detector.
The detector measures the abundance of a substance
at a given time, and this data is plotted on a graph.
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Using gas chromatography
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Testing for banned substances using GC
All kinds of athletes are banned from taking performanceenhancing drugs – including racehorses!
High-level competition horses are regularly tested for banned
substances, such as painkillers that help them run through
injury, or steroids that reduce inflammation.
Urine samples are
collected from the horses
at events, and then sent
to labs to be tested by
gas chromatography.
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Testing for banned substances
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What is spectroscopy?
Spectroscopy is the process of
investigating substances using
electromagnetic radiation.
There are many different
spectroscopic techniques, each
using a different frequency of
electromagnetic radiation,
including UV and visible light,
infrared, radio waves and x-rays.
Spectroscopy allows chemists to
identify elements and investigate the detailed structure of
compounds (including bonding and atom arrangement).
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How do chemists use spectroscopy?
Spectroscopic techniques are often the first thing a chemist
might turn to in the analysis of a new chemical.
When they find an interesting natural
product, such as a molecule from tree
bark which may have anti-cancer
applications, they need to
understand its structure.
Spectroscopy will allow them
to get a very detailed view of
how its atoms fit together.
When chemists carry out a reaction, they need to find out
what they have made, and spectroscopy is the quickest and
most reliable way of doing so.
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Atomic absorption spectroscopy
Atomic absorption spectroscopy (ABS) is a technique that
allows elements to be identified, and their concentration
measured down to just a few parts per billion.
ABS has many uses:
 environmental chemistry – to analyse
pollutant concentrations in air and water
 medicine – to analyse concentrations of
toxic chemicals in blood and urine
 building – to check for impurities in
concrete and steel
 mining – to check how much
metal is in an ore.
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How does spectroscopy work?
All spectroscopy uses the principle that electromagnetic
radiation can be absorbed by atoms and molecules. Different
parts of a molecule absorb different frequencies of radiation:
Electromagnetic
radiation
Absorbed by
Spectroscopic
technique
radio waves
protons in nuclei
nuclear magnetic
resonance
spectroscopy
ultra violet and
visible light
electrons in atoms
atomic absorption
spectroscopy
infrared
electrons in bonds
infrared
spectroscopy
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How does IR spectroscopy work?
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MRI and NMR
Magnetic resonance
imaging (MRI) scans are
often used in hospitals to
provide images of bone and
tissue. The images are made
by investigating the nuclei of
atoms with radiowaves.
Nuclear magnetic resonance
spectroscopy (NMR) is
another name for MRI.
Why do you think doctors
choose to use the term
MRI instead of NMR?
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NMR and Nobel prizes
Four Nobel prizes have been awarded to scientists for their
work with NMR spectroscopy.
 Nobel Prize for Physics – awarded to Felix Bloch and
Edward Purcell in 1952, for demonstrating the principles of
the technique.
 Nobel Prize for Chemistry – awarded to Richard Ernst in
1991 for his development of better NMR techniques;
and in 2002 to Kurt Wüthrich for his investigation of the
structures of complex biological molecules.
 Nobel prize for Medicine – awarded to
Paul Lauterbur and Peter Mansfield in
2003, for their work on developing the
technique of MRI scans.
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Spectroscopy: summary
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Glossary
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Anagrams
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Multiple-choice quiz
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