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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.

Without instrumental techniques, none of Hodgkin’s, or her fellow scientists’, work would have been possible.

She later used the same technique to investigate other biological molecules, including cholesterol and insulin.

She did this by developing an instrumental technique called x-ray crystallography.

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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.

As the solvent moves up the paper, the pattern of the single dyes can be compared to that of the mixture.

Dots of single dyes are placed alongside a dot of the unknown mixture.

Which dyes does the mixture contain?

chromatogram

The solvent is drawn up the paper by capillary action.

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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.

The mobile phase is a solvent which travels up the plate, carrying the substances.

In paper chromatography, what are the stationary and mobile phases?

glass plate

silica gel

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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 performance-enhancing drugs – including racehorses!

Urine samples are collected from the horses at events, and then sent to labs to be tested by gas chromatography.

High-level competition horses are regularly tested for banned substances, such as painkillers that help them run through injury, or steroids that reduce inflammation.

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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 toidentify 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.

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.

Spectroscopy will allow them to get a very detailed view of how its atoms fit together.

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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 muchmetal 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:

radio waves

ultra violet and visible light

infrared

Electromagnetic radiation

protons in nuclei

electrons in atoms

electrons in bonds

Absorbed by

nuclear magnetic resonance spectroscopy

atomic absorption spectroscopy

infrared spectroscopy

Spectroscopic technique

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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 in2003, 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