ANALYSING SUBSTANCES A guide for GCSE students KNOCKHARDY PUBLISHING 2010 SPECIFICATIONS.

ANALYSING ANALYSING SUBSTANCESSUBSTANCES

A guide for GCSE studentsA guide for GCSE students

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2010 2010

SPECIFICATIONSSPECIFICATIONS

ANALYSING SUBSTANCESANALYSING SUBSTANCES

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected GCSE Chemistry topics. It is based on the requirements of the AQA specification but is suitable for other examination boards.

Individual students may use the material at home for revision purposes and it can also prove useful for classroom teaching with an interactive white board.

Accompanying notes on this, and the full range of AS and A2 Chemistry topics, are available from the KNOCKHARDY WEBSITE at...

www.knockhardy.org.uk

All diagrams, photographs and any animations in this Powerpoint are original and created by Jonathan Hopton. Permission must be obtained for their use in any work that is distributed for financial gain.

All diagrams, photographs and any animations in this Powerpoint are original and created by Jonathan Hopton. Permission must be obtained for their use in any work that is distributed for financial gain.

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CONTENTSCONTENTS

• What is chromatography?

• Paper chromatography

• Gas liquid chromatography

• Mass spectrometry

• Questions

ANALYSING SUBSTANCESANALYSING SUBSTANCES

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CHROMATOGRAPHYCHROMATOGRAPHY

Chromatography is used to separate and analyse small amounts of mixtures

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Methods involve a stationary phase (stays where it is) and a mobile phase (moves)

There are several forms of chromatography

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Methods involve a stationary phase (stays where it is) and a mobile phase (moves)

There are several forms of chromatography

TYPE STATIONARY PHASE MOBILE PHASE

paper solid (filter paper) liquid

thin layer (tlc) solid (silica) liquid

column solid (silica) liquid

gas liquid (glc) solid or liquid gas

However, they all work in the same way

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PAPER CHROMATOGRAPHYPAPER CHROMATOGRAPHY

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Stationary phase chromatography paper

Mobile phase suitable solvent (water, ethanol, organic solvent)

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Separation As the solvent moves up the paper it dissolves thecomponents and moves them up the paper. The

more soluble a component is, the further it moves.

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Uses Separating the colours in Smarties®

Separating the colours in, and identifying, inks

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Place small a spot of the mixture to be analysed (and any possible component for comparison purposes) on the paper. Dip the paper in the solvent.

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Allow the solvent to rise up the paper. Each component dissolves in the solvent. Those which are more soluble travel further up the paper.

SOLVENT FRONT

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Allow the solvent to rise up the paper. Each component dissolves in the solvent. Those which are more soluble travel further up the paper.

Finished chromatogram

SOLVENT FRONT


Rf value Under similar conditions, a component should always travel at the same speed.

Its identity can be found by comparing the distance it moves relative to the solvent.

Rf = distance travelled by the component = Y distance travelled by the solvent X

X Y

SOLVENT FRONT

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Rf value Under similar conditions, a component should always travel at the same speed.

Its identity can be found by comparing the distance it moves relative to the solvent.

Rf = distance travelled by the component = Y distance travelled by the solvent X

Comparison can be a problem if…

a) components have similar Rf values

b) the unknown substance is new and there is no previous chemical to compare it with

X Y

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GAS LIQUID CHROMATOGRAPHY (GLC)GAS LIQUID CHROMATOGRAPHY (GLC)

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Stationary phase liquid adsorbed onto a solid support material

Mobile phase gas

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Mobile phase gas

Method

• a very small amount of a sample is injected into the machine

• the injector is contained in an oven

• the sample boils and is carried along a thin column by an inert carrier gas

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Mobile phase gas

Method

• a very small amount of a sample is injected into the machine

• the injector is contained in an oven

• the sample boils and is carried along a thin column by an inert carrier gas

• the column contains a liquid stationary phase, adsorbed on an inert solid

• the time taken to travel through the tube will depend on how much time is spent moving with the gas rather than being attached to the liquid.

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Retention time The time taken for a compound to travel through thecolumn to the detector.

It is measured from the time the sample is injected tothe time its peak shows maximum height.

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For a particular compound, the retention time depends on...

boiling point high boiling point = long retention time

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solubility in the liquid phase greater solubility = long retention time

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solubility in the liquid phase greater solubility = long retention time

ANIMATION

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Interpretation

• each compound in the mixture will produce a peak

• the areas under the peaks are proportional to the amount of a compound

• retention times are used to identify compounds – they are found out by putting known compounds through the system under similar conditions

The area under a peak is proportional to the amount present.

Because each compound responds differently, the machine is calibrated beforehand to show the actual mount.

Because each compound responds differently, the machine is calibrated beforehand to show the actual mount.

Each component has a different retention time.

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GAS CHROMATOGRAPHY – MASS SPECTROMETRY (GCMS)GAS CHROMATOGRAPHY – MASS SPECTROMETRY (GCMS)

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Process When a peak is detected in gas chromatography, some ofthe component is sent to a mass spectrometer

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Mass spectrometers were first used to identify thepresence of different isotopes.

Today, they are mainly used to work out relative molecular (formula) mass and identify unknown molecules.

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A mass spectrometer has three main parts...

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IONISER

ANALYSER DETECTOR

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Ioniser - the sample is bombarded with electrons and ionised- a positive molecular ion is formed- the molecular ion can break up into smaller ions

IONISER

ANALYSER DETECTOR

PARTICLES MUST BE IONISED SO THEY CAN BE ACCELERATED AND DEFLECTEDPARTICLES MUST BE IONISED SO THEY

CAN BE ACCELERATED AND DEFLECTED




Ioniser - the sample is bombarded with electrons and ionised- a positive molecular ion is formed- the molecular ion can break up into smaller ions- positive ions are accelerated towards the analyser

IONISER

ANALYSER DETECTOR

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Analyser - positive ions separate according to mass/charge ratio- higher mass/charge ratio (heavier) = smaller deflection

IONISER

ANALYSER DETECTOR

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Analyser - positive ions separate according to mass/charge ratio- higher mass/charge ratio (heavier) = smaller deflection

Detector - records the identity and abundance of each ion- compounds have a unique mass spectrum- the final peak (molecular ion) gives the molecular mass

IONISER

ANALYSER DETECTOR

IONISATION

FRAGMENTION

FRAGMENTIONRE-ARRANGEMENT

Mass spectrometry is used to identify unknown or new compounds.

The mass spectra of compounds are much more complicated than that of atoms because of fragmentation.

When a molecule is ionised it forms a MOLECULAR ION which can undergo FRAGMENTATION or RE-ARRANGEMENT to produce particles of smaller mass.

Only particles with a positive charge will be deflected and detected.

The resulting spectrum has many peaks.

The final peak shows the molecular ion and indicates the molecular mass. The rest of the spectrum provides information about the structure.

MOLECULAR ION

MASS SPECTROMETRY - FRAGMENTATIONMASS SPECTROMETRY - FRAGMENTATION

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QUESTIONSQUESTIONS

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MASS SPECTRUM OF AN ORGANIC COMPOUNDMASS SPECTRUM OF AN ORGANIC COMPOUND

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Which signal is caused by the molecular ion?


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The final signal in the spectrum

is that of the molecular ion.

The final signal in the spectrum

is that of the molecular ion.

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What is the relative molecular (formula) mass of the compound?


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If the compound is a hydrocarbon, what is its molecular formula?

If the compound is a hydrocarbon, what is its molecular formula?

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C4H10C4H10What is the relative molecular

(formula) mass of the compound?What is the relative molecular

(formula) mass of the compound?

ANALYSING ANALYSING SUBSTANCESSUBSTANCES

THE ENDTHE END

©©2011 JONATHAN HOPTON & KNOCKHARDY PUBLISHING2011 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

ANALYSING SUBSTANCES A guide for GCSE students KNOCKHARDY PUBLISHING 2010 SPECIFICATIONS.

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