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48 Cards in this Set
- Front
- Back
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What is chromatography? |
An analytical technique that separates components in a mixture |
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In chromatography, the mobile phase will be a...? |
A liquid or gas |
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In chromatography, the stationary phase may be...? |
• a solid (e.g. TLC) • a liquid or solid on a solid support (e.g. gas chromatography) |
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What are the different types of chromatography? |
• Thin-layer chromatography (TLC) • Column chromatography (CC) • Gas chromatography (GC) |
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Describe in extremely simple terms what happens in TLC? |
• a glass plate is coated with a solid (gel) • a solvent moves up the plate |
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Describe in very simple terms what happens in column chromatography? |
• a column is packed with a solid • a solvent moves down the column |
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Describe in very simple terms what happens in gas chromatography? |
• a column is packed with a solid (or a solid coated by a liquid) • a gas is passed through the column under pressure at high temperature |
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Describe in very simple terms what happens in gas-liquid chromatography? |
• the stationary phase is a liquid on an inert solid • the mobile phase is an inert gas e.g. Nitrogen, helium, argon |
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What does the distance that each substance moves up the plate depend on? |
• the solubility of the substance in the mobile phase • the retention of the substance by the stationary phase |
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Components that are more soluble in the mobile phase are said to have a high affinity for the mobile phase. Why might a substance have a higher affinity for the mobile phase? |
If the mobile phase and the substance are both polar |
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Components that are more soluble in the mobile phase will move further up the plate |
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Why do the components in a mixture separate out? |
They all have different solubilities and retentions And so travel different distances |
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Define - a mobile phase? |
Where the molecules can move (A liquid or gas) |
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Define - a mobile phase? |
Where the molecules can move (A liquid or gas) |
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Define - a stationary phase? |
Where the molecules can’t move (A solid, or liquid on a solid support) |
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In a column chromatography. • the stationary phase is polar • the mobile phase is non-polar A mixture contains ethane and water. Which of these components will move furthest? |
ethane (Non-polar compounds will move faster through the column as they have a greater solubility in the non-polar mobile phase) |
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In a column chromatography. • the stationary phase is polar • the mobile phase is non-polar A mixture contains ethane and water. Which of these components will move furthest? |
ethane (Non-polar compounds will move faster through the column as they have a greater solubility in the non-polar mobile phase) |
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In a column chromatography. • the stationary phase is polar • the mobile phase is non-polar A mixture contains ethane and water. Which of these components will move furthest? |
ethane (Non-polar compounds will move faster through the column as they have a greater solubility in the non-polar mobile phase) |
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Describe how TLC is carried out? |
• stationary phase = a thin layer of silica (SiO2) or Alumina (Al2O3) fixed to a glass plate • draw a pencil line near the bottom of the TLC plate (the baseline) + put a very small drop of each mixture to be separated on the line • allow the spots to dry • place the plate in a beaker so that the end is in solvent (the mobile phase) • ensure the solvent level is below the baseline (so it doesn’t dissolve your samples away) • the solvent starts to move up the plate, as it moves the substances are carried in the mixture with it, some components are carried faster than others (travel further up the plate) • leave until the solvent almost reaches top of plate • remove the plate from the beaker, and mark (with a pencil) the solvent front • place the plate in a fume cupboard, leave to dry (fume cupboard prevents toxic/flammable fumes from escaping into the room) • leaves you with a chromatogram. Use the positions of the spots to identify the chemicals.
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The spots left on the chromatogram after TLC will either be coloured or colourless. What do you do if the spots are colourless? |
• some TLC plates can have a special fluorescent dye added to the silica/alumina layer. Glows when UV light shines on it. Draw around the dark patches to show the spots. OR • Chromatogram is exposed to iodine vapour (plate in sealed jar with iodine crystals). Iodine is a locating agent - sticks to the chemicals on the plate - show up as brown/purple spots |
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How can you increase the accuracy of results obtained from TLC? |
Let solvent rise to near the top of the plate. (The higher the solvent = the more accurate the results) |
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How can you increase the accuracy of results obtained from TLC? |
Let solvent rise to near the top of the plate. (The higher the solvent = the more accurate the results) |
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Why does the depth of the solvent need to be below the pencil line/sample spots? |
If solvent too deep - it will dissolve the sample spots from the plate |
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How can you increase the accuracy of results obtained from TLC? |
Let solvent rise to near the top of the plate. (The higher the solvent = the more accurate the results) |
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Why does the depth of the solvent need to be below the pencil line/sample spots? |
If solvent too deep - it will dissolve the sample spots from the plate |
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Why should you wear gloves in TLC? |
Prevent contamination of the plate by hands |
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Why dry in a fume cupboard in TLC? |
Toxic fumes from solvent |
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Why use tiny drops of samples on the pencil line? |
Too big a drop will cause different spots to merge |
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Why use tiny drops of samples on the pencil line? |
Too big a drop will cause different spots to merge |
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Why use a lid in TLC? |
Prevent evaporation of toxic solvent |
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Formula for Rf values? |
(Distance moved by component)/(distance moved by solvent) |
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Describe how to do column chromatography? |
• a glass tube is filled with the stationary phase (usually silica/alumina in powder form) NB. a filter or plug is used to retain the solid in the tube • solvent is added to cover all the powder • the mixture to be analysed is dissolved in a minimum of solvent and added to the column • a solvent (or mixture of solvents) is then run through the column • the time for each component in the mixture to reach the end of the column is recorded (retention time) |
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Describe how to do column chromatography? |
• a glass tube is filled with the stationary phase (usually silica/alumina in powder form) NB. a filter or plug is used to retain the solid in the tube • solvent is added to cover all the powder • the mixture to be analysed is dissolved in a minimum of solvent and added to the column • a solvent (or mixture of solvents) is then run through the column • the time for each component in the mixture to reach the end of the column is recorded (retention time) |
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In Column chromatography, why is the stationary phase (usually silica/alumina) in powder form? |
Increase the surface area |
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What does HPLC stand for? Where is it commonly used? What are the stationary and mobile phases? |
High performance liquid chromatography (a type of column chromatography) • used in industry • stationary phase = solid silica • mobile phase = a liquid |
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Here is a diagram of column chromatography! |
Back (Definition) |
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On a TLC plate, how can you tell how many chemicals are present in the mixture? |
Count the number of spots |
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If you want to find out which chemical caused each spot on a TLC plate what would you do? |
• calculate an Rf value for the spot • look up the Rf value in a table of standard Rf values to identify the substance Very important! In your experiment you must use the same conditions as were used when the standard Rf values were calculated or you’ll get different Rf values • same composition of TLC plate • same temperature • same solvent |
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TLC If you suspect that you were unable to keep the conditions identical to the conditions used to calculate the standard Rf values, what can you do? |
If you know that a mixture contains a substance (e.g. chlorphyll) • put a spot of chlorophyll (or other substance) on the baseline of the same TLC plate as the mixture + run them both at the same time |
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What is gas chromatography mainly used for? |
Purifying an organic product (Separation of the product from unreacted chemicals and by-products |
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What is gas-liquid chromatography used for? |
To separate mixtures of volatile liquids |
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How can you identify substances using gas-liquid chromatography? |
Time taken for a particular compound to travel from the injection of the sample to where it leaves the column to the detector is known as its retention time. |
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Gas-liquid chromatography Not all substances can be identified using this method. Why? |
Some compounds have similar retention times so will not be distinguished. |
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If you do gas-chromatography on a sample. What will you be told about the sample? |
• the number of components in the mixture (by the number of peaks) • the abundance of each substance (the area under each peak is proportional to the abundance) |
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It is also possible for gas-liquid chromatography machine to be connected to a mass spectrometer, IR or NMR machine, enabling all the components in a mixture to be identified. |
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What is the mobile phase in gas-liquid chromatography? |
A gas such as helium |
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What is the stationary phase in gas-liquid chromatography? |
A high boiling point liquid absorbed onto a solid |
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What is GC-MS used in? |
Used in: • analysis • forensics • airport security • space probes |
