AQA A Level Chemistry - Chromatography

Question 1

What does separation depend on?

Answer 1

• Separation depends on the balance between solubility in the moving phase and retention by the stationary phase

Question 2

What are the key principles of chromatography?

Answer 2

• Chromatography is a whole family of separation techniques
• Though chromatography takes a number of different forms, they each rely on similar principles
• In each case:
• There is a mixture of two or more substances which need to be separated
• There is a solid substance called the stationary phase which the mixture passes over in order to be separated
• The mixture is carried over the solid material by a mobile phase. - This is either a liquid or a gas
• The mixture is separated either due to different affinities to the stationary phase, or differences in solubility in the mobile phase
• If a component of the mixture has a high affinity to the stationary phase it will travel slowly
• If a component of the mixture has a low affinity to the stationary phase it will travel quickly
• If a component of the mixture is highly soluble in the mobile phase it will travel along quickly with the solvent
• If a component of the mixture has low solubility in the mobile phase it will travel slowly with the solvent
• In all forms of chromatography, a suitable combination of mobile and stationary phases must be selected to ensure the mixture separates

Question 3

What is Thin-Layer Chromatography?

Answer 3

• A thin piece of aluminium or glass is coated in silica gel or aluminia which acts as the stationary phase
• This is known as the TLC plate
• Samples of the mixture to be separated are dropped on a line on the bottom of the TLC plate
• The TLC plate is suspended in a small amount of solvent (also called the eluant) which soaks up the plate. The solvent is the mobile phase
• Once dry, the plate is examined under ultra-violet light, or developed using chemical reagents to make the spots visible
• The number of spots on the finished chromatogram gives the number of components present in the mixture

Example:
- Sample B is more soluble in the mobile phase as it has travelled further up the TLC plate
- Sample A is less soluble in the mobile phase as it has travelled a shorter distance up the TLC plate
- Sample B has a low affinity to the stationary phase as it has travelled further up the TLC plate
- Sample A has a greater affinity to the stationary phase as it has travelled a shorter distance up the TLC plate

Question 4

What is the method for Thin-Layer chromatography?

Answer 4

• Wearing gloves, draw a pencil line 1cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line
• Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry
• Add solvent to a chamber or large beaker with a lid so that it is no more than 1cm in depth
• Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal
• When the level of the solvent reaches about 1cm from the top of the plate, remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard
• Place the plate under a UV lamp in order to see the spots. Draw around them lightly in pencil.
• Calculate the Rf values of the observed spots
• Rf value = distance moved by component/distance moved by solvent front

Question 5

What is the importance of the steps in the method for Thin-Layer Chromatography?

Answer 5

• Wear plastic gloves to prevent contamination from the hands to the plate
• Pencil line - will not dissolve in the solvent
• Tiny drop - too big a drop will cause different spots to merge
• Depth of solvent - if the solvent is too deep it will dissolve the spots from the plate
• Lid - to prevent evaporation of toxic solvent
• Will get more accurate results if the solvent is allowed to rise to near the top of the plate but the Rf value can be calculated if the solvent front does not reach the top of the plate
• Dry in a fume cupboard as the solvent is toxic
• UV lamp used if the spots are colourless and not visible

Question 6

What are the advantages and disadvantages of Thin-Layer Chromatography?

Answer 6

• Advantages of TLC:
• Faster than paper chromatography
• Will work on very small samples
• Can be used to determine when a chemical reaction is complete
• Disadvantages of TLC:
• Similar compounds may have similar Rf values
• Conditions must be kept the same in order to fairly compare Rf values
• New and unknown compounds have no reference Rf values
• It can be difficult to find a solvent which separates all components in a mixture
• Cannot be used to separate large quantities

Question 7

What is Column Chromatography?

Answer 7

• Column Chromatography uses a narrow glass tube with a spout and tap (like a burette). This is called the column
• The column is packed with a powder such as silica or aluminia to act as a stationary phase
• The mixture to be separated is loaded onto the top of the column. A solvent (eluant) is poured onto the top of the column and soaks down through the stationary phase. This solvent is the mobile phase
• The different components of the mixture travel down through the solid phase at different rates. One will reach the bottom and can be collected into a beaker first, and the next can then be collected into a separate beaker some time later
• The advantage of column chromatography is that larger quantities can be separated

Question 8

The Polarity of the Stationary Phase and Mobile Phase

Answer 8

• If the stationary phase was polar and the moving phase was non polar eg, hexane, then non polar compounds would pass through the column more quickly than polar compounds as they would have a greater solubility in the non-polar moving phase

Question 9

What is Simple Column Chromatography?

Answer 9

• A glass tube is filled with the stationary phase usually silica or alumina in powder form to increase the surface area.
• 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 a 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)
• HPLC stands for high performance liquid chromatography and it is a type of column chromatography commonly used in industry
• HPLC : stationary phase is a solid silica
• HPLC : mobile phase is a liquid

Question 10

What is Gas-Liquid Chromatography?

Answer 10

• Gas-Liquid Chromatography can be used to separate mixtures of volatile liquids
• The mobile phase in GC is a gas, known as a carrier gas. This gas is typically an unreactive gas such as nitrogen or helium, it is important to use an inert gas as these will not react with the compounds being separated in the GC column
• This carrier gas flows through a long and very thin tube (around 0.5mm in diameter) - This is known as a capillary column which can be as long as 100m cooled up. The temp can be varied
• The column is made of glass or metal and has a thin coating of a liquid or a solid which acts as the stationary phase
• A typical liquid stationary phase in GC is a high molecular mass alkane with a high boiling point
• Silicone polymers can be used as a solid stationary phase in GC
• The mobile and stationary phases can be changed to suit the mixture to be separated
• The sample to be separated is injected into the column. The carrier gas transports the components of the mixture along the column at different rates causing them to separate out
• As the mobile phase is not a solvent, you cannot say that the components separate due to different solubilities. You can only explain separation in terms of affinity to the stationary phase
• In gas chromatography, you don’t measure an Rf value for the distance travelled, you record the retention time. This is the time it takes from the point of injection for the component to the detector
• When the components leave the column and reach the detector, an electrical signal is produced
• The components move into the detector at different times, and a retention time is recorded for each component. A typical chromatography is shown below
• Components can be identified by comparing their retention times with known compounds. This area of a peak in a gas chromatogram is proportional to the amount of a component in a mixture
• This then enables the relative amounts of components in a mixture to be determined by comparing peak areas