Chromatography (A-level only) (AQA A Level Chemistry)

A bottle of butan-2-ol was found by infrared spectroscopy to be contaminated with butanone. The contents on this bottle were then separated by column chromatography.

Using your data booklet, explain how the chemist was able to determine that the butan-2-ol was no longer pure using IR spectroscopy and suggest what chemical process would have led to this contamination. 

Column chromatography was used to separate the contents of the bottle containing butan-2-ol and butanone. The column contained silica gel.

The contents of the bottle were dissolved in hexane and added to the column. Hexane continued to be added to the column. Samples of the eluent were collected.

Suggest why butanone was present in the samples of eluent collected first.

Identify the stationary and mobile phases in this chromatography column. 

Another method of chromatography is thin layer chromatography, TLC. A mixture of benzaldehyde and benzyl alcohol is placed on a TLC plate using 7:3 pentane/diethyl ether as a solvent.

The two compounds travel a certain distance up the plate as shown in Figure 1.

Figure 1

Calculate the R_f values for both compounds in the chromatogram shown in Figure 1.

Thin layer chromatography, TLC, is used routinely in the laboratory to both monitor reactions and analyse the purity of pain killing drugs. An example of a drug is called ibuprofen, shown in Figure 1.

Calculate the M_r of ibuprofen to 1 decimal place.

Figure 1

Chromatography is used to identify a sample of ibuprofen.

The sample is analysed by silica gel thin-layer chromatography (TLC) using a toxic solvent. The steps to produce the chromatogram of ibuprofen using TLC including safety precautions are outlined below.         

1. Draw a pencil baseline 1.5 cm from the bottom of the TLC plate
2. Use a capillary tube to drop a small amount of solution of sample to the origin marked on the pencil line
3. Allow the spot to dry completely
4. Add the developing solvent to a depth of no more than 1 cm below pencil line
5. Cover the chromatography tank/beaker to prevent the solvent evaporating
6. Allow the developing solution to rise up the TLC plate
7. Remove the TLC plate and mark position of the solvent front with a pencil
8. Place the plate under a UV lamp in order to see the spots and draw around them lightly in pencil

State two precautions the chemist should take when carrying out TLC and give a different justification for each precaution taken.

State two advantages of using TLC for analysis of samples of pain killings drugs. 

Thin-layer chromatography was performed on samples of ibuprofen and paracetamol.
On the ibuprofen TLC plate, only one spot was expected but two were shown.
Suggest why some chromatograms may have two or more spots present.

A mixture of dipeptides can be analysed using gas chromatography, coupled with mass spectrometry.

Summarise how each method contributes to the analysis.

Define the term retention time.

A mixture of two dipeptides, shown in Figure 1, is analysed by gas chromatography followed by mass spectrometry.

Figure 1

Explain why the mixture of dipeptides can be separated by gas chromatography.

Explain why mass spectrometry using electrospray ionisation does not identify the component amino acids for the dipeptides shown in Figure 1.

Column chromatography and gas chromatography work by the same principles of components travelling through the column at different rates.

State the difference between the eluents involved in column chromatography and gas-liquid chromatography.

A gas chromatogram trace is shown in Figure 1.

Figure 1

Identify if compound A, B or C has the greatest affinity for the solid phase. Justify and explain your answer.

Use the gas chromatogram shown in Figure 1, part (b), to identify the most abundant compound in the sample. Justify your answer.

An oil tanker is travelling through the English Channel. The tanker has a slight leak which is not large enough to result in an oil slick but some oil is noticed on a beach.

Suggest how gas chromatography could be used to identify the tanker as the source of crude oil pollution on the beach.

A student is asked to identify a mixture using thin-layer chromatography.

Draw the labelled experimental set up that the student should use to identify the number of compounds in the mixture.

The student runs their thin-layer chromatography experiment and plans to determine the compounds from their R_f values.

Describe the steps that the student needs to perform to determine the identity of the compounds.

The student’s results are shown in Figure 1.

Figure 1

For their measurements, the student locates the centre of each spot by estimating its rough position by eye.

Suggest an improved method to locate the centre of each spot.

Explain why the maximum R_f value of a compound cannot exceed 1.

Draw the isomers of dinitrobenzene and name the type of isomerism.

In general terms, state the factors that determine the distance travelled by a spot in thin-layer chromatography.

A student spotted samples of 1,2-dinitrobenzene and 1,4-dinitrobenzene on a TLC plate coated with silica gel. The solvent used to run the TLC experiment was hexane.

Figure 1 shows the student’s chromatogram with the spot for 1,4-dinitrobenzene.

Figure 1

Draw the expected position of the spot for 1,2-dinitrobenzene. Explain your answer.

Explain what the student could do to reverse the relative positions of the 1,2-dinitrobenzene and 1,4-dinitrobenzene spots.

The R_f value calculated from a chromatography experiment can be used to determine the potential identity of a compound.

Suggest why tables of R_f values must state the solvent as well as the stationary phase.

In the wider Chemistry community, R_f values are often written with the solvent as shown.

R_f = 0.25 (Ethanol - Methanoic Acid - Propanone mixture {4:3:1})

A column chromatography experiment was set up that required 50.0 cm³ of solvent.

Outline a brief method to make 50.0 cm³ of the required solvent.

Draw a labelled chromatogram for the compound detailed in part (b), assuming that the solvent front was 5.2 cm from the baseline.

R_f values for a compound are specific for that compound with a particular combination of mobile and stationary phase chemicals.

State four variables that should be controlled to ensure a reproducible R_f value can be obtained for a compound.

A polypeptide was hydrolysed to form a mixture of its component amino acids.

Suggest a suitable reagent to perform this hydrolysis.

Thin-layer chromatography of the hydrolysed polypeptide was performed by the following method.

• The mixture was spotted on to a TLC plate, 1.5 cm from the left hand edge
• The plate was placed into a container with a small volume of solvent A
• The plate was allowed to run
• The plate was removed when the solvent front was 0.5 cm from the top and allowed to dry
• The plate was then rotated 90^o to the left and placed into a container with a small volume of solvent B
• The plate was allowed to run
• The plate was removed when the solvent front was 0.5 cm from the and allowed to dry

State three possible ways in which the positions of the component amino acids could be located on the TLC plate.

The chromatogram from part (b) is shown in Figure 1.

Suggest why two different solvents were used to produce the chromatogram shown in part (c).

A burette was set up for column chromatography as shown in Figure 1.

Explain why this chromatography column may not work to separate a mixture.

A student used the following method to load the chromatography column, in part (a), with their mixture.

• The burette tap was opened to drain the solvent so that it was level with the top of the silica
• The mixture was carefully added to the top of the silica, taking care not to disturb the silica
• The burette tap was opened to drain the column until the sample was absorbed into the silica
• Solvent was then added, taking care not to disturb the mixture absorbed into the silica
• The chromatography column was then run to separate the mixture
• When necessary, more solvent was added to ensure that the column did not dry out

Suggest a piece of equipment that could be used to add the mixture or solvent without disturbing the top of the silica.

The student decides to adapt the burette from column chromatography to flash chromatography by attaching a large gas syringe via a delivery tube and bung. The student then compresses the gas syringe forcing air into the column.

Suggest how this setup will affect the separation of the student’s mixture.

The mixture that the student is separating contains colourless, photosensitive components.

Describe how the student could complete their chromatography experiment to separate their mixture into the pure components.

Partial hydrolysis of a tripeptide with concentrated hydrochloric acid produces the amino acids alanine (ala), lysine (lys) and serine (ser) along with two dipeptides.

The chromatogram produced when separating the dipeptides is shown in Figure 1.

Figure 1

Calculate the R_f values of components A and B.

The R_f values of some dipeptides are shown in Table 1.

Table 1

Use Table 1 and your answers to part (a) to identify the dipeptides responsible for spots A and B.

Use the data sheet to draw the structure of the original tripeptide.

Table 1 is repeated to help you answer this question.

Table 1

A second tripeptide lys-ser-ala undergoes partial hydrolysis to form two dipeptides.

Explain why chromatography of the component dipeptides may not be able to prove the identity of the tripeptide.