CAIE A-Level Chemistry 37.1 Thin-Layer Chromatography

Practise identifying TLC phases, calculating Rf and interpreting spot movement against reference samples.

Exam points

identify the solid stationary phase, solvent mobile phase, baseline, spots and solvent front on a plate
calculate Rf as distance travelled by the spot divided by distance travelled by the solvent front
match an unknown to references under the same conditions and explain solvent-polarity effects on movement

Covered syllabus points

37.1.1—And understand the terms 37.1.2—Rf values 37.1.3—Differences in Rf values

[Maximum number: 3]

A student uses thin-layer chromatography (TLC) to analyse a mixture containing different metal cations. The student repeats the experiment using different solvents.

Fig. 6.1 shows the chromatogram obtained by the student using water as a solvent.

Fig. 6.1

(a)

Suggest a compound that could be used as the stationary phase in this experiment.

[ 1 ]

$\mathrm{SiO}_{2}$ OR $\mathrm{Al}_{2} \mathrm{O}_{3}$ OR silica OR alumina OWTTE

(b)

Table 6.1 shows the $R_{\mathrm{f}}$ values for different metal cations when separated by TLC using water as a solvent.

Table 6.1

Suggest the identity of the cation that causes the spot at M in Fig. 6.1.

Explain your answer.

[ 1 ]

$\mathrm{Cd}^{2+}$ AND $R_{\mathrm{f}}$ of M(=2 / 5)=0 . 4 0 / same $R_{\mathrm{f}}$ as in the Table 6.1

(c)

The student repeats the experiment using butan-1-ol as a solvent.

The metal cations do not travel as far up the TLC plate in this experiment.
Suggest why the metal cations do not move as far up the TLC plate with butan-1-ol as a solvent.

[ 1 ]

[Maximum number: 2]

(a)

Thin-layer and gas-liquid chromatography can be used to analyse mixtures of substances.

Each type of chromatography makes use of a stationary phase and a mobile phase.

[ 2 ]

(i)

Complete Table 6.1 with an example of each of these.

Table 6.1

[ 1 ]

aluminum oxide / silica (on solid support) AND inert gas / named inert gas e.g. $\mathrm{N}_{2}$

(ii)

An unknown amino acid is analysed using thin-layer chromatography. Two chromatographs of the unknown amino acid and four reference amino acids, P, Q, R and S, are obtained using two different solvents.

Fig. 6.4

Identify the unknown amino acid. Justify your answer.

[ 1 ]

S AND $R_{\mathrm{f}}$ is the same as the unknown amino acid in both solvents

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Covered syllabus points

37.1 Thin-layer chromatography question 1

Question (a)

Question (b)

Question (c)

37.1 Thin-layer chromatography question 2

Question (a)

Question (i)

Question (ii)