A2 Chemistry 9701: Rf Values and GLC Retention Times for a Mixture of Acids

A student separates a mixture of three carboxylic acids, P, Q and R, first by thin-layer chromatography (TLC) and then by gas-liquid chromatography (GLC).

In the TLC experiment, a polar silica stationary phase and a non-polar mobile solvent are used. After development, the solvent front has travelled $8.0\ \text{cm}$ from the baseline. Spot P has travelled $6.4\ \text{cm}$ , spot Q has travelled $4.0\ \text{cm}$ , and spot R has travelled $1.6\ \text{cm}$ .

(a) Calculate the $R_f$ value of each of P, Q and R. [3]

(b) State which of P, Q or R interacts most strongly with the stationary phase in this TLC experiment. Explain your answer. [2]

The same mixture is then analysed by GLC, using a high-boiling-point non-polar liquid (on a solid support) as the stationary phase and an unreactive gas as the mobile phase. Three peaks are obtained, with retention times of $1.8$ minutes for P, $3.5$ minutes for Q and $6.2$ minutes for R, and with peak areas in the ratio $3 : 8 : 9$ for P : Q : R.

(c) (ii) Explain, in terms of interaction with the stationary phase, why R has the longest retention time in this experiment. [2]

Part (a): Rf values from the TLC data

$R_f = \frac{\text{distance travelled by spot}}{\text{distance travelled by solvent front}}$

$R_f(\text{P}) = \frac{6.4}{8.0} = 0.80$

$R_f(\text{Q}) = \frac{4.0}{8.0} = 0.50$

$R_f(\text{R}) = \frac{1.6}{8.0} = 0.20$

Part (b): Which compound interacts most strongly with the stationary phase

The smallest $R_f$ value belongs to the spot that has travelled the least distance relative to the solvent. Meaning it has been held back the most by the stationary phase. Here R has the smallest $R_f$ ( $0.20$ ), so R interacts most strongly with the polar silica stationary phase, for example through stronger hydrogen bonding or dipole interactions with the silica surface, and/or because it is less soluble in the non-polar mobile solvent than P or Q.

Part (c)(i): Percentage composition from GLC peak areas

In GLC, peak area is proportional to the amount (in moles) of each component present. The total number of “parts” is:

$3 + 8 + 9 = 20$

$\%\,\text{P} = \frac{3}{20}\times100 = 15\%$

$\%\,\text{Q} = \frac{8}{20}\times100 = 40\%$

$\%\,\text{R} = \frac{9}{20}\times100 = 45\%$

Part (c)(ii): Why R has the longest retention time in GLC

The stationary phase in this GLC experiment is a non-polar liquid, and retention time depends on how strongly (and for how long) each compound interacts with, effectively dissolves in, that stationary liquid, rather than travelling with the moving carrier gas. Since R has by far the longest retention time ( $6.2$ minutes), R interacts most strongly with the non-polar stationary phase of the three acids: it may, for instance, have the lowest volatility (highest boiling point) or offer the greatest surface area for van der Waals interactions with the stationary liquid, so it is carried through the column the slowest.

Note that this is a separate physical property from the polar interaction with silica tested in part (b): TLC and GLC use chemically different stationary phases, so a compound’s ranking need not be the same in both techniques.

Final answers

(a) $R_f(\text{P}) = 0.80$ , $R_f(\text{Q}) = 0.50$ , $R_f(\text{R}) = 0.20$
(b) R interacts most strongly with the (polar) stationary phase, since it has the smallest $R_f$ .
(c)(i) P = 15%, Q = 40%, R = 45%
(c)(ii) R interacts most strongly with the non-polar stationary phase in GLC, giving it the longest retention time.