(a)     State the stationary phase and an example of a mobile phase used in paper chromatography.

Stationary phase:

Mobile phase:

(b)     The identity of two sugars in a mixture can be determined by measuring their ${R_{\text{f}}}$ values, after staining.

(i)     Describe how an ${R_{\text{f}}}$ value can be calculated.

(ii)     Calculate the ${R_{\text{f}}}$ value for sugar 2 in the chromatogram below.

(c) Explain how the ${R_{\text{f}}}$ value of sugar 2 could be used to identify it.

(a)     Stationary phase:

water in the paper;

Mobile phase:

water / any other non corrosive solvent or solvent mixture;

(b)     (i)     ${R_{\text{f}}} = \frac{{{\text{distance moved by substance}}}}{{{\text{distance moved by solvent / solvent front}}}}$;

(ii)     ${R_{\text{f}}} = \frac{{3.0}}{{5.7}} = 0.53$;

Accept answers between 0.5 and 0.55 (± 1mm on measurements).

(c)     compare ${R_{\text{f}}}$ of unknown to known values;

(conducted) under the same conditions (of stationary/mobile phase);

Allow second mark for specifying a particular condition to keep the same (e.g. solvent).

This was generally well done with most candidates displaying a good comprehension of the ${R_{\text{f}}}$ concept. In Part (c) many candidates correctly suggested comparing the ${R_{\text{f}}}$ value to that of standard samples, but very few pointed out that these must be obtained under identical conditions.