Comparing pigments with chromatography

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Comparing pigments with chromatography

Chromatography separates components because they interact differently with a stationary phase and moving solvent. Leaf extracts may contain several pigments even when the intact leaf appears one colour. The number and position of resolved bands depend on the system; one band can contain unresolved substances.

Rf is the distance travelled by a pigment from the origin divided by the distance travelled by the solvent front from the same origin. Both distances use the same units, so the ratio has no unit. Comparable references require the same solvent and stationary-phase conditions. An Rf match can support identification, but is not an unconditional proof of molecular identity.

Measure consistently along the direction of movement and record the solvent-front position. A diffuse band introduces uncertainty in the chosen centre. Comparing shade and sun leaves also requires consideration of species and starting material, rather than attributing every difference to light. Solvents and plant-material preparation require supervised approved methods; the practical study competence is not recorded by this calculation lesson.

Worked example: A pigment moves 6.0 cm and the solvent front moves 10.0 cm from the origin. Find Rf.

Use the same origin for both distances.

Calculate 6.0 / 10.0.

Answer: Rf = 0.60, with no distance unit.

Check your understanding: Can Rf values from different solvent systems identify a pigment directly?

Answer: Not without suitable comparable evidence.

Why: Changing the phases changes the interactions and migration relationship.