Explain separation
Components can differ in their affinity for the stationary phase and their behaviour in the mobile phase. They therefore travel different distances. A statement such as “the pigment is lighter” does not by itself explain movement in this separation method.
Establish the origin
Apply a small starting spot at a defined origin and keep it above the initial solvent level. A submerged starting spot can dissolve directly into the reservoir. A pencil line can avoid introducing an ink mixture that might itself separate and confuse the chromatogram.
Record the front
Mark the solvent front promptly because its position can become hard to locate after evaporation. Measure solute and solvent distances from the same origin. Use a defined centre for each spot and a consistent measurement rule if spots are broad.
Compare under the same conditions
Solvent composition, stationary phase and other conditions affect Rf. Run suitable references under comparable conditions if identification is intended. A matching Rf and appearance can support an identification but do not exclude two different substances with similar behaviour.
Interpret biological samples
A leaf extract can show several pigment components. More resolved spots provide evidence of more separated components under those conditions, but the absence of a clear spot does not establish absolute absence: low abundance, poor extraction or overlapping spots may matter.
Worked data example
The solvent front travels 80 mm from the origin. Two pigment spot centres travel 24 mm and 56 mm. Calculate their Rf values.
Rf values are 24/80 = 0.30 and 56/80 = 0.70. Both are dimensionless. The second component travelled further relative to the front in this system.
Can an Rf value from a different solvent system identify a pigment reliably on its own?
No. Rf depends on the separation conditions. Use comparable conditions and suitable reference evidence rather than assuming the value is a universal molecular constant.