Paper Chromatography: Separating Plant Pigments in Lab

Paper chromatography is a separation technique in which a drop of leaf-pigment extract is placed on filter paper and a solvent creeps up the paper, dragging the pigments along at different speeds. Because each pigment sticks to the paper differently, the single green spot separates into distinct bands: orange-yellow carotene, yellow xanthophyll, blue-green chlorophyll a and yellow-green chlorophyll b.

It matters because it proves with your own eyes that "chlorophyll" is actually a mixture of pigments, each catching different wavelengths of light. The technique is also the ancestor of all modern chromatography used in drug testing, forensics and food safety.

Key Concepts

• Stationary phase: the filter paper, which stays put; pigments adsorb (stick) to it with different strengths.
• Mobile phase: the solvent mixture (e.g. petroleum ether and acetone) that travels up the paper by capillary action.
• Adsorption: the sticking of pigment molecules to paper fibres — stronger sticking means slower travel.
• Rf (retention factor) value: the ratio of a pigment's travel distance to the solvent front's distance; each pigment's fingerprint under fixed conditions.
• Solvent front: the farthest line the solvent reaches — marked immediately before it dries and disappears.

How It Works

1. Grind fresh green leaves (e.g. spinach) with a little acetone and filter to get a concentrated dark-green extract.
2. Draw a light pencil line (the origin) about 2 cm from the bottom of a filter-paper strip — never use ink, which would separate too.
3. Spot the extract on the line, let it dry, and repeat 3–4 times for a concentrated spot.
4. Stand the strip in a jar with a shallow layer of solvent, keeping the spot above the solvent level, and cover the jar.
5. Let the solvent rise by capillary action until near the top, then remove the strip and immediately mark the solvent front with pencil.
6. Let it dry and observe the separated bands from bottom (most adsorbed) to top (least adsorbed): chlorophyll b, chlorophyll a, xanthophyll, carotene.

Worked Example

Question: On a developed chromatogram the solvent front travelled 10.0 cm from the origin. Carotene's band centre is at 8.4 cm and chlorophyll a's at 5.9 cm. Calculate both Rf values and state which pigment is more strongly adsorbed.

1. Rf = distance by pigment ÷ distance by solvent.
2. Rf (carotene) = 8.4 ÷ 10.0 = 0.84.
3. Rf (chlorophyll a) = 5.9 ÷ 10.0 = 0.59.
4. Lower Rf means the pigment lagged behind — stuck more strongly to the paper.
5. Answer: carotene 0.84, chlorophyll a 0.59; chlorophyll a is more strongly adsorbed to the paper than carotene.

Common Misconceptions

• "One green band means pure chlorophyll." Wrong — a single band may just mean poor separation; proper chromatography always resolves four pigments from a healthy leaf.
• "The fastest band is the most abundant pigment." Wrong — speed reflects solubility and weak adsorption, not quantity; chlorophyll a is usually the most abundant despite travelling slower than carotene.
• "Rf values are the same in every solvent." Wrong — Rf depends on the exact solvent, paper and temperature; that is why conditions are standardised when comparing.

Real-World Applications

• Pharmaceuticals: checking drug purity and spotting contaminants before medicines reach patients.
• Food safety: detecting illegal artificial colours in sweets and spices.
• Forensics: comparing ink or dye samples from crime scenes.
• Plant science: studying how pigment ratios change with seasons, stress and disease.

Paper chromatography turns one green dot into a rainbow of proof: carotene, xanthophyll, chlorophyll a and chlorophyll b, each with its own Rf fingerprint. Simple enough for a school lab, yet the ancestor of modern analytical chemistry — all from paper, solvent and a leaf's pigments.