A practical guide to separating plant pigments by column chromatography
Plant leaves contain several coloured molecules that absorb different wavelengths of light. Chlorophyll a and b give leaves their green appearance, while carotenoids contribute yellow and orange tones. Other tissues may contain anthocyanins, which produce red, purple, or blue shades. Column chromatography can separate these compounds so their colours, movement, and chemical behaviour can be examined individually.
The method is especially useful for teaching organic chemistry, natural products analysis, and introductory analytical chemistry. It connects polarity, adsorption, solvent selection, and visual observation in one experiment. A small sample of spinach from a local Australian supermarket or a common garden plant is usually enough for a demonstration.
For Nepali students and chemists working in Australia, the experiment also offers a practical way to connect familiar plant science with laboratory technique. Leaf colour can vary with species, season, light exposure, and soil conditions. Background reading on soil pH testing helps explain why growing conditions may influence pigment production without treating colour alone as a reliable diagnostic.
The procedure requires careful handling of solvents and glassware. Petroleum ether, hexane, acetone, and ethanol can be flammable, while silica dust can irritate the respiratory system. Work in a properly ventilated laboratory, follow the relevant Australian laboratory rules and safety data sheets, and use suitable waste containers rather than disposing of solvents down a sink.
What the pigments are
Chlorophyll a is blue-green and chlorophyll b is yellow-green. These molecules contain a large conjugated system that absorbs visible light, with chlorophyll a generally moving differently from chlorophyll b during separation. Carotenoids such as beta-carotene are yellow to orange and are less polar than chlorophylls.
Anthocyanins are water-soluble pigments found in many red, purple, and blue plant tissues. They are often extracted more effectively with aqueous alcohol or mildly acidified alcohol than with non-polar solvents. For a straightforward silica-column experiment, green leaves are usually preferable because chlorophylls and carotenoids produce clear bands.
Preparing the plant extract
Cut fresh leaves into small pieces and grind them with a mortar and pestle. A small amount of clean sand can assist cell disruption. Add a suitable extraction solvent gradually, commonly acetone, ethanol, or a mixture selected according to the available equipment and risk assessment. Avoid heating flammable solvents directly.
Filter the dark extract through filter paper or allow coarse solids to settle before loading it. A concentrated extract gives sharper, more visible bands. Leaves collected around Sydney, Melbourne, or Brisbane may differ in moisture and pigment content, so sample mass and solvent volume should be recorded. Comparing spinach with a locally collected eucalyptus leaf can be interesting, but eucalyptus extracts may contain additional compounds that complicate the separation.
Packing the chromatography column
A glass column is packed with silica gel, the stationary phase. First place a small plug of glass wool or cotton at the outlet, then add a thin layer of sand. Slurry-packing silica with the starting solvent generally produces a more uniform bed than pouring dry powder into the column.
Keep the silica wet and avoid allowing the solvent level to fall below its surface. Air bubbles, cracks, and an uneven top cause bands to spread or travel irregularly. Add another thin sand layer above the silica before applying the sample. A Pasteur pipette can help place the extract carefully on the surface without disturbing the bed.
Choosing the solvent system
Silica is polar, so polar compounds interact with it more strongly and usually move more slowly. Non-polar compounds travel faster in a non-polar mobile phase. A common starting system for chlorophyll and carotenoid separation is petroleum ether with a small proportion of acetone, although the exact ratio depends on the silica, sample, and column dimensions.
Begin with a less polar solvent and increase polarity gradually if the bands remain near the top. Too much acetone can make every pigment move together, while a solvent that is too weak can leave the chlorophylls stuck on the column. Thin-layer chromatography is useful for testing solvent ratios before committing to a larger column.
Running and collecting the bands
Load the extract slowly, then add solvent in small portions. Maintain a steady flow rather than forcing the liquid through under high pressure. Distinct coloured zones may appear as yellow-orange carotenoids, green chlorophylls, and sometimes pale or mixed bands. Collect the eluate in labelled test tubes or small flasks.
Keep the fractions in order and note their colour, volume, and collection time. If a band is broad, collect smaller fractions because adjacent pigments may overlap. The Australian summer can increase solvent evaporation, especially in warm laboratories, so containers should remain covered when they are not being used.
Interpreting results and improving reliability
Separated fractions can be examined by visible colour, UV-visible spectroscopy, or thin-layer chromatography. A single coloured fraction may still contain several compounds, so colour is evidence of separation rather than proof of chemical purity. Running a TLC spot from each fraction provides a simple check for remaining mixtures.
The table compares common approaches used with plant pigments:
| Method | Main advantage | Typical limitation | Useful application |
|---|---|---|---|
| Paper chromatography | Low cost and simple setup | Lower resolution | School demonstrations |
| Thin-layer chromatography | Fast solvent screening and good comparison | Small sample capacity | Selecting a mobile phase |
| Column chromatography | Collects separated fractions | Uses more solvent and time | Preparative pigment isolation |
| Spectrophotometry | Provides absorbance data | Requires an instrument and standards | Estimating pigment concentration |
Good records are essential. Note the plant species, mass, extraction solvent, silica amount, solvent composition, flow rate, fraction numbers, and observations. Students working in university laboratories in Canberra, Perth, or elsewhere in Australia should also record instrument settings and disposal procedures so the experiment can be repeated and compared across laboratories.
The most useful next step is to run a small thin-layer chromatography trial on the leaf extract, record the band positions, and use that solvent ratio for the packed column.