A practical guide to paper chromatography for amino acids
Paper chromatography is a simple separation method for identifying amino acids in mixtures. It uses a sheet of porous paper, a solvent, and a visualising reagent to reveal how far different compounds travel. Although modern laboratories often use high-performance liquid chromatography, paper methods remain valuable for teaching, preliminary analysis, and low-cost experiments.
The method is based on differences in solubility and attraction. Amino acids dissolve in the mobile solvent, then interact with the cellulose fibres and moisture held in the paper. Compounds that spend more time in the solvent usually travel further, while those retained by the paper move more slowly.
For students in Australia, this experiment fits well with senior chemistry practical work, undergraduate laboratory classes, and outreach activities at universities in Sydney, Melbourne, Brisbane, Perth, and other centres. It can also introduce concepts used in food testing, biochemistry, forensic science, and pharmaceutical analysis.
Paper chromatography is accessible, but reliable results require careful spotting, controlled solvent movement, and safe handling of reagents. A small error in sample placement or chamber setup can produce streaks, overlapping spots, or misleading identifications.
| Feature | Paper chromatography | Thin-layer chromatography |
|---|---|---|
| Stationary phase | Cellulose paper | Silica or alumina coating |
| Typical advantages | Low cost, simple, widely available | Faster and often sharper separations |
| Suitable use | Teaching, screening, basic amino acid analysis | More advanced practical and research work |
| Visualisation | Commonly ninhydrin | Ninhydrin or other stains |
| Main measurement | Distance travelled and (R_f) value | Distance travelled and (R_f) value |
The separation principle
Amino acids contain both acidic and basic functional groups, so their movement depends strongly on polarity and pH. In a typical solvent system, each amino acid partitions differently between the moving liquid and the water associated with the paper. This creates separate spots after the solvent has travelled across the sheet.
The distance is expressed using the retention factor, or (R_f):
[ R_f=\frac{\text{distance travelled by amino acid spot}}{\text{distance travelled by solvent front}} ]
An (R_f) value lies between 0 and 1. It is useful only when conditions are controlled, because solvent composition, temperature, paper type, sample concentration, and chamber saturation can all change the result.
Equipment and chemical preparation
A basic setup requires chromatography paper, a developing chamber with a lid, a pencil, a ruler, capillary tubes or fine spotting tools, amino acid standards, and an unknown mixture. A small beaker or jar can serve as the chamber. Whatman-style papers are commonly available through Australian laboratory suppliers, university stores, and education distributors.
A solvent may contain water, an alcohol such as propan-1-ol or butan-1-ol, and a small amount of acid. Exact recipes vary between laboratory manuals, so the method should follow the validated procedure used by the school, TAFE, or university. Prepare standards at suitable concentrations and label every solution clearly.
Ninhydrin is widely used because it reacts with most amino acids to form coloured products, often purple or blue-purple. It is a hazardous laboratory chemical and should be handled according to the current Safety Data Sheet. Australian laboratories should use appropriate eye protection, gloves, ventilation, and chemical-waste procedures rather than disposing of reagent into a sink.
Preparing and spotting the paper
Draw a light pencil baseline about 1.5–2 centimetres from one edge of the paper. Do not use ink, because dyes from the pen can dissolve and interfere with the chromatogram. Mark separate positions for each standard and the unknown sample, leaving enough space to prevent spots from merging.
Use a fine capillary to apply a very small drop at each mark. Allow the spot to dry before applying another drop in the same position if a stronger signal is needed. Keeping the spots compact is essential: large or wet spots spread sideways and make separation difficult to interpret.
Place the paper in the chamber with the baseline above the solvent level. The solvent must contact the lower edge of the paper without covering the sample spots. Seal the chamber promptly, since evaporation changes the solvent composition and can affect migration.
Developing the chromatogram
Before inserting the paper, allow the closed chamber to become saturated with solvent vapour. This improves reproducibility. Hang or stand the paper so it remains straight and does not touch the chamber walls. Mark the solvent front with pencil as soon as the run is complete.
Development may take from less than an hour to several hours, depending on the solvent and paper dimensions. Keep the chamber away from direct sunlight, heat sources, and strong air movement. A practical class in Melbourne or Adelaide may need extra time if the room is cool, while air conditioning in a Brisbane laboratory can alter evaporation and drying.
Remove the paper before the solvent reaches the top. Dry it in a suitable ventilated area, then apply or develop the ninhydrin stain according to the laboratory method. Some procedures use gentle heating to reveal the colours, but heating must be controlled and performed with approved equipment.
Identifying amino acids
Measure from the baseline to the centre of each visible spot, then measure the distance from the baseline to the solvent front. Calculate the (R_f) value for every standard and unknown spot. An unknown containing spots at matching positions may contain the corresponding amino acids, provided the standards were run under the same conditions.
A single (R_f) match is evidence of identity, not absolute proof. Two different amino acids can sometimes have similar values, while a concentrated sample can conceal a weaker component. Running standards beside the unknown is more dependable than comparing results with a value copied from a textbook or website.
Colour also provides supporting information, but it should not replace distance measurements. If spots overlap, repeat the separation with a different solvent system, a longer paper, or a lower sample concentration. Confirmatory analysis would require an instrumental method such as liquid chromatography.
Common errors and better practice
Streaking often results from applying too much sample, failing to dry between applications, or using a damaged paper surface. No movement may indicate that the solvent is unsuitable, the chamber was assembled incorrectly, or the baseline was submerged. A sloping solvent front usually points to poor paper positioning or an uneven chamber.
Record the solvent composition, paper type, temperature, development time, sample labels, spot distances, and calculated values. Photographing the dried chromatogram can create a useful record for a laboratory report, provided a ruler and labels are included in the image.
For Australian students, local laboratory rules still apply whether the experiment is performed at a university in Canberra, a school in regional New South Wales, or a community workshop. Check the institution’s chemical register, waste instructions, and emergency procedures before starting, and use only reagents supplied through an approved educational or scientific channel.
From classroom result to useful evidence
Paper chromatography works best as a comparative technique. It can show that an unknown behaves like a known standard under defined conditions, reveal whether a mixture contains several components, and demonstrate how molecular properties affect separation. Its low equipment cost makes it especially useful where access to advanced instruments is limited.
Begin with three clearly labelled standards and one prepared unknown. Run them on the same sheet, calculate each (R_f) value, and record the observations in a results table. That single controlled experiment provides a sound starting point for understanding amino acid separation.