Paper Chromatography: Principle, Procedure, Types and Uses

Put a single spot of black ink near the bottom of a strip of filter paper, stand the strip in a little water, and wait. Within minutes the black spot climbs and splits into separate rings of blue, pink, and yellow, because "black" ink is really a mixture of dyes, and each one is carried a different distance. Do the same with a smear of crushed green leaf and you will see the chlorophyll separate from the yellow and orange pigments hiding beneath it. That simple, beautiful experiment is paper chromatography, one of the easiest ways to separate the parts of a mixture. This article explains what it is, the principle behind it, its types, the full procedure, and where it is used.

What is paper chromatography?

Paper chromatography is a technique that separates the components of a mixture as they move along a strip of special paper carried by a solvent. A small spot of the sample is placed near one end of the paper, and the edge of the paper is dipped in a solvent. As the solvent soaks through the paper by capillary action, it carries the components of the mixture with it, and because each component travels at a different speed, they separate into distinct spots.

Although the paper looks like the important part, the real stationary phase is the water held within the fibers of the paper. The moving solvent is the mobile phase. Paper chromatography is one of the simplest members of the chromatography family and is closely related to thin layer chromatography (TLC).

Why paper chromatography matters

Paper chromatography is popular because it is extremely cheap, simple, and needs almost no equipment, just paper, solvent, and a jar. That makes it the classic first experiment for learning how separation works, and it is still genuinely useful for quickly separating and identifying colored substances such as pigments, dyes, and inks, and for teaching how the Rf value works.

Principle of paper chromatography

Paper chromatography works by partition. This is a key point that students often get wrong: the separation is not mainly the paper "adsorbing" the sample, but the sample dividing itself between two liquids.

The paper naturally holds a thin layer of water trapped in its cellulose fibers, and this water is the stationary phase. The solvent that climbs the paper is the mobile phase. As the solvent rises, each component of the mixture constantly partitions (shares itself) between the stationary water and the moving solvent:

• A component that is more soluble in the moving solvent spends more time in the mobile phase and travels far up the paper.
• A component that is more attracted to the stationary water stays behind and travels only a short distance.

Because different substances have different solubilities, they move at different speeds and separate into distinct spots.

The Rf value

The movement of each spot is described by its Rf value (retention factor), exactly as in TLC:

Rf = distance traveled by the component / distance traveled by the solvent front

Both distances are measured from the origin (the starting spot). The Rf value has no units and is always between 0 and 1, because a spot can never move faster than the solvent carrying it.

Worked example: if the solvent front rises 12 cm from the origin and a spot travels 6 cm, then Rf = 6 / 12 = 0.5. Under the same paper and solvent, that substance will always give an Rf near 0.5, so the value helps identify it.

Types of paper chromatography

• Ascending: the solvent moves upward through the paper against gravity. This is the most common and simplest form.
• Descending: the solvent is placed in a trough at the top and moves downward, helped by gravity, which makes it faster.
• Ascending-descending: a combined form where the solvent first rises and then descends, giving a longer run.
• Radial (circular): the sample is spotted at the center of a circular paper and the solvent spreads outward, separating the components into rings.
• Two-dimensional (2D): the sample is run with one solvent, the paper is turned 90 degrees, and it is run again with a second solvent. This separates complex mixtures such as amino acids far more clearly.

Parts and materials

• Chromatography paper: a high-quality filter paper (such as Whatman paper) that holds water in its fibers.
• Stationary phase: the water held within that paper.
• Mobile phase (solvent): a solvent or a mixture of solvents chosen to suit the sample.
• Developing chamber: a closed jar or tank with a lid, which keeps the air inside saturated with solvent vapor.
• Capillary tube or micropipette: to apply a small, neat sample spot.
• Detecting agent: a UV lamp or a spray reagent (such as ninhydrin for amino acids) to reveal colorless spots.

Procedure of paper chromatography (step by step)

1. Prepare the paper. Cut a strip of chromatography paper and draw a faint pencil origin line about 2 cm from one end.
2. Spot the sample. Using a capillary tube, place a small, concentrated spot of the sample on the origin line and let it dry. Keep the spot small for a sharp result.
3. Add the solvent. Pour a shallow layer of solvent into the jar. The solvent level must stay below the origin line, so the spot is not dipped directly into the solvent.
4. Develop the chromatogram. Stand the paper in the jar and close the lid. The solvent rises through the paper by capillary action, carrying the components upward and separating them.
5. Mark the solvent front. Remove the paper before the solvent reaches the top, and immediately mark how far the solvent traveled (the solvent front). Then let the paper dry.
6. Detect the spots. Colored spots are seen directly; colorless ones are revealed under a UV lamp or with a spray reagent.
7. Calculate the Rf values. Measure each spot's distance from the origin, divide by the solvent-front distance, and record the Rf values to identify the components.

Where students get confused

• The stationary phase is water, not the paper itself. The paper is only the support; the water trapped in its fibers is what holds the sample back. This is why the mechanism is partition, not adsorption.
• The spot must sit above the solvent. If the origin line dips into the solvent, the sample simply washes into the solvent instead of climbing the paper.
• Rf is always 1 or less. A spot cannot outrun the solvent that carries it, so an Rf above 1 means the two distances were swapped.
• Paper chromatography versus TLC. Both use the Rf value and look similar, but paper uses water in cellulose (partition), while TLC uses a thin layer of adsorbent (often silica) on a plate. TLC is faster, sharper, and more sensitive.
• Measure from the origin, not the bottom of the paper. Both distances start at the origin line.

How to remember

• "Water hides in the paper." The real stationary phase is the water held in the paper's fibers, so paper chromatography is a partition method.
• Rf = "Ran far?" A high Rf means the component ran far up the paper because it preferred the moving solvent.
• Radial spreads like ripples. In radial paper chromatography the spots spread outward in rings, like ripples from a stone dropped in water.

Applications of paper chromatography

• Separating plant pigments: splitting a leaf extract into chlorophyll a, chlorophyll b, carotenoids, and xanthophylls, a classic biology experiment.
• Separating amino acids and sugars: widely used in biochemistry teaching, often with two-dimensional runs and a ninhydrin spray for amino acids.
• Analyzing inks and dyes: identifying the dyes in pens and food colorings, and in forensic document work.
• Qualitative analysis: checking whether a sample contains a particular component by comparing Rf values with known standards.
• Teaching: demonstrating the principle of chromatography simply and cheaply.

Advantages of paper chromatography

Paper chromatography is very cheap and simple, needs minimal equipment, uses tiny samples, and is easy to perform, which makes it ideal for teaching and for quick qualitative checks. Several samples can be run side by side for direct comparison.

Limitations of paper chromatography

It is slow compared with TLC, gives less sharp separation and lower sensitivity, and is mainly qualitative rather than quantitative. It works best for small, mostly colored samples, is not suited to large-scale separation, and results can be affected by humidity and the quality of the paper. For sharper, faster results, TLC is usually preferred.

Key exam points

References

1. Consden R, Gordon AH, Martin AJP. Qualitative analysis of proteins: a partition chromatographic method using paper. Biochemical Journal. 1944;38(3):224-232.
2. Harris DC. Quantitative Chemical Analysis. 9th ed. New York: W. H. Freeman; 2015.
3. Sherma J, Fried B. Handbook of Thin-Layer Chromatography. 3rd ed. New York: Marcel Dekker; 2003.
4. Wilson K, Walker J. Principles and Techniques of Biochemistry and Molecular Biology. 7th ed. Cambridge University Press; 2010.