Separation by Different Types of Chromatography: Paper Chromatography

Chromatography represents a set of laboratory techniques used for the separation, identification, and purification of the components in a mixture. There exist different types of chromatography such as gas chromatography, liquid chromatography, column chromatography, thin-layer chromatography, and paper chromatography among others. Each method uses a different stationary phase and mobile phase to separate substances based on their physical or chemical properties.

Introduction to Chromatography

In essence, chromatography separates components of a mixture by distributing them between two phases: a stationary phase and a mobile phase. The stationary phase is a solid or a viscous liquid supported on a solid, whereas the mobile phase can be a liquid or a gas that moves over or through the stationary phase. Depending on how each substance interacts with these phasessuch as affinity, polarity, or sizedifferent components travel at different rates, achieving separation.

Chromatography is widely used in chemical analysis, biochemistry, pharmaceuticals, forensic science, food industry, and environmental investigations.

Paper Chromatography: An Overview

Paper chromatography is one of the simplest and most accessible types of chromatography. It utilizes a special kind of filter paper as the stationary phase and a solvent or mixture of solvents as the mobile phase. Since the stationary phase is water trapped in the cellulose fibers of the paper, paper chromatography is sometimes considered a form of partition chromatography.

Paper chromatography was first widely used in the early 20th century and still persists as an excellent educational tool to demonstrate principles of separation and identification of chemical substances.

Principle of Paper Chromatography

The basic principle behind paper chromatography is the difference in partitioning of substances between the paper (stationary phase) and the solvent (mobile phase). When a drop of mixture is applied on the paper and the paper is placed in the solvent, the solvent moves upward through capillary action.

As the solvent migrates, components of the mixture dissolve in the solvent to varying extents and interact differently with water molecules held within the paper fibers. Components more soluble in the mobile phase travel further, while those more attracted to the stationary phase (paper) move slowly.

Materials Used in Paper Chromatography

• Chromatography paper: Special absorbent filter paper designed for chromatography, typically made from cellulose.
• Solvent (mobile phase): The liquid that moves through the paper, such as water, ethanol, acetone, or a mixture chosen based on the compounds to be separated.
• Sample mixture: The mixture of substances to be separated, which can be dyes, amino acids, plant pigments, inks, etc.
• Capillary tubes or micropipettes: To apply small drops of sample on the paper.
• Developing chamber: A sealed container where the chromatography paper stands vertically with the bottom immersed in solvent.

Procedure of Paper Chromatography

The general steps to perform paper chromatography are as follows:

1. Preparation of the paper: Cut a strip of chromatography paper, typically 1520 cm long and about 23 cm wide.
2. Mark a baseline: Using a pencil, draw a line about 2 cm from the bottom edge of the paper; this is where the sample spots will be applied.
3. Spot the sample: Apply small drops of the sample mixture on the baseline at spaced intervals, using a capillary tube or micropipette. Allow the spots to dry before adding more layers to concentrate the spot.
4. Prepare the solvent: Pour the chosen solvent into the developing chamber to a depth of about 12 cm. The solvent level must be below the baseline on the paper to avoid dissolving the spots directly in the solvent.
5. Develop chromatogram: Suspend the paper in the chamber vertically, with the baseline above the solvent level. Seal the chamber to saturate the environment with solvent vapor, improving solvent migration.
6. Allow solvent to migrate: The solvent will ascend the paper by capillary action, carrying different components at different rates.
7. Dry and analyze the chromatogram: Once the solvent front reaches near the top of the paper (or after a suitable time), remove the paper, mark the solvent front line immediately, and allow it to dry.

Interpreting the Results

After the chromatogram dries, different separated spots should be visible. Depending on the sample, spots may be colored or colorless (requiring detection methods like UV light or staining agents).

The position of each spot relative to the baseline and solvent front is quantified by the retention factor, R_f, calculated as:

R_f = (Distance traveled by the substance) (Distance traveled by the solvent front)

The R_f value is always between 0 and 1 and is characteristic of a substance under specific conditions. Comparing R_f values with known standards helps identify compounds.

Types of Paper Chromatography

There are a few variants of paper chromatography based on the nature of the mobile phase and mechanism of separation:

• Ascending paper chromatography: The solvent moves upward against gravity due to capillary action; most commonly used method.
• Descending paper chromatography: The solvent moves downward due to gravity, typically with the paper suspended over the solvent reservoir.
• Radial paper chromatography: Sample is applied at the center of a circular paper and solvent moves radially outward.
• Two-dimensional paper chromatography: Two different solvents are used successively in perpendicular directions to improve separation.

Applications of Paper Chromatography

Paper chromatography finds diverse applications in chemistry and biology due to its simplicity and low cost:

• Separation and identification of plant pigments: Like chlorophyll, carotenoids, and xanthophylls in green leaves.
• Analysis of amino acids and proteins: Identifying amino acid composition after hydrolysis.
• Detection of drugs and pharmaceuticals: Screening for presence of drugs in biological fluids.
• Ink and dye analysis: Used in forensic science to identify pen inks.
• Food industry: Identification of additives and dyes.

Advantages of Paper Chromatography

• Simple and inexpensive: Requires minimal equipment and materials.
• Visual and easy to perform: Suitable for educational demonstrations and quick qualitative analysis.
• Low sample quantity: Only minute amounts of sample are needed.
• Non-toxic solvents: Many solvents used are relatively safe.
• Adaptable: Can separate mixtures of different chemical classes like pigments, amino acids, sugars, and more.

Limitations of Paper Chromatography

• Limited resolution: Less efficient for complex mixtures or substances with very similar properties.
• Slow process: Migration and separation can take a long time.
• Qualitative rather than quantitative: Mostly used for identification; precise quantification is challenging.
• Limited to relatively polar and water-soluble substances: Non-polar compounds are harder to separate.
• Reproducibility issues: Environmental factors like humidity and temperature can affect results.

Comparison with Other Chromatography Types

While paper chromatography is excellent for simple qualitative analysis, other chromatography methods provide more power for complex separations:

• Thin layer chromatography (TLC): Uses a thin layer of silica gel or alumina on a glass plate instead of paper. Offers faster separation and better resolution.
• Column chromatography: Employs a column packed with stationary phase allowing preparative isolation of larger quantities.
• Gas chromatography (GC): Suitable for volatile compounds, providing high resolution and quantitative capabilities.
• High-performance liquid chromatography (HPLC): Highly precise and sensitive, widely used for pharmaceuticals and biochemical analysis.

Despite these alternatives, paper chromatography remains important for basics, educational labs, and simple qualitative tests.

Summary

Paper chromatography is a classical technique for separating components of mixtures based on differential partitioning between a stationary phase (water trapped in cellulose fibers) and a mobile phase (solvent). It is easy to perform, economical, and visually illustrative, making it ideal for educational purposes and quick preliminary analysis.

This technique has helped scientists identify plant pigments, analyze amino acids, and detect adulterants in inks and foods. While it lacks the sophistication and resolution of advanced chromatographic techniques, paper chromatography remains a foundational tool in analytical chemistry and biochemistry.