What is the fundamental difference between the mobile phase and the stationary phase in chromatography?

The mobile phase is the solvent that moves through the system carrying the solutes, while the stationary phase is the fixed material (like paper) that the solutes interact with. Separation occurs because different solutes distribute themselves differently between these two phases.

Why must the solvent level be below the origin line at the start of the experiment?

If the solvent level is above the origin line, the sample spots will wash off the paper and dissolve into the bulk solvent in the beaker. This prevents the solutes from traveling up the paper via capillary action, making separation impossible.

Define the $R_f$ value and state its formula.

The $R_f$ (Retardation Factor) value is a ratio used to quantify the movement of a solute relative to the solvent. It is calculated as $R_f = \frac{\text{distance moved by solute}}{\text{distance moved by solvent front}}$.

How does molecular size typically affect the migration of a solute in paper chromatography?

Generally, larger molecules move more slowly through the stationary phase because they experience more physical resistance within the fiber matrix of the paper. Smaller molecules can navigate the pores more easily and thus tend to have higher $R_f$ values if their solubility is similar.

Why is a pencil used to draw the origin line instead of a pen?

Ink from a pen is a mixture of dyes that are soluble in the chromatography solvent and would separate, interfering with the sample results. Pencil graphite is insoluble and will not move or separate during the process.

What is a 'locating agent' and when is it required?

A locating agent is a chemical (like ninhydrin) used to make colorless spots visible on a chromatogram. It is required when separating substances that do not naturally absorb visible light, such as amino acids or certain sugars.

If two substances have the same $R_f$ value in one solvent, are they definitely the same substance?

Not necessarily; they might have similar solubilities in that specific solvent but different properties in another. To confirm identity, the substances should be tested using different solvents or stationary phases to see if they remain inseparable.

What does a very low $R_f$ value (close to 0) indicate about a solute's properties?

A low $R_f$ value indicates that the solute has a very high affinity for the stationary phase and low solubility in the mobile phase. It suggests the molecule is either very large or possesses functional groups that bind strongly to the paper.

How does the solubility of a solute in the mobile phase relate to the distance it travels?

There is a direct relationship: the more soluble a solute is in the mobile phase, the further it will travel. This is because the molecule spends a greater proportion of time in the moving solvent rather than being adsorbed onto the stationary phase.

What error occurs if the solvent front is allowed to reach the very top of the chromatography paper?

If the solvent front reaches the top, it becomes impossible to measure the total distance traveled by the solvent. This makes the calculation of $R_f$ values inaccurate because the denominator of the ratio is no longer known.

Chromatography | OCR A-Level Biology

2. Foundations in Biology

Chromatography

Summary

Chromatography is a versatile analytical technique used to separate, identify, and quantify the individual components within a complex biological mixture. It operates on the principle of differential distribution between a mobile phase and a stationary phase, allowing for the precise isolation of molecules such as amino acids, sugars, and plant pigments based on their unique physical and chemical properties.

1. Definition & Core Concepts

Chromatography is a laboratory technique used to separate a mixture into its constituent parts, known as solutes. This is essential in biology for purifying substances like proteins or identifying unknown metabolites in a sample.
The process involves two distinct components: the stationary phase, which remains fixed in place (such as paper or a silica gel layer), and the mobile phase, which is a solvent that moves through the stationary phase.
Separation occurs because different solutes have different affinities for the two phases. A substance that is more soluble in the mobile phase will move faster and further than one that adheres more strongly to the stationary phase.

Diagram of a paper chromatography setup showing the stationary phase (paper), mobile phase (solvent), origin line, and separated solute spots.

2. Underlying Principles

The separation is driven by differential solubility. Solutes that are highly soluble in the mobile phase spend more time moving with the solvent, resulting in a greater distance traveled from the origin.
Molecular size and charge also play critical roles, especially in the separation of amino acids. Larger molecules often encounter more resistance within the stationary phase matrix, causing them to move more slowly than smaller, more agile molecules.
The interaction between the solute and the stationary phase is often referred to as adsorption. Stronger adsorption forces (like hydrogen bonding or ionic attractions) will retard the movement of the solute, keeping it closer to the starting point.

3. Methods & Techniques

Preparation: A straight line is drawn in pencil near the bottom of the chromatography paper to mark the 'origin'. Pencil is used because graphite is insoluble and will not interfere with the chromatogram, unlike ink.
Loading: A small, concentrated spot of the mixture is placed on the origin line and allowed to dry. This ensures the components start from the same point and do not smear excessively.
Development: The paper is suspended in a container with the solvent level below the origin line. Capillary action draws the solvent up the paper, carrying the solutes with it.
Visualization: If the components are colorless (like amino acids), a locating agent such as ninhydrin must be sprayed on the dried paper to produce visible colored spots (typically purple or brown).

4. Quantitative Analysis: The $R_f$ Value

5. Key Distinctions

6. Exam Strategy & Tips