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

The mobile phase is the solvent that moves through the system carrying the solutes, while the stationary phase is the fixed material (the paper) that the solutes interact with to slow down their movement.

How does the solubility of a monosaccharide in the mobile phase affect its position on a chromatogram?

Higher solubility in the mobile phase means the monosaccharide spends more time dissolved in the moving solvent. Consequently, it will travel a greater distance from the origin line compared to less soluble components.

Why must the starting line on chromatography paper be drawn in pencil rather than ink?

Ink contains various dyes that are soluble in the mobile phase and would separate, interfering with the sample results. Pencil lead (graphite) is insoluble and remains at the origin, providing a clean reference point.

What is the purpose of using 'known standard solutions' when performing chromatography on an unknown mixture of sugars?

Standards provide a reference for identification. Since a specific sugar will always travel the same relative distance under identical conditions, matching the distance of an unknown spot to a standard spot identifies the sugar.

What would happen if the solvent level in the beaker was higher than the pencil line where the sugar spots were placed?

The sugar samples would dissolve into the bulk solvent in the beaker rather than being carried up the paper. This prevents any separation from occurring and results in a failed chromatogram.

Define the term 'chromatogram' in the context of this technique.

A chromatogram is the visual result of a chromatography run, consisting of the stationary phase (paper) with the separated components visible as distinct spots at various heights.

Why is a staining agent necessary when analyzing monosaccharides via paper chromatography?

Monosaccharides are colourless molecules and are not visible to the naked eye on the paper. A staining agent reacts with the sugars to produce colored spots, allowing their positions to be measured and analyzed.

How does molecular size influence the separation of different sugars on the paper?

Smaller molecules generally move faster through the fibrous structure of the chromatography paper. Larger molecules face more resistance and move more slowly, leading to separation based on size as well as solubility.

When comparing two spots on a chromatogram, if Spot A is higher than Spot B, what can you conclude about their solubility?

Spot A is more soluble in the mobile phase than Spot B. Because it is more soluble, it remained in the moving solvent for a longer duration, allowing it to be carried further up the paper.

What is a common mistake made during the 'setup' phase that prevents the spots from being visible at the end?

Forgetting to apply a staining reagent or dye to the colourless monosaccharides. Without this step, the sugars remain invisible on the paper even if they have successfully separated.

Chromatography: Monosaccharides | AQA A-Level Biology

Chromatography: Monosaccharides

Summary

Chromatography is a fundamental analytical technique used to separate and identify the individual components of a mixture, such as different monosaccharides. It relies on the differential distribution of solutes between a mobile phase and a stationary phase, where separation is driven by variations in solubility and molecular size.

1. Definition & Core Concepts

Chromatography is a laboratory technique designed to separate a complex mixture into its constituent parts based on their physical properties. In the context of carbohydrates, it allows scientists to distinguish between similar simple sugars like glucose, fructose, and galactose.
The process involves a stationary phase, which is a fixed material that the mixture is applied to, and a mobile phase, which is a solvent that moves through the stationary phase. In paper chromatography, the paper itself acts as the stationary phase while a liquid solvent serves as the mobile phase.
A chromatogram is the final visual output of the process, showing distinct spots or bands at different positions. Each spot represents a specific component of the original mixture that has been isolated during the run.

A 2D diagram of a paper chromatography setup showing the stationary phase paper, the mobile phase solvent at the bottom, the origin line, and separated spots at different heights.

2. Underlying Principles

Separation occurs due to the differential solubility of the solutes in the mobile phase. Components that are more soluble in the solvent will spend more time in the mobile phase and thus travel a greater distance up the stationary phase.
Molecular size also plays a critical role in the speed of migration through the stationary phase. Generally, smaller molecules can navigate the pores of the chromatography paper more easily and move faster than larger, more bulky molecules.
The interaction between the solute and the stationary phase (adsorption) acts as a retarding force. The final position of a substance on the chromatogram is the result of the balance between its affinity for the solvent versus its affinity for the paper.

3. Methods & Techniques

The process begins by drawing a baseline in pencil near the bottom of the chromatography paper. Pencil is used because graphite is insoluble and will not interfere with the chromatogram, unlike ink which would separate into its own colors.
Because monosaccharides are naturally colourless, the mixture and the standards must be treated with a staining agent to make them visible. This is often done by spraying the paper with a specific reagent after the solvent has finished traveling.
Standard solutions of known monosaccharides are spotted alongside the unknown sample on the same baseline. These serve as a reference point, allowing for the identification of unknown components by comparing the distances they travel under identical conditions.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions