Interpreting Chromatograms

Basics of Chromatograms

A chromatogram is a visual output from chromatography, showing the separation of components in a mixture. It helps chemists identify and analyse substances based on how far each component travels on the stationary phase when moved by the mobile phase.

The stationary phase is the solid or liquid that stays fixed in place, such as paper or a thin layer of solid on a plate. The mobile phase is the solvent or gas that moves through the stationary phase, carrying the components of the mixture with it.

Different substances travel different distances because they interact differently with the stationary and mobile phases. This creates separate spots or peaks on the chromatogram, each representing a component.

Retention factor (Rf) values are used to compare how far substances move relative to the solvent front. A simple Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front. Detailed calculations of Rf values are covered in Higher Tier topics.

Interpreting Chromatogram Spots

The number of spots on a chromatogram tells you how many different components are in the mixture. For example, one spot means the sample is likely pure or contains only one substance, while multiple spots show a mixture.

The position of each spot depends on the polarity of the substance. Less polar substances tend to travel further with the mobile phase, while more polar substances stick more to the stationary phase and travel less.

The size or intensity of a spot relates to the concentration of that component in the mixture. A larger or darker spot usually means a higher concentration.

For instance, if a chromatogram shows three spots at different heights, it means the sample contains three components with different polarities and concentrations.

Using Chromatograms to Assess Purity

A pure substance produces only one spot on a chromatogram because it contains only one component. If more than one spot appears, it indicates impurities or a mixture of substances.

To check purity, you can compare the chromatogram of an unknown sample with chromatograms of known pure substances. If the sample’s spots match exactly in position and number with a known pure substance, it is likely pure.

If extra spots appear or the spots do not match the known sample, the substance is impure or a mixture.

For example, if a sample shows only one spot matching a pure substance, it is likely pure.

For example, a sample of food colouring might show one spot if pure, but if it shows two or three spots, it contains different dyes or impurities.

Applications in Formulations

Chromatograms are useful in analysing formulations, which are mixtures designed for specific purposes, like medicines, cleaning products, or food.

By identifying the components in a formulation, chemists can check if the correct substances are present and in the right proportions. This ensures the product works properly and is safe.

Chromatography also helps check the consistency of products from batch to batch. If chromatograms differ, it may indicate problems in production or contamination.

Quality control uses chromatograms to confirm that formulations meet required standards before they reach consumers.

For example, a pharmaceutical company uses chromatography to verify that a medicine contains the correct active ingredient and no harmful impurities.

Learning Example

A student runs a chromatogram of a sample of ink and sees three spots. The known blue ink sample shows only one spot at the same position as the first spot in the sample. What does this tell us about the sample?

Since the sample has three spots but the known blue ink has only one, the sample is a mixture containing the blue ink and two other substances. It is not pure blue ink.