What precautions are needed when using a UV lamp to view chromatograms?

Ultraviolet light can damage the eyes and skin. UV-blocking glasses are worn and the lamp is never looked at directly.

How can TLC identify the components of a painkiller?

The painkiller and reference samples of aspirin, paracetamol and caffeine are spotted side by side. A component matches a reference when their spots travel the same distance.

A painkiller spot travels 2.4 cm and another 5.6 cm, with the solvent front at 8.0 cm. What are their Rf values?

The Rf values are 0.30 and 0.70. The working is 2.4 ÷ 8.0 = 0.30 and 5.6 ÷ 8.0 = 0.70.

CCEA A-Level Life and Health Sciences 0008

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Analytical Chemistry Techniques

Visualising chromatograms and identifying painkillers and pigments

3 things to know
A2 9

Overview

UV and ninhydrin visualisation, identifying painkillers with Rf values and co-spotting, amino acids in a mixture, leaf pigments and carotene, impurities and improvements.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why must some chromatograms be visualised?

Key term

How is UV light used to visualise a TLC plate?

Key term

Why must the dried plate be heated after spraying with ninhydrin?

Questions from this subtopic

Each one with the model answer underneath.

1
Why must some chromatograms be visualised?
Model answer
Many substances, such as amino acids and painkillers, are colourless. The spots must be revealed before their positions can be measured.
2
How is UV light used to visualise a TLC plate?
Model answer
Many plates contain a fluorescent indicator that glows under ultraviolet light. Substances that absorb UV block the glow and appear as dark spots, which are circled in pencil.
3
Why must the dried plate be heated after spraying with ninhydrin?
Model answer
Heat speeds up the reaction between ninhydrin and the amino acids. Purple spots then appear clearly.
4
What precautions are needed when using a UV lamp to view chromatograms?
Model answer
Ultraviolet light can damage the eyes and skin. UV-blocking glasses are worn and the lamp is never looked at directly.
5
How can TLC identify the components of a painkiller?
Model answer
The painkiller and reference samples of aspirin, paracetamol and caffeine are spotted side by side. A component matches a reference when their spots travel the same distance.
6
A painkiller spot travels 2.4 cm and another 5.6 cm, with the solvent front at 8.0 cm. What are their Rf values?
Model answer
The Rf values are 0.30 and 0.70. The working is 2.4 ÷ 8.0 = 0.30 and 5.6 ÷ 8.0 = 0.70.

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