Chromatography and Purity

Introduction

In chemistry, a pure substance contains only one element or compound. Earlier in this unit, purity was tested with melting and boiling points. Chromatography offers a different test that works with tiny samples and does not need heating: count the spots. It is so sensitive and so simple that it is used to check medicines, detect food fraud and link evidence to suspects at crime scenes.

Core explanation

One spot or many. When a sample is run on a chromatogram:

- a pure substance gives one spot , because it contains only one component; - an impure substance or mixture gives two or more spots , one for each component that separates and can be seen.

A pure substance also gives a spot with the same Rf value as a genuine reference sample of that substance. So chromatography can confirm both what a substance is and whether anything else is present.

Limits of the test. One spot is good evidence of purity, but not absolute proof:

1. An impurity might have the same Rf value as the main substance in that solvent, so the two spots overlap. 2. An impurity might stay on the baseline or be colourless and go unnoticed. 3. A very small amount of impurity may give a spot too faint to see.

Running the sample in two or more different solvents, and treating the paper to reveal colourless spots, makes the test much more reliable. Combining chromatography with a melting-point test gives stronger evidence still.

Forensic uses. Forensic scientists use chromatography to:

- compare the ink on a forged cheque or threatening letter with inks from suspects' pens; - identify drugs or poisons in samples from a crime scene or a body; - compare dyes in fibres found at a scene with fibres from a suspect's clothing.

A match does not prove guilt on its own, but it can link pieces of evidence together.

Food testing. Food scientists use chromatography to:

- check that only permitted colourings have been used, and none that are banned; - detect adulteration , such as cheaper oils mixed into expensive olive oil or sugar syrups added to honey; - measure pesticide residues and check that they are within legal limits.

Medicines. Drug manufacturers run chromatography on every batch to confirm that the active ingredient is present and that impurities are below safe limits.

Step-by-step reasoning

To decide whether a sample is pure:

1. Run it beside a reference sample of the pure substance. 2. Count the spots in the sample column. 3. One spot matching the reference suggests purity; extra spots show impurities. 4. Repeat in a second solvent to confirm.

Visual explanation

Picture three columns on a chromatogram. The reference aspirin shows one spot at Rf 0.60. Batch 1 shows a single spot at exactly 0.60. Batch 2 shows the same spot plus a small faint spot at 0.20 — an impurity that fails the quality check.

Real-world analogy

Checking purity with chromatography is like shaking a bag of what should be only white rice onto a tray. If everything looks identical, the rice is probably pure. A few different grains or bits of grit stand out immediately. But a grain that looks exactly like rice could still slip through, so a second, different check is wise.

Real-world example

Forensic document examiners have used chromatography to expose forged documents. When an extra line or figure was added to a genuine document with a different pen, running tiny samples of each part of the writing showed different dye patterns, revealing that two inks had been used.

Why?

Why does a pure substance give only one spot? A pure substance is made of only one kind of particle. All its particles have the same balance between the mobile and stationary phases, so they all travel the same distance and stay together as a single spot.

Common misconception

"If a sample shows one spot, it is definitely pure." It may contain an impurity that runs at the same Rf, stays invisible or is present in too small an amount to see. One spot in several different solvents is much stronger evidence of purity.

Worked example

Question: A sample of a food colouring labelled as the single dye "brilliant blue" gives two spots: a large blue spot at Rf 0.55 and a small yellow spot at Rf 0.80. A reference sample of brilliant blue gives one spot at Rf 0.55. What can you conclude?

Reasoning: The blue spot matches the reference, so brilliant blue is present. The extra yellow spot shows another substance.

Answer: The colouring contains brilliant blue but is not pure; it contains at least one other substance (the yellow dye).

Quick check

1. How many spots does a pure substance give on a chromatogram? Answer: One.

Exam focus

Link chromatography to purity: pure gives one spot, impure gives more than one. Be ready to explain why one spot is not complete proof, and to describe one forensic and one food-testing use. Questions may combine chromatography with melting-point evidence.

Advanced insight

In industry, "pure" often means pure to a stated standard. A medicine may be required to contain, for example, no more than 0.1% of any single impurity. Modern chromatography can measure spot or peak sizes precisely, so it can report not just whether impurities are present but how much of each there is.

Summary

A pure substance gives a single spot on a chromatogram, while an impure substance or mixture gives two or more. One spot is strong but not absolute evidence of purity, so results are confirmed with different solvents or other tests. Chromatography is used to check medicines, detect food colourings and adulteration, and compare inks, drugs and fibres in forensic investigations.

Practice questions

1. A sample gives three spots. Is it pure? Explain. Answer: No. Three spots show it contains at least three different substances. 2. Give two reasons why a single spot does not prove a substance is pure. Answer: An impurity could have the same Rf value and overlap, or it could be colourless, stay on the baseline or be too faint to see. 3. Describe one way chromatography is used in forensic science. Answer: Comparing the ink on a document with inks from suspects' pens to see whether the dye patterns match. 4. How could chromatography show that honey has been adulterated? Answer: A sample of the honey would show extra spots from the added syrup that are absent in a genuine honey sample.