Mixtures and Separation
5. Chromatography
Learning outcomes
- I can explain how chromatography separates substances in a mixture.
- I can identify the stationary phase and mobile phase in a chromatography experiment.
- I can interpret simple chromatograms.
- I can calculate and use values where appropriate.
- I can describe applications of chromatography in science, medicine, and forensic investigations.
Introduction
Many substances that appear to be a single colour or material are actually mixtures of several different substances. For example, black ink may contain blue, purple, and green dyes, while plant leaves contain several different pigments. To separate and identify these substances, scientists often use a technique called chromatography.
Chromatography is one of the most powerful separation methods used in chemistry. It allows scientists to separate tiny amounts of substances, identify unknown chemicals, test the purity of compounds, and analyse complex mixtures. Today, chromatography is widely used in medicine, environmental science, food testing, and forensic investigations.
What Is Chromatography?
Chromatography is a separation technique used to separate the components of a mixture.
It works because different substances move at different speeds through a material.
As the substances move, they separate into distinct spots or bands.
Chromatography is commonly used to separate:
- Inks
- Food colourings
- Plant pigments
- Medicines
- Biological samples
Figure 1. Chromatography separates the different substances in a mixture as they move across the paper.
How Chromatography Works
In paper chromatography:
- A small spot of the mixture is placed near the bottom of the paper.
- The paper is placed in a solvent.
- The solvent rises up the paper.
- The substances dissolve in the solvent.
- Different substances travel different distances.
- The mixture separates into individual spots.
The separated pattern is called a chromatogram.
The Stationary and Mobile Phases
Chromatography uses two important components.
Stationary Phase
The stationary phase does not move.
In paper chromatography:
- The chromatography paper is the stationary phase.
Some substances stick strongly to the paper and move slowly.
Mobile Phase
The mobile phase moves through the stationary phase.
In paper chromatography:
- The solvent is the mobile phase.
It carries the dissolved substances up the paper.
Different substances travel at different speeds depending on how strongly they are attracted to the paper and how well they dissolve in the solvent.
Figure 2. The stationary phase remains fixed while the mobile phase carries substances through it.
Why Substances Separate
Each substance has different properties.
Some:
- Dissolve well in the solvent.
- Travel further.
Others:
- Stick more strongly to the paper.
- Travel more slowly.
The balance between these two effects causes the mixture to separate.
This allows scientists to identify individual substances within a mixture.
Interpreting a Chromatogram
A chromatogram is the final pattern of separated spots.
Scientists can use it to determine:
- How many substances are present.
- Whether two samples contain the same components.
- Whether a sample is pure or a mixture.
For example:
A pure substance usually produces:
- One spot.
A mixture usually produces:
- Two or more spots.
If two samples produce spots at the same height under identical conditions, they may contain the same substance.
Figure 3. A chromatogram can reveal whether a sample is pure or contains multiple substances.
Rf Values
Scientists often compare substances using their Rf value (Retention factor).
The formula is:
Rf = Distance travelled by the substance ÷ Distance travelled by the solvent front
The Rf value is always between 0 and 1.
For example:
Distance travelled by spot = 6 cm
Distance travelled by solvent front = 8 cm
Rf = 6 ÷ 8
Rf = 0.75
Rf values help scientists compare substances tested under the same conditions.
Calculating an Rf Value
Example
Distance travelled by substance:
4.5 cm
Distance travelled by solvent front:
7.5 cm
Rf = 4.5 ÷ 7.5
Rf = 0.60
Scientists compare calculated Rf values with known values to help identify unknown substances.
Figure 4. The Rf value compares the distance travelled by a substance with the distance travelled by the solvent front.
Applications of Chromatography
Chromatography is widely used in science.
Medicine
Scientists analyse:
- Blood samples.
- Urine samples.
- Medicines.
Forensic Science
Investigators compare:
- Ink from documents.
- Paint samples.
- Drug samples.
- Poison residues.
Food Industry
Manufacturers test:
- Food colourings.
- Flavourings.
- Additives.
- Contaminants.
Environmental Science
Scientists monitor:
- Water pollution.
- Air pollution.
- Soil contamination.
Chromatography helps detect substances even when only tiny amounts are present.
Figure 5. Chromatography is widely used in forensic science, medicine, environmental monitoring, and food testing.
Advantages of Chromatography
Chromatography is valuable because it:
- Separates complex mixtures.
- Identifies unknown substances.
- Requires only small samples.
- Is highly accurate.
- Can detect tiny amounts of material.
Modern laboratories use many advanced forms of chromatography for scientific research and quality control.
Worked Example
Question
A chromatography experiment gives the following results:
- Distance travelled by solvent front = 10 cm
- Distance travelled by a spot = 7 cm
Calculate the Rf value.
Solution
Rf = Distance travelled by substance ÷ Distance travelled by solvent front
Rf = 7 ÷ 10
Rf = 0.70
Real-World Connection
Chromatography is an essential tool in forensic laboratories. If police recover a handwritten note from a crime scene, scientists can separate the dyes in the ink and compare the chromatogram with inks from suspected pens. Similar techniques are used to identify drugs in blood samples, detect banned substances in athletes, and monitor pollutants in rivers and drinking water.
Did You Know?
Modern analytical laboratories often use highly advanced forms of chromatography, such as gas chromatography (GC) and high-performance liquid chromatography (HPLC). These techniques can detect substances present in amounts as small as a few billionths of a gram, making them invaluable in medicine, environmental monitoring, and scientific research.
Key Terms
Chromatogram – The pattern of separated substances produced during chromatography.
Chromatography – A technique used to separate the components of a mixture.
Mobile phase – The moving substance (usually a solvent) that carries the sample through the stationary phase.
Pure substance – A material that produces a single spot under suitable chromatography conditions.
Retention factor (Rf) – The ratio of the distance travelled by a substance to the distance travelled by the solvent front.
Solvent – A liquid used to dissolve and transport substances during chromatography.
Solvent front – The furthest point reached by the solvent during a chromatography experiment.
Stationary phase – The material that remains fixed while the mobile phase moves through it.
Key Takeaways
- Chromatography separates the components of a mixture because different substances move at different speeds.
- The stationary phase remains fixed, while the mobile phase carries substances through it.
- A chromatogram can show whether a sample is pure or a mixture.
- Rf values are calculated by dividing the distance travelled by the substance by the distance travelled by the solvent front.
- Chromatography is widely used to identify substances and test purity.
- Important applications include medicine, forensic science, environmental monitoring, and food analysis.