GCSE Chemistry Practical: Chromatography

Chromatography is an important analytical technique that’s used to separate a mixture into its individual components. 

Its invention has contributed to scientific advances in various fields including the agricultural sector, biotechnology, food science, pharmaceuticals, and the chemical manufacturing industry.

In this article, we explain the principles behind chromatography and provide step-by-step instructions so you can perform your own simple experiment. It’s a great way to enhance your analytical skills and support your GCSE chemistry studies.

What is chromatography?

Chromatography is a physical process of separating a mixture into its distinct chemical components. 

Once separated, the individual substances can then be analysed more closely. A mixture can be a solution, a colloid, or a suspension. It could also be in various states of matter such as solid, liquid, and gas. 

So, exactly how does the chromatography process work? 

Firstly, the mixture that’s being analysed is dissolved in a fluid solvent. This fluid, which can be a liquid or a gas, is known as the ‘mobile phase’. It carries the mixture’s components through a system (e.g. a sheet of paper or a steel column) on which the material is fixed. This is called the ‘stationary phase’. The constituent substances separate because they interact differently with the mobile and stationary phases.

Chromatography was originally developed in 1903 by the botanist Mikhail Tswett, who separated plant pigments like chlorophyll and carotenoids. 

Today, chromatography is used to analyse the chemical composition and molecular properties of a variety of mixtures. Its versatility means it has a wide range of uses in both chemistry and everyday life (more on this later).

The objective of a practical chromatography experiment

This practical experiment aims to help you understand the principles behind chromatography and improve your laboratory and analytical skills. Using simple materials and equipment from the school chemistry laboratory, you’ll learn how to separate the components of a mixture for the purpose of analysis.

There are various types of chromatography techniques, including gas chromatography and reverse-phase chromatography. However, many of these require expensive lab equipment. For simplicity and practicality purposes, we’ll be using paper chromatography to investigate whether certain pigments can be further separated.

Materials and equipment needed

Below is a list of the materials and equipment you’ll need to perform this simple paper chromatography experiment. You should be able to get hold of most of these materials in your school laboratory or classroom.

Materials:

• Chromatography paper
• Gel pens
• Eluting solution

Equipment:

• 500 mL beaker
• Pencil
• Ruler
• Plastic wrap
• Tape
• Paper towels

Step-by-step breakdown: the procedure

This experiment can be performed either individually or with a laboratory partner. It’s important to follow the instructions and record the results carefully.

Preparing the setup

Step 1. Wash your hands thoroughly to remove excess oil. Hold the edges of chromatography paper but be careful not to leave any fingerprints. 

Step 2. Put the chromatography paper on a paper towel to prevent it from being contaminated with dirt. The paper should be landscape. Using a ruler, draw a straight line across the paper about 1.5 cm from the bottom. 

Next, draw a parallel line 10 cm from the bottom edge. The first line will serve as the starting line while the second one will act as the finish line.

Applying the sample

Step 1. Measure about 2.5 cm from the starting line on the right side. Lightly mark the measured segment with a small “X”. From this, measure 1.5cm intervals along the starting line. Mark each with an X and the following colour inks:

• Black
• Burgundy
• Red
• Pink
• Violet
• Turquoise
• Green
• Blue

Step 2. Get a small piece of tape. Form a cylindrical roll with the chromatography paper (the spots should be located on the outside). Tape the top and bottom edges of the paper but be careful not to allow the two edges of the paper to overlap.

Developing the chromatogram

Step 1. Pour 25mL of eluting solution into the 500mL beaker and then get a piece of plastic wrap to use as a cover.

Step 2. Take the paper cylinder you have prepared and gently place it into the beaker. You can then cover the beaker with the plastic wrap; just make sure the ink spots are above the liquid level. You then need to wait for about 45 to 90 minutes to observe the movement of the eluent upwards to the finish line, slowly taking with it some of the colour components of the ink spots.

Step 3. When the solvent gets to the finish line, take the paper out of the beaker. Allow some excess eluents to drip into the beaker. Carefully remove the tape and lay the chromatography paper, now called the chromatogram, on a paper towel. Put it inside the fume hood and let it dry (you can use the heat lamp in the fume hood for five to 10 minutes to expedite the drying process).

Analysing the results

Once the chromatogram has dried, you can analyse the results using both qualitative and quantitative approaches (you’ll need a ruler for this part).

Step 1. Label each original colour beneath the starting “X” mark based on the order in which you have initially marked each colour spot. You will observe that various colour spots have formed between the starting line and the finish line. Circle each distinct colour spot.

Step 2. Get the ruler and use it to draw a plus sign at the centre of each spot. Now measure the distance of each spot from the starting line and record this in your laboratory notebook (use centimetre as the unit of measurement). We can designate it as the D values. 

Step 3. Measure the farthest distance that the front of the eluent has travelled for each spot from the starting line. Record your measurements. We can designate it as the F values.

Step 4. Compute the retention factor or Rf for each spot. This is simply the ratio between the D values and the F values for each colour spot. The D values show how far the solute has travelled, while the F values show how far the solvent has travelled. Individual components of a mixture have unique Rf values. As the ratio becomes closer to the value of 1.0, it indicates lower retentiveness of the components. 

Once you’ve collected your data you can put it into a table like the one below:

Step 5. Write your report and analysis of the data based on the Rf values and the descriptive colours of each distinctive spot.

Safety precautions when experimenting with chromatography

There are no hazards associated with this experiment because you’ll be using non-toxic, non-flammable, and non-corrosive substances. The main precaution you should take is to ensure there are no fingerprints and contaminants on the chromatography paper. It’s also important to handle the glassware properly to prevent breakages.

Extra: Real-world applications of chromatography

Chromatography isn’t just useful in the academic study of chemistry – it has a wide range of applications in everyday life too. 

• Agricultural industry – chromatography is used to analyse, characterise, and authenticate the contents of various agricultural products. It helps experts identify the presence of certain types of chemical compounds in fruits, vegetables, and herbs.
• Environmental science – pollutants such as microplastics and greenhouse gases can be identified with the help of chromatography techniques. For example, gas chromatography is commonly used to identify and measure the concentration of volatile organic compounds in a sample of air.
• Forensic science – the most common types of chromatography used in forensic science include high-performance liquid chromatography (HPLC) and planar chromatography. These techniques can be used to identify poison in blood samples and traces of other toxic contaminants in the sample tissues.
• Biotechnology – chromatography is very useful in biotechnology when analysing the presence of certain biochemical compounds such as nucleic acids, vitamins, fats, carbohydrates, and protein.
• Food science – one prominent case where chromatography was used in food science was the 2013 horse meat scandal in Europe. The technique helped to expose the illegal and unethical practice of mixing undeclared horse meat in supposedly beef meat products.
• Pharmaceuticals – chromatography is used to analyse the toxicity and efficacy of various pharmaceutical products and medicines. It’s also useful in quality control analysis.

Summary

Chromatography is a versatile analytical technique that’s used to separate a mixture into its constituent components. Conducting your own simple chromatography experiment is an excellent way to improve your analytical skills and enhance your understanding of this important scientific method. Chromatography has a variety of applications in the real world including in sectors like forensics, environmental science, pharmaceuticals and biotechnology.

You can find GCSE chromatography sample questions and revision guides here.