Chromatography is a clever technique used to separate a mixture into its individual components, revealing hidden ingredients that were once blended together. The word chromatography comes from Greek words meaning “color writing,” which hints at the process’s ability to show different components as colorful bands or spots. This powerful technique provides a way to see the unseen parts of a substance, which is why it is used by researchers all over the world.
The Science of Separation
Chromatography works by exploiting the differences in how substances interact with two main parts: the stationary phase and the mobile phase. The stationary phase stays still, often consisting of absorbent paper or a solid material. The mobile phase is typically a liquid, such as water or alcohol, that travels through the stationary phase, carrying the substance being analyzed.
When the mobile phase moves, it carries the mixture’s components along with it. Components that are more attracted to the stationary phase “stick” to it and travel more slowly; this attraction is known as adsorption. Other components that are more soluble in the mobile phase dissolve easily and are carried quickly to the finish line.
The varying degrees of attraction and solubility cause the separation. Over time, the faster-moving components race ahead while the slower-moving ones lag behind, resulting in the mixture separating into distinct bands or spots. The speed at which each component travels is unique, allowing scientists to identify and analyze the individual parts of the original mixture.
How Scientists Use Chromatography
Chromatography’s ability to separate complex mixtures makes it a foundational technique across many scientific fields. In forensic science, for instance, a technique like thin-layer chromatography can be used to analyze minute samples left at a crime scene. A forensic scientist might use it to separate the dyes in ink or identify unknown substances recovered as evidence. This separation provides a distinct chemical fingerprint that can help link a suspect to a location or item.
Food safety is another area that relies heavily on this technique, particularly with methods like high-performance liquid chromatography. Scientists use it to test for contaminants, such as pesticides or additives, in food products. In medicine, laboratories use chromatography to analyze compounds in biological samples, checking for hormones, vitamins, or drugs. Pharmaceutical companies also use large-scale chromatography to purify and isolate specific compounds when manufacturing new medicines.
Your At-Home Color Experiment
The most accessible way to see chromatography in action is with a simple experiment using common household materials. This activity, known as paper chromatography, demonstrates the separation of ink dyes, often revealing that a single color is actually a blend of several others.
Materials Needed
For this project, you will need absorbent paper (such as a coffee filter), a water-soluble black marker, a shallow dish, and water. Rubbing alcohol can be used if you want to test permanent markers.
Setting Up the Experiment
Cut a strip of the absorbent paper about one inch wide and long enough to stand up in your dish. Draw a thick, dark dot of black marker ink about one inch from the bottom edge of the strip. This spot of ink is the mixture you will be separating, and it must be placed above the level of the liquid. Let the ink spot dry completely before moving to the next step.
Running the Separation
Pour just enough water into the shallow dish to cover the bottom, ensuring the depth is less than the distance from the bottom of the paper to the ink spot. Carefully stand the paper strip upright in the dish so that only the very bottom edge touches the water, and the ink spot remains completely dry. The water will slowly begin to move up the paper via capillary action. As the water rises, it acts as the mobile phase, dissolving the ink and carrying the different colored dyes upward.
Observing the Results
As you watch, the black ink spot will start to blur and separate into its constituent colors, often showing shades of purple, blue, and red. The dye molecules that are easily dissolved in the water and are less attracted to the paper will travel the farthest up the strip. Conversely, the dyes that stick more strongly to the paper will separate out lower down. After 10 to 20 minutes, remove the paper and let it dry to see your completed chromatogram, a visual record of the separation.