Chromatography is a set of techniques used to separate the parts of a mixture. It matters because many real samples, such as inks, plant pigments, medicines, and pollutants, contain several substances mixed together. By separating them, scientists can identify components, compare samples, and check purity.
A simple paper chromatography strip shows the main idea clearly as colored spots spread into bands at different heights.
Key Facts
- Chromatography separates substances based on their different attractions to a stationary phase and a mobile phase.
- Stationary phase = the material that stays in place, such as paper, silica gel, or a coated column.
- Mobile phase = the solvent or gas that moves through the stationary phase and carries sample components.
- Rf = distance traveled by solute / distance traveled by solvent front.
- A component with stronger attraction to the mobile phase usually travels farther in the same time.
- A component with stronger attraction to the stationary phase usually moves more slowly and stays closer to the starting line.
Vocabulary
- Chromatography
- Chromatography is a laboratory method that separates mixture components by how they move between a stationary phase and a mobile phase.
- Stationary phase
- The stationary phase is the fixed material that mixture components interact with as the mobile phase moves past it.
- Mobile phase
- The mobile phase is the moving solvent, liquid, or gas that carries mixture components through the system.
- Solvent front
- The solvent front is the farthest point reached by the mobile phase on a chromatography strip or plate.
- Rf value
- The Rf value is a ratio that compares how far a solute travels to how far the solvent front travels.
Common Mistakes to Avoid
- Putting the starting spot below the solvent level is wrong because the sample can dissolve directly into the solvent chamber instead of traveling up the stationary phase.
- Measuring Rf from the bottom of the paper is wrong because distances must be measured from the original baseline to the center of the spot and to the solvent front.
- Using ink for the baseline is wrong because ink can dissolve and create extra spots that contaminate the chromatogram.
- Assuming the top spot is always the heaviest molecule is wrong because movement depends mainly on attraction to the phases and solubility, not mass alone.
Practice Questions
- 1 In a paper chromatogram, a dye spot travels 4.2 cm from the baseline while the solvent front travels 7.0 cm. Calculate the Rf value.
- 2 A TLC plate has a solvent front 8.5 cm from the baseline. Compound A has Rf = 0.40 and compound B has Rf = 0.75. How far did each compound travel?
- 3 Two pigments are placed on the same paper strip. Pigment X stays near the baseline, while pigment Y moves close to the solvent front. Explain which pigment is more strongly attracted to the stationary phase and why.