Answer
🧪 Normal-Phase Chromatography: Eluent Strength
Question
In normal-phase chromatography, the eluent strength of the solvent increases as the solvent becomes ________?
✔ More Polar
What is Normal-Phase Chromatography?
Normal-phase chromatography uses a polar stationary phase (such as silica) and a non-polar or weakly polar mobile phase. Compounds are separated based on their polarity:
- Polar analytes interact more strongly with the stationary phase → elute more slowly
- Non-polar analytes elute more quickly
What is Eluent Strength?
Eluent strength refers to how effectively a solvent can elute (wash out) analytes from the stationary phase. A more polar solvent competes more effectively with analytes for adsorption sites on the stationary phase, increasing eluent strength.
Solvent Polarity vs. Eluent Strength
In normal-phase chromatography, eluent strength increases with solvent polarity. Here’s a general trend:
| Solvent | Polarity | Relative Eluent Strength |
|---|---|---|
| Hexane | Non-polar | Weakest |
| Toluene | Low | ↑ |
| Dichloromethane | Moderate | ↑↑ |
| Ethyl acetate | Moderate | ↑↑↑ |
| Acetone | Polar | ↑↑↑↑ |
| Methanol | Very polar | Strongest |
Key Concept Summary
- Eluent strength increases as solvent becomes more polar.
- Polar solvents elute polar analytes more quickly in normal-phase chromatography.
- Weaker solvents result in longer retention times for polar compounds.