DATE: 14-APRIL-2020 Last Updated: 14-JUNE-2025
If you're just getting started in HPLC or chemistry, you might come across the terms
adsorption
and
absorption
. They sound similar, but they mean very different things — and understanding the difference is important for interpreting how molecules behave in chromatography.
🧲 Adsorption – Think “Sticking to the Surface”
- Definition: Adsorption is when molecules stick to the surface of a solid or liquid.
- In HPLC: Water molecules might stick to the surface of a hydrophilic (water-attracting) stationary phase inside the column.
- Key idea: The molecules are not going inside the material — they’re just on the surface.
🧪 Example:
A paper towel adsorbs water — the water clings to the surface fibers.
🧊 Absorption – Think “Soaking In”
- Definition: Absorption is when molecules are taken into the volume of a material — like being soaked up.
- In chemistry: A compound might dissolve into a liquid phase during an extraction.
- Key idea: The molecules are inside the material, not just on the surface.
🧪 Example:
Your skin absorbs sunlight — the energy goes into your skin, not just on top.
🧠 Easy Way to Remember
| Term | Where the Molecule Goes | Can It Be Recovered Easily? | Everyday Example |
|---|---|---|---|
| Adsorption | On the surface | ✅ Yes, often by elution | Water on a paper towel |
| Absorption | Into the material | ❌ No, not easily removed | Sunlight into your skin |
💡 Rule of Thumb:
If you can wash it off or elute it — it’s probably adsorption.
If it’s soaked in or dissolved — it’s absorption.
Understanding this difference is especially helpful in HPLC, where adsorption plays a key role in how compounds interact with the stationary phase and how they separate during analysis.
To put this in layman's terms and as a rule of thumb, with adsorption you can regain the molecule with elution, with absorption, the molecules are not recoverable. Sunlight is absorbed by skin and water is adsorbed by a paper towel.