Chromatography — TLC, column, GC, HPLC (principles)
WHY does chromatography exist?
The problem: Real samples (a plant extract, a drug batch, exhaled breath) are mixtures. To analyse or purify, we must physically separate components.
The trick: Set up a tug-of-war between two phases:
- A stationary phase (fixed: paper, silica, a coated tube wall) that grabs molecules.
- A mobile phase (moving: solvent or gas) that carries molecules along.
Each component constantly hops between "stuck on stationary" and "riding the mobile". A molecule spends a fraction of time stuck; the more it sticks, the more it lags behind. Over a long path these tiny differences add up to full separation.
HOW does a molecule's speed depend on ? (Derivation from scratch)
What we want: the velocity of an analyte band compared to the mobile phase.
A molecule only moves when it's in the mobile phase. So its average forward speed is:
Let the fraction of molecules in the mobile phase at any instant equal the fraction of time one molecule spends there (ergodicity). If = volume of mobile phase in the column and = volume of stationary phase:
- Amount in mobile
- Amount in stationary
TLC — Thin Layer Chromatography
Polarity rule (normal phase silica): Silica is polar, so polar compounds stick more → small ; non-polar compounds run fast → large .
Column Chromatography (preparative, gravity)
HOW: Load mixture on top → run solvent → least-retained (least polar) component elutes (exits) first; collect successive fractions in test tubes. Why first-out = least sticky? Smallest ⇒ fastest .
GC — Gas Chromatography
WHY a gas? Gases diffuse fast → very sharp, narrow bands → superb resolution. Limit: sample must be volatile and thermally stable (it has to evaporate without decomposing).
Separation basis in GC: mostly boiling point / volatility + affinity for the stationary film. Low-boiling (volatile) compounds spend more time as vapour ⇒ smaller ⇒ elute first.
HPLC — High-Performance Liquid Chromatography
WHY high pressure? Tiny particles give huge surface area & many equilibration steps (great separation) but resist flow. A high-pressure pump forces solvent through at usable speed.
- Normal phase: polar stationary (silica), non-polar mobile → polar analytes retained longer.
- Reverse phase (common): non-polar stationary (C₁₈ chains), polar mobile (water/MeCN) → non-polar analytes retained longer, polar elute first. (Reverses the TLC rule!)
WHY HPLC over GC? For non-volatile / heat-sensitive samples (proteins, sugars, drugs) you can't vaporise — HPLC keeps everything in solution.
Comparison table
| TLC | Column | GC | HPLC | |
|---|---|---|---|---|
| Mobile phase | liquid (capillary) | liquid (gravity) | inert gas | liquid (pumped) |
| Stationary | silica plate | silica in column | coated capillary film | packed fine particles |
| Main use | quick analysis | purification | volatile mixtures | non-volatile / precise analysis |
| Output | spots | fractions | peaks | peaks |
Recall Explain to a 12-year-old (hidden)
Imagine a river carrying a crowd of swimmers downstream, but the riverbed is sticky like honey. Some swimmers are slippery and barely touch the honey — they shoot ahead. Some are sticky and keep clinging to the bottom — they fall behind. Even though everyone started together, by the end of the river they arrive at different times, all spread out and separated. The honey-bed is the stationary phase, the river is the mobile phase, and how sticky each swimmer is, is its .
Flashcards
What two phases define any chromatography?
What does the partition coefficient measure?
Derive analyte velocity in terms of retention factor .
Define in TLC.
On normal-phase silica, does a polar compound have high or low ?
Which component elutes first in column chromatography?
What is the mobile phase in GC?
What sample property does GC require?
Why does HPLC need high pressure?
Difference between normal and reverse-phase HPLC?
A large means a compound elutes first or last?
What does retention time tell you?
Connections
- Polarity and Intermolecular Forces — sets (why molecules stick).
- Capillary Action — drives solvent rise in TLC.
- Distribution / Partition Equilibrium — the idea.
- UV-Vis Spectroscopy — common HPLC detector.
- Mass Spectrometry — coupled as GC-MS / LC-MS for identification.
- Purification Techniques — column chromatography as a prep method.
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, chromatography ka core idea bahut simple hai: ek mixture ko separate karna hai, toh hum do phases ke beech ek "tug-of-war" laga dete hain. Ek hota hai stationary phase (jaise silica, jo fix rehta hai aur molecules ko pakadta hai) aur doosra mobile phase (solvent ya gas, jo aage badhta hai aur molecules ko kheechta hai). Har molecule kabhi chipakta hai, kabhi bahta hai. Jo zyada chipakta hai (bada ), woh slow chalta hai aur peeche reh jaata hai; jo kam chipakta hai woh aage nikal jaata hai. Bas yahi differential sticking se sab alag-alag time pe bahar aate hain.
Formula bhi rattana mat — samajhna. Molecule sirf tab move karta hai jab woh mobile phase me hai. Isliye uski speed ko multiply karo us fraction se jitna time woh bahta hai, jo nikalta hai . Toh . Yaad rakho: bada matlab last me elute hoga, pehle nahi — ye sabse common galti hai.
Types ki baat karein: TLC ek quick test hai, silica plate pe solvent capillary se upar chadhta hai, aur hum (spot distance)/(solvent front distance) nikaalte hain. Polar silica pe polar compound chipakta hai toh chhota. Column wahi chemistry hai par gravity se, grams purify karne ke liye. GC me mobile phase gas hota hai — sample ko vaporise karna padta hai, toh sirf volatile cheezon ke liye. HPLC me high-pressure liquid pump karte hain, non-volatile aur heat-sensitive samples (drugs, proteins) ke liye perfect, aur reverse-phase me rule ulta ho jaata hai (non-polar zyada rukta hai).
Exam tip (80/20): bas teen cheezein pakad lo — (1) aur ka matlab, (2) "like sticks to like" polarity rule, aur (3) kaunsa technique kab use hota hai. Inhi se zyadatar questions ban jaate hain.