CHROMATOGRAPHY

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THE CHROMATOGRAPHY COMPENDIUM

From Beginner Paper to Advanced HPLC: The Ultimate Separation Protocol

The Universal Rule of All Chromatography

No matter how fancy the machine gets, ALL chromatography relies on a race between two things:

• 1. The Stationary Phase (The Track): A solid or coated material that molecules stick to.
• 2. The Mobile Phase (The River): A liquid or gas that flows over the track, pushing the molecules forward.

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1. Thin Layer Chromatography (TLC)

Principle: Separation by Polarity. The stationary phase is polar Silica Gel. The mobile phase is a non-polar solvent running up the plate via capillary action. Non-polar molecules race to the top; Polar molecules stick to the bottom!

Fig 1: TLC Plate. Red (Non-polar) runs high; Green (Polar) stays low.

Deep Protocol: TLC Execution

1. Preparation: Draw pencil line 1.5 cm from bottom of Silica plate. (Hold plate by edges only).
2. Spotting: Apply 1-2 µL sample. Let dry. (Spot diameter < 2mm).
3. Development: Add 0.5 cm Mobile Phase to chamber. Place plate in chamber. Do not submerge the origin line! Wait until solvent is 1 cm from top.
4. Visualization: Mark solvent front, dry, and view under 254 nm UV light or spray with Ninhydrin. Calculate Retention Factor (Rf).

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2. Ion Exchange Chromatography (IEX)

Principle: Separation by Charge. The column uses charged resin beads. Cation Exchange catches positive proteins; Anion Exchange catches negative proteins. Repelled proteins wash straight out!

Fig 2: IEX Principle. Negative bead binds positive proteins.

Deep Protocol: IEX Execution

1. Equilibration: Wash column with 5 Column Volumes (CV) of starting buffer (pH 1 unit below protein pI for Cation exchange).
2. Loading & Washing: Load sample. Unbound proteins wash through.
3. Elution: Flush column with an increasing **Salt Gradient** (e.g., 0M to 1M NaCl). Na+ or Cl- ions compete with the protein, forcing it to unstick and elute.

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3. Size Exclusion (SEC)

Principle: Separation by Mass. The column is a maze of porous beads. BIG molecules come out FIRST. They bypass the maze, while tiny molecules get trapped wandering around inside the bead pores.

Fig 3: SEC Maze. Large (Red) bypass beads; Small (Blue) get trapped.

Deep Protocol: SEC Execution

1. Equilibration: Equilibrate column with desired final buffer (e.g., PBS).
2. Loading (CRITICAL): Load small sample volume (2% to 5% of Column Volume) to prevent smearing.
3. Elution: Pump buffer continuously. Big molecules elute first (Void Volume), tiny molecules elute very last.

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4. Affinity Chromatography

Principle: Separation by Biological Lock-and-Key. Beads are coated with a specific "Ligand" (Nickel, Antibody). ONLY the target protein with the perfect matching "Lock" (His-Tag, Antigen) will stick. All others wash away.

Fig 4: Lock-and-Key. Target fits ligand, all others wash away.

Deep Protocol: IMAC (His-Tag) Execution

1. Binding: Load lysate over Ni-NTA column. His-tagged target protein sticks. Wash thoroughly.
2. Elution: Flush column with competitive inhibitor (e.g., high concentration **Imidazole**, 250-500 mM), forcing the pure protein off the resin.

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5. HPLC System

Principle: Machine-Driven extreme resolution. Uses microscopic beads (high efficiency), high-pressure pumps (6000 psi), and automated detection. RP-HPLC uses a non-polar column (C18) and a polar mobile phase.

Fig 5: High-Performance Liquid Chromatography (HPLC) System. The pump mixes polar water and non-polar organics under high pressure to force the sample through the tightly packed C18 column.

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🧠 Biotech Viva & Pop Quiz!

Tap the questions below to reveal the correct answers for your lab exams.

1. Why must we use a pencil instead of a pen to mark the Origin Line in TLC?

✅ Answer: Pen ink contains a mixture of dyes!

If you use a ballpoint pen, the solvent will pick up the pen ink and carry it up the paper, creating a massive smear of blue and black colors that will completely mix with and ruin your actual sample. Graphite pencils do not dissolve in organic solvents.

2. In Ion Exchange, why do we elute with a Salt Gradient instead of pushing pure salt all at once?

✅ Answer: To sequentially separate different proteins.

If you have three different proteins stuck to the column (weakly bound, moderately bound, and strongly bound) and you hit them with 100% salt immediately, they will all wash off at the exact same time, ruining the separation! By using a gradient (slowly increasing salt from 0% to 100%), the weakly bound protein falls off first, followed later by the stronger ones.

3. In Size Exclusion (Gel Filtration), why do HUGE proteins exit the column BEFORE tiny proteins?

✅ Answer: Because the tiny ones get lost in the maze!

The beads inside the column are full of microscopic tunnels. Tiny proteins wander into these tunnels and take a very long, zigzagging path to get out. Massive proteins are too big to enter the tunnels, so they just fall straight down the empty space between the beads and exit immediately (the Void Volume).

4. Why must we "degas" the liquid mobile phase before putting it into the HPLC pump?

✅ Answer: To prevent bubble explosions under high pressure.

Normal water has microscopic air dissolved inside. When the HPLC pump subjects this water to 4000+ psi of pressure, those tiny air bubbles compress and expand violently. This ruins the flow rate, breaks the pump valves, and creates massive, erroneous "ghost peaks" on the UV detector chromatogram.