TLC & Paper Chromatography Analysis

🧪 1. What is Chromatography?

Chromatography is a technique for separating mixtures based on how components partition between two phases:

Stationary phase — a solid or a liquid bound to a solid (silica gel for TLC; cellulose paper for paper chromatography).
Mobile phase — a solvent (or solvent mixture) that moves through the stationary phase by capillary action.

Compounds in the sample distribute themselves between the two phases according to their polarity. Compounds that prefer the mobile phase travel farther; those that prefer the stationary phase travel less.

Key rule of thumb: "Like dissolves like." Polar compounds stick to the polar stationary phase and travel short distances (low Rf). Non-polar compounds prefer the less-polar mobile phase and travel far (high Rf).

📄 2. TLC vs Paper Chromatography

Aspect	TLC	Paper
Stationary phase	Silica gel (or alumina) on glass/aluminium	Cellulose paper (water-bound)
Mobile phase	Organic solvent mixtures (BAW, EA/FA/W, etc.)	Water-rich mixtures (BAW, phenol:water)
Speed	Fast (10–30 min)	Slow (1–20 hours)
Resolution	High — sharp bands	Moderate — diffuse bands
Sample size	Micrograms	Micrograms to milligrams
Typical use	Lipids, alkaloids, flavonoids, drug purity	Amino acids, sugars, dyes
Detection	UV, iodine, ninhydrin, specific reagents	Ninhydrin (AA), aniline (sugars), UV

Both techniques use the same principle and produce the same metric — the Rf value. In practice, TLC is preferred for organic compounds; paper is preferred for very polar compounds like amino acids and sugars.

📐 3. The Rf Factor — Definition & Formula

Rf (Retention factor, or Ratio-to-front) is the ratio of the distance traveled by a compound to the distance traveled by the solvent front:

Rf = distance moved by compound ÷ distance moved by solvent front
Both distances measured from the origin (the spotting line).

Key facts about Rf

Rf is always between 0 and 1 under normal conditions.
Rf = 0 → the compound does not move at all (very polar or strongly adsorbed).
Rf = 1 → the compound moves with the solvent front (very non-polar).
Rf is constant for a given compound under fixed conditions (same stationary phase, mobile phase, temperature, and saturation).
Rf is dimensionless — the units (mm, cm) cancel out.
Rf is used for identification by comparing to reference standards run on the same plate.

Example calculation

If the solvent front moved 80 mm and a compound moved 32 mm:

Rf = 32 / 80 = 0.400

🧴 4. Choosing a Solvent System

The mobile phase determines what you can separate. Its polarity must be matched to the compounds of interest.

Solvent system	Polarity	Good for
n-Hexane : Ethyl acetate (4:1)	Very low	Lipids, fatty acids, terpenes
Petroleum ether : Ethyl acetate (7:3)	Low	Steroids, carotenoids
Toluene : Ethyl acetate (8:2)	Low	Alkaloids, aromatic compounds
Chloroform : Methanol (9:1)	Low–moderate	Lipids, alkaloids
BAW (4:1:5, upper phase)	Moderate	Amino acids, sugars, flavonoids, general
Ethyl acetate : Acetic acid : Water (3:1:1)	Moderate–high	Flavonoids, phenolic acids
Methanol : Water (7:3)	High	Very polar compounds
Water (paper)	Very high	Amino acids, sugars, dyes
Phenol : Water (4:1)	High	Amino acids, carbohydrates

Tip: If your compound has Rf > 0.8, use a less polar solvent. If Rf < 0.2, use a more polar solvent. Aim for Rf between 0.3 and 0.7 for best resolution.

🔍 5. Interpreting Rf Values

Rf range	Polarity	Interpretation
0.00 – 0.10	Very polar	Compound strongly adsorbed; consider more polar solvent.
0.10 – 0.30	Polar	Typical of amino acids, sugars in BAW.
0.30 – 0.60	Moderately polar	Ideal range for good resolution.
0.60 – 0.85	Non-polar	Flavonoids, alkaloids — good separation.
0.85 – 1.00	Highly non-polar	Nearly co-elutes with solvent front; use less polar solvent.

Using Rf for identification

Run your unknown compound alongside a reference standard on the same plate. If they have the same Rf (±0.02), they are likely the same compound.

Important: Matching Rf alone is not proof of identity. Confirm with co-spotting (mix of standard + unknown gives a single spot) and a second solvent system.

💡 6. Detection & Visualization

Method	Detects	Notes
UV light (254 nm)	UV-absorbing compounds (aromatics, flavonoids, alkaloids)	Use fluorescent silica plates — dark spots on green background.
UV light (365 nm)	Fluorescent compounds (chlorophyll, coumarins)	Colored fluorescence.
Iodine vapour	Most organic compounds (reversible)	Brown spots — non-destructive.
Ninhydrin spray	Amino acids, amines	Purple/violet spots after heating.
Aniline-diphenylamine	Sugars	Colored spots.
Iodoplatinate / Dragendorff	Alkaloids	Orange/brown spots.
Natural colors	Pigments (chlorophyll, carotenoids)	Visible without reagent.

In this tool, colored bands are visualised using the actual color of the compound when visible; UV-absorbing compounds are shown as gray, and colorless compounds as gray faint bands.

✏️ 7. Worked Examples

Example 1 — Amino acid separation by paper chromatography

Solvent front = 80 mm; measured distances:

Glycine: 21 mm → Rf = 21 / 80 = 0.263
Alanine: 26 mm → Rf = 26 / 80 = 0.325
Valine: 40 mm → Rf = 40 / 80 = 0.500
Leucine: 57 mm → Rf = 57 / 80 = 0.713

Interpretation: Rf increases from glycine (smallest side chain) → leucine (largest aliphatic side chain). The larger the non-polar side chain, the farther the amino acid migrates in BAW.

Example 2 — Caffeine in a beverage sample

Solvent front = 65 mm; sample spot moved 42 mm → Rf = 42 / 65 = 0.646.

A standard caffeine spot on the same plate at 41.5 mm gives Rf = 0.638. The two Rf values differ by only 0.008, so the sample likely contains caffeine.

Example 3 — Pigment separation (TLC)

Solvent: petroleum ether : acetone (9:1). Spinach extract shows 4 bands:

β-Carotene (yellow-orange, top) — Rf ≈ 0.95
Chlorophyll a (blue-green) — Rf ≈ 0.75
Chlorophyll b (yellow-green) — Rf ≈ 0.55
Xanthophyll (yellow, bottom) — Rf ≈ 0.35

Non-polar carotene travels highest; more polar xanthophyll stays closest to the origin.

🛠 8. Troubleshooting & Best Practices

Problem	Likely cause	Fix
All compounds at origin	Solvent too non-polar	Add polar component
All compounds at front	Solvent too polar	Use less polar solvent
Tail-shaped bands	Overloading or adsorption	Load less sample; use better plate
Diffuse / smeared spots	Solvent front running too fast; old plate	Pre-equilibrate tank; use fresh plate
Curved solvent front	Chamber not saturated, uneven plate	Line tank with filter paper; allow equilibration
Streaky bands	Sample too concentrated	Dilute; apply smaller volume
Rf not reproducible	Temperature, solvent composition or chamber saturation varies	Standardize conditions; always run standard
Poor separation of close compounds	Wrong solvent	Try a different ratio or 2D chromatography

Pre-run checklist

✔ Stationary phase is dry and fresh (if self-coated)
✔ Origin line drawn in pencil (never ink)
✔ Sample spotted small (1–2 mm) and dried
✔ Chamber lined with filter paper and pre-equilibrated
✔ Solvent level below the origin line
✔ Run stopped when solvent front reaches ~1 cm from top
✔ Solvent front marked immediately after the run (before it evaporates)
✔ Reference standard run on the same plate
✔ Distances measured from origin to band center, in mm

Safety: Many solvents used in chromatography (chloroform, toluene, phenol, methanol) are toxic or carcinogenic. Work in a fume hood, wear gloves and goggles, and dispose of waste according to your institution's guidelines.

Reporting checklist

✔ Stationary phase used (silica gel 60 F254, Whatman No. 1 paper, etc.)
✔ Solvent system with exact proportions
✔ Solvent front distance (mm)
✔ Distance moved by each compound (mm)
✔ Calculated Rf values (3 decimal places)
✔ Detection method (UV, iodine, ninhydrin, etc.)
✔ Reference standards used
✔ Representative photograph or diagram of the chromatogram

🧪 TLC & Paper Chromatography Analysis Toolkit

No analysis yet

📑 On this page