Which one of the following techniques is not used to spot components of a mixture separated on thin layer…

Step 1: Understand How TLC Spots are Detected

In TLC, after running the chromatogram, the separated spots are often invisible to the naked eye (colourless compounds). Various techniques are used to visualise the spots:

Step 2: Evaluate Each Detection Technique

(a) $\ce{I2}$ (Solid) — Iodine Staining ✅ (valid method)

• The developed TLC plate is placed in a closed jar containing iodine crystals
• Iodine vapour adsorbs on organic compounds → forms yellow-brown spots
• Non-destructive method

(b) U.V. Light ✅ (valid method)

• TLC plates with fluorescent indicator (silica gel GF₂₅₄) fluoresce under UV light
• Compounds that absorb UV appear as dark spots on the green fluorescent background
• Works at 254 nm or 365 nm

(c) Visualisation agent as a component of mobile phase ❌ (NOT a valid method)

• The mobile phase is the solvent system that carries compounds through the TLC plate
• Adding a visualisation agent (like iodine) to the mobile phase would interfere with the separation process
• It would alter the $R_f$ values and contaminate the stationary phase
• This is NOT a valid or used technique — the visualisation is always done AFTER the run

(d) Spraying of an appropriate reagent ✅ (valid method)

• Chemical reagents are sprayed on the plate after development
• Examples: ninhydrin (for amino acids → purple), ceric sulphate (for most organic compounds), anisaldehyde

Step 3: Conclusion

The technique NOT used is: (c) Visualisation agent as a component of mobile phase

Key Points to Remember:

• TLC visualisation methods: UV light, iodine vapour, chemical spray reagents
• Visualisation is always done AFTER the chromatographic run (post-development)
• Adding detection agents to the mobile phase would corrupt the separation — never done
• Fluorescent silica (GF₂₅₄) allows UV detection without any chemical treatment