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

We need to identify which technique is not used to spot (visualize) components on a thin layer chromatographic (TLC) plate.

Common Techniques for Visualizing TLC Spots:

Option A: $$I_2$$ (Solid) — Used

Iodine vapour chamber is a widely used technique. The TLC plate is placed in a closed chamber with iodine crystals. The iodine vapour gets adsorbed by the organic compounds, forming brown spots.

Option B: U.V. Light — Used

Many TLC plates contain a fluorescent indicator. Under UV light (254 nm or 366 nm), the spots appear as dark patches against a fluorescent background.

Option C: Visualisation agent as a component of mobile phase — Not Used

The mobile phase (solvent) is used to separate the components by carrying them up the plate. The visualization agent is never added as part of the mobile phase because it would interfere with the separation process. Visualization is done after the separation is complete.

Option D: Spraying of an appropriate reagent — Used

Spraying reagents like ninhydrin (for amino acids), sulfuric acid, or other specific reagents onto the plate is a standard technique to make invisible spots visible.

Hence, the correct answer is Option C: Visualisation agent as a component of mobile phase.