When should TLC development be stopped?

When should TLC development be stopped?

• A. When the solvent front is about 1 cm from the top.
• B. After 1 minute.
• C. When the plate dries completely.
• D. When the baseline disappears.

Answer: (A)

The key idea is to stop TLC development while the solvent front is still well below the top edge so that separation is preserved and you can accurately measure how far the spots have moved. Stopping when the solvent front is about 1 cm from the top keeps enough room for the front to push the spots apart without risking it running off the plate, which would blur or smear the spots and ruin the Rf values. After you stop, you promptly remove the plate, mark the solvent front with a pencil, then dry and visualize. Waiting for a fixed time isn’t reliable because how far the solvent travels depends on the solvent system, plate, temperature, and humidity. Stopping after the plate dries completely or when the baseline disappears isn’t a meaningful criterion for when to halt development, since the important change is the position of the solvent front, not the plate’s dryness or the fate of the baseline.

The key idea is to stop TLC development while the solvent front is still well below the top edge so that separation is preserved and you can accurately measure how far the spots have moved. Stopping when the solvent front is about 1 cm from the top keeps enough room for the front to push the spots apart without risking it running off the plate, which would blur or smear the spots and ruin the Rf values. After you stop, you promptly remove the plate, mark the solvent front with a pencil, then dry and visualize.

Waiting for a fixed time isn’t reliable because how far the solvent travels depends on the solvent system, plate, temperature, and humidity. Stopping after the plate dries completely or when the baseline disappears isn’t a meaningful criterion for when to halt development, since the important change is the position of the solvent front, not the plate’s dryness or the fate of the baseline.