How far above the bottom edge of a TLC plate is the baseline drawn?

How far above the bottom edge of a TLC plate is the baseline drawn?

• A. 1.5 cm.
• B. 0.5 cm.
• C. 2.5 cm.
• D. 3.5 cm.

Answer: (A)

Baseline marks the starting line for the spots and serves as the reference for calculating how far a compound travels (Rf). It needs to be set a short distance from the bottom edge so the applied droplets have room to dry and form a defined origin before development. If the line is too close to the edge, spots can smear or be washed off as the solvent front rises. If it’s placed much farther up, you waste plate space and the spotting area becomes awkward to use, reducing practical working area for good separation. About 1.5 cm above the bottom is a common, practical distance that balances drying room with efficient use of the plate, making it the best choice.

Baseline marks the starting line for the spots and serves as the reference for calculating how far a compound travels (Rf). It needs to be set a short distance from the bottom edge so the applied droplets have room to dry and form a defined origin before development. If the line is too close to the edge, spots can smear or be washed off as the solvent front rises. If it’s placed much farther up, you waste plate space and the spotting area becomes awkward to use, reducing practical working area for good separation. About 1.5 cm above the bottom is a common, practical distance that balances drying room with efficient use of the plate, making it the best choice.