In paper chromatogaphy, why should the level of the sample spot be above the level of solvent in the beaker?

Question

Why is it important to cover the beaker tightly with aluminum foil while the chromatogram is developing?

Answer 1

There are several factors which effect the elution of the various components in paper chromatography such as the solvent, temperature and the rate of solvent flow. By placing the spot above the level of the solvent in the beaker, you slow the flow of solvent which results in improved separation of the components in the spot. If you placed it below the level of the solvent in the beaker, the solvent would just flow at a much faster rate and instead of separating the components in the spot, they would simply be washed away. The reason for keeping the beaker covered during the elution process is many fold. One reason is to keep the solvent from evaporating before the components in the spot are completely eluted, another reason is to keep other things from getting into your beaker and ruining your project. Then, in the case of highly volatile solvents such as alcohol, it keeps the paper in a saturated vapor atmosphere and keeps it from drying out. It also keeps the vapors in the beaker instead of filling the room with vapors. Containment of the vapors is also a safety consideration

Answer 2

The solvent travels up the paper by capilary action, similar to the way a whole towel gets wet if the corner dips in the bath tub.The spot should be above the liquid level so that it will not dissolve out into the solvent. The beaker is covered to prevent evaporation of the solvent out into the room where it is being done. Some solvents are very volatile.

Answer 3

to protect the spots from dissolving in the solvent and we must close the beaker to make the pressure stable inside

Answer 4

if the spot touches the solvent it will bleed in all directions and you'll ge a mess.

you want the tights seal so the environment is saturated with solvent (it helps trust me)