Briefly explain the problem a student might encounter if the following procedura

Question

Briefly explain the problem a student might encounter if the following procedural changes were made while performing this experiment. Instead of using a pencil, the student marked the location of the dye spots with a pen. The student neglected to cover the chamber while the mobile phase solution ascended the plate. The student was late starting the experiment, and allowed the solvent front to move only 20 mm up the 7.5-cm TLC plate. The two components in the mixture, A and B, had R_f s of 0.40 and 0.60 respectively. The student ran out of mobile phase solution after testing the samples but before testing the FD&C; dye standards. Because the isoamyl alcohol bottle was empty, the student made the new mobile phase solution using an 8:1:2 mixture of 95% ethanol, concentrated NH_3 solution, and water.

Explanation / Answer

Analysis of TLC

1. If the student used pen insetad of pecil to mark tlc plate, the plate would have interference of the ink used in the pen. Ink contains organic matter which would also develop on tlc along with the desired spots and may cause error due to it.

2. If the solvent chamber is uncovered while running the tlc, solvent would evaporate and chamber is no longer saturated with solvent vapors. The compound would not move as expected up the plate and we would get uneven movement of spots on the plate.

3. If the student allowed only a short distance run on the plate, the spots would not be separate enough. The spots A and B would be very close to one another on the plate and difficult to distinguish properly.

4. Water is more polar. The new solvent system used as mobile phase would have spots move faster in it and the Rf values would thus be higher than the previous iso-amyl alcohol run.

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