Thin Layer Chromatography experiment post-lab questions 1. What is the effect of

Question

Thin Layer Chromatography experiment post-lab questions

1. What is the effect of solvent polarity on the separation of the herbs extract?

2. (1.5 point) Why is it a problem to have the spotting line on a TLC plate below the solvent level in the developing chamber?

3. (1.5 points) A mixture of compounds is spotted on a TLC plate and developed in 90:10 hexane : ethyl acetate. Visualization with both UV and iodine shows a single spot at the origin. What mixture of hexane and ethyl acetate should you try next? Explain.

4. (1.5 points) On the Developed TLC plate, match each spot to one of the compounds shown. The developing solvent was 90:10 ethyl acetate:hexane.

5. What are possible sources of error in this experiment? How can you solve them?

Explanation / Answer

1. In the separation of compounds on the tlc plate, the solvent used as the movile phase plays a major role. the separation works on the simple rule of like-dissolves-like. So when a low polarity solvent is employed the compounds with lowest polarity would elute first and the most polar say acids would stay close to the origin. In a non-polar or low polarity solvent polar compiunds have strong interaction with the stationary phase adsorbed on the solica plate and thus elute at the end or stay at the bottom.

2. If we spot the samples below the origin line, two possibilites exist for error. the first being the solvent level might be now very close or be in direct conatct with the spots, which would not let the spots to run properly on the tlc plate. Secondly and most important, even If solvent level is below the Rf value calculated might not be accurate. The Rf value is measured as distance travelled by both solute and the solvent from the origin, which is different now and would give error in the calculated value.

3. Since both the spots are seen at the bottom by the 10% ethylacetate in hexane mixture, it means the polairty of the compounds is more. So we would have to use higher polarity of the mobile phase. We would say use 20% ethylacetate in hexane to run the tlc.

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5. Possbile errors might be, we may have spotted the samples too less on the plate, so the concentration might be too low to run and be seen on the tlc plate. We may have spotted the sample too many times, which would increase the concentration of compounds and thus trailing of spots will be observed. We might have forgotten to remove the plate in time and the solvent might have run beyond the marked level. this would give an error in Rf values calculated. The solvent level might be too high or too low which would create problems in proper running of tlc. To solve the problems we have to make sure the samples are spotted at the right concentration and numbr of times, the solvent fron tis marked and tlc is removed as it reaches the marked line. The solvent level is checked and the developing chamber is saturated with solvent vapor.

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