1) Two components have Rf values of 0.2 and 0.6 respectively when TLC is perform
ID: 894319 • Letter: 1
Question
1) Two components have Rf values of 0.2 and 0.6 respectively when TLC is performed on silica gel in 70% CH2Cl2-30% CH3OH. Would you expect these Rf values to increase or decrease if the experiment were repeated and the developing liquid was changed to 90% CH2Cl2 - 10% CH3OH. Explain.
2) A mixture containing three colored components are added to the top of an alumina column that has been equilibrated with 70% ligroine-30% acetone. As this same solvent is continually added to the top of the column, an orange component elutes in 3min; a blue component in 7min; and a green one in 17min. Which component is presumably the most nonpolar?
If the experiment is done again with 60% ligroine-40% acetone, would the colored components move slower or faster down the column? Explain.
3) In performing the TLC experiment, what problem will arise if too much of the mixture is spotted on the line near the bottom of the chromatogram.
Explanation / Answer
Spotting sizes of your sample should be not be larger than 1-2 mm in diameter. The component spots will never be larger than or smaller than your sample origin spot. If you have an over-large spot, this could cause overlapping of other component spots with similar Rf values on your TLC plate. If overlapping occurs, it would prove difficult to resolve the different components. The sample should be above the solvent level. If the solvent level covers the sample, the sample spot will be washed off into the solvent before it travels up the TLC plate.
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