BIOL 1406
PreLab 7.3
How can I compare the migration rate of molecules separated
by thin layer chromatography (TLC)?
| During TLC, the migration rate of each pigment depends on its relative affinity
for the stationary and mobile phases. Pigments that are more strongly attracted
to the mobile phase will move up the plate faster, while pigments that are more
strongly attracted to the stationary phase will move up the plate slower. This
relative affinity for the 2 phases depends only on the molecular structure of
the pigment. Therefore, it remains a constant property of the pigment for
any given stationary-mobile phase combination.
Note, however, that the actual distance that a pigment travels
during TLC is not a constant property of the pigment because it can be
affected by a number of variables, such as how long the chromatography plate
remains immersed in the developing solution. Until the solvent front reaches the
top of the plate, the longer the plate remains immersed in the developing
solution, the farther each pigment will travel. Because the migration
distance of a pigment can vary, scientists find it
more useful to look at the ratio of the migration distance of the pigment to the
migration distance of the solvent front. This ratio is called the Rf value, and
for any given stationary-mobile phase combination it remains constant. Because the Rf value
a pigment is constant, it can be
used to help identify unknown pigments.
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Use the interactive exercise below to calculate Rf values for 3 pigments separated by TLC. |
| Your Turn | ||
| Use the information from the interactive exercise above to answer the following questions: | ||
| Which of the 3 pigments has the greatest affinity for the mobile phase? | Hint | Check your answer. |
| Which of the 3 pigments has the greatest affinity for the stationary phase? | Hint | Check your answer. |
| What is the Rf value for the dark green pigment when using silica gel as the immobile phase, hexane as the mobile phase, and a run time of 50 minutes? | Hint | Check your answer. |
| What is the Rf value for the dark green pigment when using silica gel as the immobile phase, acetone as the mobile phase, and a run time of 50 minutes? | Hint | Check your answer. |
| Using silica gel as the immobile phase and hexane as the mobile phase, you plan to run your chromatography plate until the solvent front travels 10 cm. What is the predicted migration distance for the dark green pigment? | Hint | Check your answer. |
| What happens to the migration distance of
the yellow pigment if the run time is increased from 50 to 90 minutes while
keeping the mobile and immobile phases constant? Explain your answer:
|
Hint | Check your answer. |
| What happens to the Rf value of the yellow
pigment if the run time is increased from 50 to 90 minutes while keeping the
mobile and immobile phases constant? Explain your answer:
|
Hint | Check your answer. |
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