You are carryingout TLC on normal phase plates. Compound A has an Rf of 0.4 and compound B of 0.6. What would happen to the Rf values if you increase the polarity of the mobile phase?
For the compounds in question 1, you change to running them on reverse phase plates. What would you expect to happen.
A compound has a retention time of 3 minutes on GLC at a column tep. of 120 C. What would happen if you increased the column temp. to 160 C.
1. If you increase the polarity of the mobile phase in TLC, it will generally decrease the Rf values of the compounds. This means that the compounds will have a lower migration distance on the TLC plate relative to the solvent front.
2. If you change to running the compounds on reverse phase plates, the Rf values might change differently for compound A and compound B. Generally, reverse phase plates have higher polarity than normal phase plates. This means that more polar compounds (like compound B with an initial Rf value of 0.6) will have lower Rf values on reverse phase plates. On the other hand, less polar compounds (like compound A with an initial Rf value of 0.4) might still have relatively similar Rf values or slightly lower Rf values, depending on the exact polarity of the reverse phase plates.
3. If you increase the column temperature in gas liquid chromatography (GLC) from 120°C to 160°C, it will generally decrease the retention time of the compound. This means that the compound will spend less time retained on the column and the elution time (or retention time) will decrease. In other words, the compound will elute faster at the higher column temperature.
To answer these questions, we need to understand the principles of both thin-layer chromatography (TLC) and gas-liquid chromatography (GLC) and how changes in polarity and temperature affect these techniques.
1. Regarding the first question about TLC and increasing the polarity of the mobile phase:
In TLC, the retention factor (Rf) is a measure of how far a compound travels on the stationary phase (in this case, the TLC plate) relative to the mobile phase. It is calculated as the distance traveled by the compound divided by the distance traveled by the mobile phase. Increasing the polarity of the mobile phase can affect the Rf values in the following ways:
- If the mobile phase becomes more polar, it will have a stronger affinity for polar compounds and may cause them to travel further up the TLC plate. This can result in an increase in the Rf value for polar compounds like compound A (which originally had an Rf of 0.4) since they will be more easily eluted from the stationary phase.
- On the other hand, less polar compounds like compound B (which originally had an Rf of 0.6) may not be as strongly retained by the stationary phase with the increased polarity of the mobile phase. This can lead to a decrease in the Rf value for less polar compounds.
Therefore, increasing the polarity of the mobile phase in TLC can potentially increase the Rf value for polar compounds and decrease the Rf value for less polar compounds.
2. For the second question about running the compounds on reverse phase plates:
Reverse phase chromatography is the opposite of normal phase chromatography. In reverse phase chromatography, the stationary phase contains hydrophobic groups, while the mobile phase is polar. In this case, you are running the compounds on reverse phase plates instead of normal phase plates. In such a scenario, you would expect the following to happen:
- More polar compounds like compound A, which originally had an Rf of 0.4 on normal phase plates, would have a smaller Rf value on reverse phase plates. This is because the stationary phase in reverse phase chromatography is more hydrophobic and will retain more polar compounds, which will result in a decrease in their Rf values.
- Conversely, less polar compounds like compound B, which originally had an Rf of 0.6 on normal phase plates, would have a larger Rf value on reverse phase plates. This is because the hydrophobic stationary phase will not retain less polar compounds as strongly, leading to an increase in their Rf values.
Therefore, running the compounds on reverse phase plates in TLC can lead to decreased Rf values for polar compounds and increased Rf values for less polar compounds.
3. Regarding the third question about increasing the column temperature in GLC:
In GLC, the retention time refers to the time it takes for a compound to travel through the gas chromatography column. The retention time is influenced by various factors, including the column temperature. Increasing the column temperature from 120°C to 160°C can have the following effect:
- As the temperature increases, the kinetic energy of the compound also increases, leading to faster movement through the column. Consequently, increasing the column temperature will generally decrease the retention time of the compound. Therefore, if the column temperature is increased from 120°C to 160°C, the retention time of the compound is likely to decrease.
Overall, increasing the column temperature in GLC typically results in decreased retention times for compounds as they move more quickly through the column.