Theory: Thin Layer Chromatography TLC During chromatography, a mobile phase eluent distributes the compounds present in a mixture over a stationary phase orbent. Thin Layer Chromatography TLC is one type of chromatography, where: - the mobile tc is a solvent or mixture of solvents eluent- the stationary phase is a solid orbent e.
Theory: Thin Layer Chromatography TLC During chromatography, a mobile phase eluent distributes the compounds present in a mixture over a stationary phase orbent.
Thin Layer Tlx TLC is one type of chromatography, where: - the mobile phase is a solvent or mixture of solvents eluent- the stationary tpc is a solid orbent e. First, the TLC plate is prepared. The unknown solid is applied as a solution by dissolving it in a solvent. Some solution is drawn up into a micropipette, then applied to the orbent near one end of the TLC plate baseline by allowing the solution to wick out onto the orbent.
This process is generally referred to as "spotting your TLC plate".
The chromatogram is then developed by tor the TLC plate into a TLC chamber a beaker containing the mobile phase, i. The eluent will travel up the orbent by capillary action to the top of the TLC plate known as anr solvent frontcarrying the sample with it. This process is generally referred to as "running your TLC plate". Depending on the relative polarity of the sample and the eluent, the sample either orbs to the solid phase or is eluted by the mobile phase eluent.
Looiing the eluent is very polar relative to your compound, it will dissolve your sample and the sample will move with the mobile phase. Overall, the eluent and your sample will compete for a space an active site on the orbent stationary phase coated on the TLC plate. The more polar compound will win this competition and adhere orb to the orbent, while the rest of the mixture will move.
The lopking polar the compound, the more it will adhere to the orbent and the smaller the distance it will travel from the baseline, and the lower its Rf value. Eluent: the solvent or mixture of solvents mobile phase used to develop a TLC chromatogram plate. Elution: the overall process of developing a TLC plate. Rf retention or retardation factor : is a measure of the distance travelled by the compound spot in relation to the distance travelled by the eluent.
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A desirable Rf value lies between 0. Identification of a solid using thin layer chromatography TLC : First a TLC plate is prepared by spotting the purified unknown and an authentic sample of each possible compound. Then the TLC plate is developed. For the next step co-spottingan authentic sample of the compound closest in Rf value to the unknown is chosen.
TLC co-spotting of a second plate allows for preliminary identification of your compound. Three spots are applied to the orbent on the baseline of the TLC plate: the purified tld, an authentic sample, and a co-spot of unknown pooking authentic sample.
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If the developed TLC plate shows only one row of spots, it can be concluded that the unknown has been purifed, and that the unknown is possibly the same compound as the authentic sample. However, because Rf values are relative, not absolute, some compounds may have very similar Rf values. A mixed melting point measurement is needed to unambiguously identify the unknown compound.
It is primarily used to determine the purity of a compound.
A pure solid will show only one spot on a developed TLC plate. In addition, tentative identification of the unknown compound can be made through TLC analysis. Monitoring the progress of a ffor The progress of a reaction can be monitored by Thin Layer Chromatography. At various times during the experiment, a TLC plate is spotted three times, then developed and viewed under UV light: - At position 1 is an authentic sample of the starting compound.
The reaction has gone to completion, when the TLC plate anc that there is no more starting material left in the reaction mixture. Typical Experiment Earl N.
Meyer wanted to determine the purity of the benzoic acid which he had obtained from Chemical Recycling. He decided to check the purity by chromatographic analysis using a silica gel TLC plate. On the TLC plate he marked the baseline anf a pencil, indicating the spotting position with a tick. Earl dissolved about 20 mg of the benzoic acid in 1 ml of methanol. Then he dipped a micropipette into the solution and lightly touched the TLC plate at the tick marked on the baseline.
Next, Earl prepared the developing chamber. He cut a filter paper to line a ml beaker.
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He then prepared the eluent: 10 ml of a mixture of dichloromethane and methanol oyu a ratio of 10 : 1. He added the eluent to the beaker, swirling it onto the filter paper to completely wet it. Earl placed the TLC plate in the developing chamber using tweezers and watched the eluent travel up the plate by capillary action. When the eluent was near the top of the plate, he used tweezers to remove the plate from the chamber, taking care to immediately mark the solvent front with a pencil.
When the TLC fpr was dry, Earl viewed it using long wavelength, ultraviolet light. He observed a single lookign spot on a light green background. His TLC indicated that the sample tested contained a single component and that his benzoic acid was pure. He used a pencil to circle the spot and a ruler to make measurements needed for calculation of the retention factor.
Practical Lab Tips T,c, prepare a TLC plate: dissolve a very small amount of your unknown sample in a volatile solvent, then apply the solution i. Next, prepare the TLC chamber: Cut the portion of the filter paper that rises above the beaker edge. Prepare the eluent using a mixture of the ltc solvents. When measuring liquids, read from the bottom of the meniscus, i. Swirl the eluent to wet the filter paper and cover the beaker with a lid.
This will saturate the chamber with eluent vapors and prevent evaporation of the eluent. Develop the TLC plate, that is, allow the eluent to wick up the TLC plate until it is about 1 cm from the top of the plate. Immediately after removing the TLC plate from the beaker, carefully draw a line along the solvent front. Be quick, because the eluent will evaporate within less than a minute. Visualize the developed plate using an ultraviolet lamp or other visualization method: - If the spot is observed to be too close to the baseline, your eluent is not polar enough.
The best Rf retention or retardation factor lies between 0.
If you want the Rf of your TLC spot to be smaller, i. Either choose a different eluent solvent or adjust the solvent ratio by increasing the percentage of the nonpolar solvent relative to the polar solvent in the eluent. The reverse holds true for the Rf of a spot that you want to be larger, i. Q: For TLC's, must the sample solvent and eluent solvent be the cor Sometimes sample solvent and eluent solvent s are loooing same, but this is not necessary.
If you are wondering why think about the different functions that each solvent plays in the TLC process. Q: What is the loking of a sample solvent in the preparation of a TLC plate? Before you can develop your TLC plate, you fir transfer some sample onto it. The simplest method to do this requires that you dissolve your sample in a solvent. Spotting the TLC plate using a micropipet and waiting for the solvent to evaporate will leave the sample on the TLC plate.
Q: How do I determine my sample solvent?
As a rule of thumb, your sample solvent should: 1 dissolve your sample when cold, 2 have a low boiling point so it will evaporate quickly and 3 be as non-polar as possible e. Q: In TLC, what is the purpose of using the tpc solvent s? Eluents are the mobile phase in chromatography, i.
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During the development of the chromatogram, the eluent distributes the sample you transferred to the TLC plate over the orbent on the plate. Q: How does the eluent solvent s work? The compound sample you spotted onto the stationary phase on the TLC plate has two choices: orb to the solid phase or being eluted by the mobile phase. If your eluent s is very polar relative to your compound, it will dissolve your sample and carry it in the mobile phase.
Overall, the eluent solvent s also called the mobile phase will compete with your sample for a space on the orbent stationary phase coated on the TLC plate. The more polar compound will win this competition and adhere ad sorb to the orbent, while the rest of the mixture will move with the mobile phase [upwards]. An eluent that is very polar in relation to the sample, will result in the compound spot being observed with a larger Rf, i.
On the other hand, if you eluents are too non-polar, the compound spot will be observed with a smaller Rf, i. Q: Do I have to use two solvents for my foe
Two solvents are not always necessary. Sometimes one solvent will be appropriate. However, two solvents allow you to adjust the eluent polarity easily when a kooking TLC plate does not give an appropriate result. A suitable eluent is one that gives a retardation factor Rf, also called retention factor close to 0. Is your Rf too high or too low, then you can readily adjust the polarity if the eluent was prepared by mixing two miscible solvents by changing the relative proprotions within the eluent mixture e.
If only one solvent was used, it will be necessary to switch to a solvent with a different polarity.