The absorbent is coated as a thin layer in a glass slide. The organic compounds are separated by manipulating the mobile phase.
TLC is a chromatography technique responsible for the separation of organic compounds on a mixture based on their relative mobility. It uses a solid stationary phase made up of a polar absorbent and a liquid mobile phase composed of a single or a mixture of organic solvents. The polar absorbent can be either finely-ground alumina or silica particles. Separation of black ink by TLC is shown in figure 1. The organic compounds and the mobile phase move over the stationary phase due to the capillary action.
Let's say that you had a plate that looked something like this. You had initially spotted two compounds. We'll call them A and compound B. And then what you saw on the plate was that your mobile phase had traveled up to about here, A had traveled to about here, and B had traveled this far. But what does that really mean? How can we even report these values? The way we'd report them if we were writing up a lab report or writing a manuscript, you'd need something known as the retardation factor, also known as the retention factor or RF for short.
RF is equal to the distance traveled by solute over the distance traveled by the solvent. So the first step you need to do is measure these distances for the different compounds and also for the solvent, also known as the mobile phase. So let's put a ruler next to our TLC plate, much like you would if you were sitting in lab. We'll say that this is 1 unit, 2 units, 3 units, and 4 units. So we can measure the distance that A has traveled, and that's from the starting line to the center of the spot.
That's two units. And for compound B, again from the starting line to the center of the spot, that's 3 units. And for the solvent, the starting line to this finish line, that is 4 units. So let's plug that into our equation. If we wanted to solve RF of A, you need the distance traveled by compound A over the distance traveled by the solvent, so let's say A over S. There are no units in the answer and it doesn't matter what units you use to measure the Rf value because an Rf value is a ratio, and therefore the units are arbitrary.
Why silica gel is used in TLC? Silica gel is by far the most widely used adsorbent and remains the dominant stationary phase for TLC. The surface of silica gel with the highest concentration of geminal and associated silanols is favored most for the chromatography of basic compounds because these silanols are less acidic.
What does an RF value of 0 mean? The highest Rf value is 1, which, by definition, means at the very top of the plate. The lowest Rf value is 0, which again, by definition, means at the very bottom of the plate. Rf values have nothing to do with polar or nonpolar, it is just another way of measuring the distance from the bottom of the plate.
Is silica gel polar? Silica gel is a polar adsorbent. This allows it to preferentially adsorb other polar materials. When it comes to polarity, materials interact more with like materials. This principle is particularly important to many laboratories, which use silica gel as the stationary phase for column chromatography separations. What are the R f values for each standard?
Now, you know the R f values for the standards. Next, on another piece of paper you place a dot of the mixture and allow the chromatogram to develop by placing it in water. Once removed, you find that there are two dots that separated from the initial dot you placed.
This likely means that there are only two components in your mixture. Next, you need to figure out which of the standards these two components correspond to. To do this, you measure the R f values. The solvent moved 4.
One component called component 1 moved 2. To see which standard these components correspond to you will need to use the retention factor formula to calculate the R f values. Given these R f values, it is likely that component 1 corresponds to Standard 3 since the R f values are very similar. Component 2 is likely Standard 2. She has an interest in astrobiology and manned spaceflight. She has over 10 years of biology research experience in academia.
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