Chromatography - Online Tutor, Practice Problems & Exam Prep

Chromatography is a technique for separating components in a mixture based on molecular attractions. It involves a stationary phase, like a silica plate, and a mobile phase, such as a solvent. The movement of components is influenced by their affinity for the solvent versus the plate. The retention factor (RF) is calculated using the formula: distance traveled by compounddistance traveled by solvent. This helps identify compounds by comparing RF values to known standards.

Chromatography

So when we say chromatography, we're going to say that this is a technique that involves the separation of components, mainly solids and liquids, within a mixture because of a difference in molecular attractions. We're going to say in the procedure a mixture is spotted on a silica plate and the progress of the components on that plate is based on their affinity to the solvent or the plate itself. Now with this process, we have 2 phases. 1 represents the silica plate which doesn't move, and it holds the mixture. Because it doesn't move, we call it the stationary phase. The other phase represents the solvent, the portion that's moving, the liquid portion. So we call it the mobile phase. So it moves up the silica plate and it moves by capillary action.

So here I have a TLC plate, I draw a line, I have a mixture, and I take a sample of this mixture and I spot the TLC plate. I place it within my mobile phase which is my solvent. And here, let's say that the solvent is 50% ethanol which is CH₃CH₂OH. Ethanol is slightly polar and then we have 50% hexanes, which is C₆H₁₄. It's a hydrocarbon, so it's nonpolar. Now, the dots can start to separate and move up. So here, as the solvent phase is moving up the TLC plate, we can see that this dot, which is made up of a mixture of 2 different compounds, starts to split. It splits into red dots and green dots to show the separation of the mixture that I originally took from my sample. Now, if the dots themselves, if the attraction to the plate is greater than the attraction to the solvent, then the dots are not going to move very far up the plate so there's going to be low movement. Now, let's say that the dots have a greater affinity for the solvent than the plate. Since they have a greater affinity or attraction to the solvent, the mobile phase, they're going to move with it. So, there's going to be high movement or higher movement.

Looking at this we can see that at the end the solvent has reached up this high in terms of the plate. I take it out, we can see that the green dots have moved up higher than the red dots. The green dots had a higher affinity for the solvent, they moved up with it. The red dots had less of an affinity for the solvent so they don't move up as high. Here I mark 1, 2, 3, and 4. 1 being the original position the starting line, 2 is the position where the red dot stopped, 3 is the position where the green dot stopped, and 4 is my mobile front that's where my solvent stopped, where I took the paper out. From these numbers, I can determine my R_F value. So, the distance traveled by the components is a method we can use to find the R_F value. This helps us in the identification of the compound.

Your R_F value equals distance traveled by compound divided by distance traveled by solvent. So, how would I do this? Well, if we're looking at the red dots here, we'd say the red dots traveled 2 and the solvent traveled 4. So the R_F value of red is 0.50. For green, it traveled 3. The solvent traveled 4, so that's 0.75. These are the R_F values of the red dot and the green dot. Usually you'd have a manual and associated with these numbers, you'd have names of actual compounds with R_F values and you choose which compound from that list matches closely with 0.50 and 0.75. And in that way, you'd identify what does the red dot represent, what compound, and what does the green dot represent. So that's how we can use basically TLC plates and spotting mobile phases and stationary phases to determine the identity of these 2 different spots. Okay. And we incorporate the R_F value to do so.

Here’s what students ask on this topic:

What is chromatography and how does it work?

Chromatography is a technique used to separate components in a mixture based on their molecular attractions. It involves two phases: a stationary phase, like a silica plate, and a mobile phase, such as a solvent. The mixture is spotted on the stationary phase, and as the mobile phase moves up the plate by capillary action, the components separate based on their affinity for the solvent versus the plate. Components with a higher affinity for the solvent move further up the plate, while those with a higher affinity for the stationary phase move less.

What are the stationary and mobile phases in chromatography?

In chromatography, the stationary phase is the phase that does not move, such as a silica plate. It holds the mixture to be separated. The mobile phase is the phase that moves, typically a liquid solvent. The mobile phase travels up the stationary phase by capillary action, carrying the components of the mixture with it. The separation of components depends on their relative affinities for the stationary and mobile phases.

How is the retention factor (RF) calculated in chromatography?

The retention factor (RF) in chromatography is calculated using the formula:

RF=distance traveled by compounddistance traveled by solvent

For example, if a compound travels 2 cm and the solvent travels 4 cm, the RF value is 0.50. This value helps identify compounds by comparing it to known standards.

What factors affect the movement of components in chromatography?

The movement of components in chromatography is affected by their affinity for the stationary phase versus the mobile phase. Components with a higher affinity for the stationary phase (e.g., silica plate) will move less, while those with a higher affinity for the mobile phase (e.g., solvent) will move more. The polarity of the solvent and the components also plays a significant role. Polar solvents will carry polar compounds further, while non-polar solvents will carry non-polar compounds further.

How can chromatography be used to identify compounds?

Chromatography can be used to identify compounds by calculating the retention factor (RF) for each component in the mixture. The RF value is determined by dividing the distance traveled by the compound by the distance traveled by the solvent. These RF values are then compared to known standards listed in a manual. By matching the RF values, the identity of the compounds can be determined.