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Chromatography TLC A-level chemistry Help

Question: What determines the distance travelled by a spot in TLC?
Answer: The balance between solubility of moving phase and retention by stationary phase.

Could anyone explain what they mean by this?

Reply 1

jmadghost

Okay I’ve not looked at this in a while, but think as the solvent and the TLC plate as two different entities. If the sample is really soluble, it is more likely to travel with the solvent up the plate right? If it’s not soluble, or has interactions with the plate, it’s much more likely to be retained! The balance between the two determines how far it moves up the plate!

Reply 2

tony_dolby

The stationary phase allows components in the mobile phase to bind to it reversibly by various intermolecular forces.

The mobile phase may be a gas or a liquid and serves to carry the component molecules through the chromatographic system.

Let’s consider we want to separate two solids, A and B, by chromatography.
Both A and B are able to interact with the mobile phase which is carrying them along and also with the stationary phase.

An equilibrium is set up:

[Am] (reversible reaction arrow) [As]

Where [Am] is the concentration of A in the mobile phase
And [As] is the concentration of A bound to the stationary phase

DA = [As] / [Am]

DA is called the distribution coefficient for A. You don’t need to know this, but we can see from the expression for DA that the bigger the size of DA, then the greater the affinity of A for the stationary phase.

What about B? The distribution coefficient for B has exactly the same form. DB = [Bs] / [Bm]

It is obvious that if DB is much larger than DA then B binds more strongly to the stationary phase than A does.

Therefore B will move more slowly than A.

TLC is very similar to paper chromatography except that the stationary phase is not paper, but alumina sprayed onto a piece of glass. TLC is very useful as a preparation for further chromatography to separate products of a reaction for example. We can quickly calculate Rf values for compounds in a particular solvent by dividing the distance moved by each spot by the distance moved by the solvent front.

(edited 1 year ago)

Reply 3

jessicanaude

Original post by tony_dolby

The stationary phase allows components in the mobile phase to bind to it reversibly by various intermolecular forces.

The mobile phase may be a gas or a liquid and serves to carry the component molecules through the chromatographic system.

Let’s consider we want to separate two solids, A and B, by chromatography.
Both A and B are able to interact with the mobile phase which is carrying them along and also with the stationary phase.

An equilibrium is set up:

[Am] (reversible reaction arrow) [As]

Where [Am] is the concentration of A in the mobile phase
And [As] is the concentration of A bound to the stationary phase

DA = [As] / [Am]

DA is called the distribution coefficient for A. You don’t need to know this, but we can see from the expression for DA that the bigger the size of DA, then the greater the affinity of A for the stationary phase.

What about B? The distribution coefficient for B has exactly the same form. DB = [Ds] / [Dm]

It is obvious that if DB is much larger than DA then B binds more strongly to the stationary phase than A does.

Therefore B will move more slowly than A.

TLC is very similar to paper chromatography except that the stationary phase is not paper, but alumina sprayed onto a piece of glass. TLC is very useful as a preparation for further chromatography to separate products of a reaction for example. We can quickly calculate Rf values for compounds in a particular solvent by dividing the distance moved by each spot by the distance moved by the solvent front.