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ehd123 ehd123
wrote...
Valued Member
Posts: 778
10 years ago
Hello everyone
Could someone please help with the question attached?

Many thanks Slight Smile
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B.Sc in Biology
M.Sc Neuroscience
PhD. Candidate in Neuroscience


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wrote...
Valued Member
On Hiatus
10 years ago
Hello,
Can you copy-paste the questions? The questions must be posted word by word.
ehd123 Author
wrote...
Valued Member
10 years ago
There are no more questions.. this is the only one :/

In a TLC experiment, the following data were obtained. All measurements are listed from the bottom of the plate.
Original solvent depth : 0.6 cm
Final solvent front: 8cm
Original position of spots: 1cm
Final position of spot A: 4cm
Final position of spot B: 7cm
What is the Rf value of spot A and B and which of the two spots is of higher polarity?
B.Sc in Biology
M.Sc Neuroscience
PhD. Candidate in Neuroscience


wrote...
Valued Member
On Hiatus
10 years ago
Wikipedia http://en.wikipedia.org/wiki/Retardation_factor :
In planar chromatography in particular, the retardation factor (Rf) is defined as the ratio of the distance traveled by the center of a spot to the distance traveled by the solvent front.
That is correct, right?
Well, you just have to calculate how much did the solvent front traveled: (since the depth of the solvent is 0,6 that is out initial value) So,we calculate 8-0,6=7,4cm.
We calculate the distance traveled by each spot:
A: dfinal - dinitial=4-1=3cm
B: dfinal - dinitial=7-1=6cm

And we make the divisions:
For A: Rf=3/7,4=0,405
For B: Rf=6/7,4=0,811


About the polarity, I found that:
For instance, if normal phase silica gel is used as the stationary phase it can be considered polar. Given two compounds that differ in polarity, the more polar compound has a stronger interaction with the silica and is, therefore, more capable to dispel the mobile phase from the binding places. As a consequence, the less polar compound moves higher up the plate (resulting in a higher Rf value).[6] If the mobile phase is changed to a more polar solvent or mixture of solvents, it is more capable of dispelling solutes from the silica binding places and all compounds on the TLC plate will move higher up the plate.
http://en.wikipedia.org/wiki/Thin_layer_chromatography


But, it states that normal phase silica is used. Do we know what gel is used for your TCL? Or it doesn't make any difference?

Anyways, assuming that the above applies to your question, the less polar substance will travel more. (A has higher polarity). And, I think that should be the correct answer.
ehd123 Author
wrote...
Valued Member
10 years ago
Hi Alexx! Thank you so much for the help..
Yep silica is what we used for the experiment, but in the question silica was not stated, so I am not sure whether it matters or not. However, the work is all right, but the distance moved by the solvent is wrong. Frowning Face could it be 8 - 1 instead of 8 - 0.6?
Why is the spot at 1 cm? and the solvent " originally" at 0.6. the solvent will eventually move up to the spot, so should we assume the solvent depth is 1 cm and not 0.6 since we are calculating the Rf for the spot when TLC actually started? I am confused as to what the solvent distance is  Frowning Face
B.Sc in Biology
M.Sc Neuroscience
PhD. Candidate in Neuroscience


wrote...
Valued Member
On Hiatus
10 years ago
Quote
the solvent will eventually move up to the spot, so should we assume the solvent depth is 1 cm and not 0.6 since we are calculating the Rf for the spot when TLC actually started?
Oh, that sounds right, we should start with 1 cm.
That is, because before the solvent reaches the spots, the spots will not move. (the substances will only move along with the solvent). So, there is no reason to add the distance before the 1cm.

The important thing is, that the depth should be less than 1cm so that the spots will not be submerged in the solvent. That's probably why it states that the depth is 0,6cm.

So yes, the solvent distance should be 8-1=7cm. (And the Rf values will change accordingly)

And, sorry for the mistake, I'm not very familiar with these techniques either.
ehd123 Author
wrote...
Valued Member
10 years ago
Yea that's what I was thinking as well  Face with Open Mouth Thanks for the help Alexx! Much appreciated!  Slight Smile
B.Sc in Biology
M.Sc Neuroscience
PhD. Candidate in Neuroscience


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