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111davey1

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Hi,
I was just looking at the limitations of TLC chromatography in my textbook and one said:

Similiar compounds have similiar rf values.

i can understand that similar compounds with similar rf values will not separate during the test.

Is this bad because you won't be able to measure the rf values of the components accurately and therefore if you wanted to compare to the rf of a pure substance this would not be possible?

secondly, it said that unknown compounds have no reference rf for comparison.

Does this mean that you have to either run a chromatogram with suspected substances alongside or compare the rf value of a suspected pure substance, and that if you have no idea what the substance is it can't be identified
Thanks

h3rmit

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(Original post by 111davey1)
Hi,
I was just looking at the limitations of TLC chromatography in my textbook and one said:

Similiar compounds have similiar rf values.

i can understand that similar compounds with similar rf values will not separate during the test.

Is this bad because you won't be able to measure the rf values of the components accurately and therefore if you wanted to compare to the rf of a pure substance this would not be possible?

A compound is one component, and would ideally produce an individual dot. If other compounds have dots at the same places, your measurement of Rf is still accurate, it just isn't as helpful because their are multiple possibilities for what the dot could be. When you compare the dot to the pure substance, the multiple possibilities are still there.

secondly, it said that unknown compounds have no reference rf for comparison.

Does this mean that if you do not run a chromatogram with known substances alongside the one you are experimenting it is possible to just measure the rf values and identify the compound.

Thanks

You can measure the Rf values of your compounds in question and try to identify by comparison to literature values, but then if your compound is unknown you've got diddly squat.

Also, your solvent might not be exactly the same as the literature value's, or the air might be colder, etc. so it's better to have a known substance chromatogram running alongside the unknown.

111davey1

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(Original post by h3rmit)
A compound is one component, and would ideally produce an individual dot. If other compounds have dots at the same places, your measurement of Rf is still accurate, it just isn't as helpful because their are multiple possibilities for what the dot could be. When you compare the dot to the pure substance, the multiple possibilities are still there.

You can measure the Rf values of your compounds in question and try to identify by comparison to literature values, but then if your compound is unknown you've got diddly squat.

Also, your solvent might not be exactly the same as the literature value's, or the air might be colder, etc. so it's better to have a known substance chromatogram running alongside the unknown.

Thanks very much.
by an unknown compound do you mean there is no literature value for it or do you mean that when you do the test you have no idea what to expect so you can't refine the search for the compound beforehand and so it would be very difficult to positively identify it.

h3rmit

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(Original post by 111davey1)
Thanks very much.
by an unknown compound do you mean there is no literature value for it or do you mean that when you do the test you have no idea what to expect so you can't refine the search for the compound beforehand?

I mean unknown unknown, as in no Rf value

111davey1

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(Original post by h3rmit)
I mean unknown unknown, as in no Rf value

Thanks for the help

111davey1

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(Original post by h3rmit)
I mean unknown unknown, as in no Rf value

Sorry to come back to this, but a question in an exam said:

'several amino acids have structures that are very similar, suggest why this could cause problems when using TLC to analyse a mixture of amino acids'

and the answer was similar compounds have similiar rf values and so may not separate. Mark scheme would allow spots often overlap.

i don't know how to interpret this. Is this assuming the practical element is not done correctly so they won't separate or is it saying that even the literature values of amino acids may me very close or the same meaning the test is not really that useful, even if you do it correctly they may not separate.
Or is it
similiar compounds have similiar rf values --> leads to no seperation or lack of seperation ---> means resolving rf value becomes tricky.
Thanks again

h3rmit

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(Original post by 111davey1)
Sorry to come back to this, but a question in an exam said:

'several amino acids have structures that are very similar, suggest why this could cause problems when using TLC to analyse a mixture of amino acids'

and the answer was similar compounds have similiar rf values and so may not separate. Mark scheme would allow spots often overlap.

i don't know how to interpret this. Is this assuming the practical element is not done correctly so they won't separate or is it saying that even the literature values of amino acids may me very close or the same meaning the test is not really that useful, even if you do it correctly they may not separate.
Or is it
similiar compounds have similiar rf values --> leads to no seperation or lack of seperation ---> means resolving rf value becomes tricky.
Thanks again

It's assuming the practical element is done correctly, but as the amino acids have similar levels of solubility in the solvent and similar levels of attraction to the stationary phase, they move to the same level. This means the compounds are indistinguishable by TLC, so TLC isn't very helpful in this case. It's like using 2,4-DNP when all the compounds you're testing for are aldehydes or ketones - you don't get any new or helpful information from the result.

The lack of separation leads to similar Rf values. As long as the dots are small, resolving (by which I assume you mean calculating) the Rf value is easy - if the dots are in the same place, it's still easy but your unknown (as in unknown to you, not to science) could just be multiple things so you get no helpful information from TLC.

111davey1

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(Original post by h3rmit)
It's assuming the practical element is done correctly, but as the amino acids have similar levels of solubility in the solvent and similar levels of attraction to the stationary phase, they move to the same level. This means the compounds are indistinguishable by TLC, so TLC isn't very helpful in this case. It's like using 2,4-DNP when all the compounds you're testing for are aldehydes or ketones - you don't get any new or helpful information from the result.

The lack of separation leads to similar Rf values. As long as the dots are small, resolving (by which I assume you mean calculating) the Rf value is easy - if the dots are in the same place, it's still easy but your unknown (as in unknown to you, not to science) could just be multiple things so you get no helpful information from TLC.

Ah that clears things up thanks

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