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Evaporation of organic solvent and leftover residue

  1. Apr 15, 2009 #1

    ~christina~

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    I have a question about evaporation of organic solvents and the leftover sample after this is done. (sample dissolved in the organic solvent)

    Does the sample, leftover after the evaporation of a organic solvent, (used to dissolve the sample) affect the analysis of the dry sample? (basically, would there still be leftover organic solvent left, or does it all evaporate off? and thus not affect the sample analysis)

    This question applies to two situations I have:

    1. I have to run a TLC plate and I was wondering if the organic solvent used to dissolve the compound mattered, since you evaporate it off before you run the plate. Does it affect the run?

    2. Would there be a residual leftover of organic solvent on the sample after evaporation that would affect the spectrum of the compound when run on a KBr pellet in IR spectroscopy?

    Thank you.
     
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  3. Apr 16, 2009 #2

    Borek

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    In general there should be no problems.

    It wouldn't be very surprising if in some very rare cases simple evaporation will be not enough, but IMHO you will be very (un)lucky to have to deal with such a situation.
     
  4. Apr 16, 2009 #3

    ~christina~

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    Thanks alot Borek :smile:
     
  5. Apr 16, 2009 #4

    chemisttree

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    In my experience, and I'm a professional chemist, you almost always have residual organic solvent in your sample unless you take pains to remove it thoroughly. Simple evaporation almost never removes all of the solvent unless the compound crystallizes.
     
  6. Apr 16, 2009 #5

    Borek

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    OK, I stand corrected :redface:

    What I was aiming at was that in my experience (past experience I must add) solvent was never a problem when preparing spectra. Sure, if you are using spectroscopy to determine contaminants, you may easily detect solvent left, but - if that's the risk - you will never use this solvent in the first place. When you use solvent for extraction, or for reaction - it usually doesn't interfere.

    Or is my experience abnormal?
     
  7. Apr 16, 2009 #6
    You need to dry a compound for at least 24 hours over night in a vacuum oven if you want a clean NMR without any traces of solvent.
     
  8. Apr 17, 2009 #7

    ~christina~

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    http://img5.imageshack.us/img5/4596/shockedb.gif [Broken] Darn, I thought that it would leave a residue.

    Situation: unknown compound(s) dissolved in solvent(s) and thus I have to evaporate solvent off before being able to analyse compound(s). I want to use TLC to run, solvent + compound to determine number of compounds in liquid. Then of course evaporate the solvent off if it's one compound and extract if it's more than one compound. Finally I want to, evaporate solvent off and make KBr pellet and run in IR spectrometer.

    Note: Using AIR evaporation in fume hood

    The big question is, (for anyone who can answer this)
    1. Will it make it difficult to determine the spectra of an unknown compound if there is
    residual solvent leftover, when analysing the spectra through KBr pellet method?

    2. Does solvent used to dissolve compound and run TLC plate matter since it evaporates? (I
    want to run the TLC plate preferably without using GC to find exact solvent first, to see
    how many drugs are in unknown liquid)

    I don't know what the solvent is and I really haven't encountered having to evaporate liquid samples to obtain a powder form of a compound. This is the reason I don't know how a IR spectra will be affected by leftover solvent residue from just air evaporation.
    Well the problem is that I don't have the availability to place something in an oven overnight and we don't have a vacuum oven, so I'm planning to use air evaporation in the fume hood.

    Thanks
     
    Last edited by a moderator: May 4, 2017
  9. Apr 18, 2009 #8

    chemisttree

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    Christina asked about IR pellets which implies that she is working with a solid. You are correct that the residual solvent won't usually interfere with an IR determination. There are some exceptions I could come up with but I'd be reaching if I called that the usual experience. If a student were trying, for example, to make a ketone or carboxylic acid from an alcohol for the first time, he or she might be confused in their interpretation of an IR spectrum if solvent peaks are present. An alcohol (starting material) has an OH stretch that would interfere with the OH absorption of water if that was used in the workup - extraction. Likewise the carbonyl stretch of a solvent like ethyl acetate might be confused with the carbonyl stretch from the expected product likely producing two closely-spaced absorbances and leading to confusion in the interpretation.
     
  10. Apr 18, 2009 #9

    chemisttree

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    For IR, probably not but be aware that you are usually only looking at certain functional groups in the IR. Keep in mind what type(s) of solvents you used that might interfere.

    If the evaporation leaves an oily residue, expect significant interferences.

    TLC is usually no problem since you are spotting them from solution anyway.
     
  11. Apr 19, 2009 #10

    ~christina~

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    We are using the fingerprint region of the spectrum and comparing that with known fingerprint region spectra of known compounds in a database.

    Define, "oily residue." I'm not sure how that would look like.

    Thank you
     
  12. Apr 19, 2009 #11

    Borek

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    Like goo.
     
  13. Apr 19, 2009 #12

    ~christina~

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    i.e. not powdery?
     
  14. Apr 20, 2009 #13

    Borek

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    Generally - not solid. Most likely you will have a drop of something that looks like oil.
     
  15. Apr 20, 2009 #14

    ~christina~

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    okay, Thanks Borek. :smile:
     
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