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chemrox
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TLC solvents
I need to adjust my TLC tank solvent system. I have Me2Cl2, EtOAc, MeCN, trace of Et3N -- 2:1:0.2:tr. Its not separating the amides from the
amines enough. The spots overlap with an rf difference of 0.5-1.0. I need to increase the separation. This system did work well separating the
primary and secondary amines and the amines from other cmpds. Thanks for your suggestions.
CRX
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Sandmeyer
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What's the structure of the compounds you want to separate? In my experience in order for amines to migrate Et3N (or highly polar eluent) is needed.
Try heptane:EtOAc 1:1 to begin with, silica is slightly acidic and without base it should be very easy to separate amines from amides since amine
should remain on the baseline. "Me2Cl2" = DCM?
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Arrhenius
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Can you try to explain the plate (draw it if necessary). Name all spots in the mixture by Rf. If they're running together, your solvent system may
be no good, or you're spotting too much.
What you need is a non-polar solvent (hexane), all of what you listed would be considered polar. Remove the Et3N and amines will have Rf of about
zero from pure EtOAc or EtOAc:hexane provided your compound isn't huge otherwise. Amides should be easily separable from amines.
If you need to separate a mixture, I would suggest acid-base extraction to remove the amines from the amides (though amides can be very water
soluble).
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smuv
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Get a small sample of the Amide/amine mixture, then take a pipette, stick it into a bottle of HCl pull just the semi-anhydrous HCl vapour sitting over
the HCl solution and then bubble this through the small sample of the amide/amine mixture. Then spot/elute this small sample. After this treatment,
the amine will definitely be at baseline with all but the most polar solvents.
Edit:
@Sandmeyer thanks for introducing me to Royksopp
@ Chemrox, upon second read, I realize that I don't fully understand what you are aksing...Forgive me if i am completely off base.
[Edited on 2-21-2009 by smuv]
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chemrox
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I'm monitoring the progress of a conversion: amine --> amide by TLC. I'm not getting big enough RF difference to separate the spots but I just
saw something in these responses I should have snapped to immediately. My spots are too damned big. If I spotted less I'd have adequate separation
to make sure the amine reactant was consumed. BTW- I visualize with I2. It seems to work will and is simplicity itself. I have a tank with I2 in it
that the plates go in when dried. I also have an H2SO4 sprayer but haven't found it necessary yet. @ Sandemeyer, yes my non-standard abbreviation for
DCM was Me2Cl2 (I have no idea why I used it ..) so while Smuv has a great idea and I will make a note on the technique involved, I simply need to
pull another sample and dilute before spotting.
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stoichiometric_steve
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| Quote: | Originally posted by chemrox
my non-standard abbreviation for DCM was Me2Cl2 |
not only non-standard but plain wrong
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Globey
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| Quote: | Originally posted by stoichiometric_steve
| Quote: | Originally posted by chemrox
my non-standard abbreviation for DCM was Me2Cl2 |
not only non-standard but plain wrong |
Nope: chemrox's abbreviation makes perfect sense. In place of CH he has Me. Unless your just being picky because of the two free -'s. But
electrons aren't generally shown in shorthand.
(For everyone's info, stoichiometric_steve just sent me an unsolicited PM, the contents of which: "You are an idiot"
Any you, SS, are obviously a nice guy, and not some antisocial miserable fuk.
Chow
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497
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Uhm... remind me again how there are there two carbons in DCM?
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Globey
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There aren't, there's 2 hydrogens one carbon and a two chlorines. It's just short hand, and there's no mistaking it because methylene chloride is the
only real compound this could be.
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HydroCarbon
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Generally the more polar the compound being tested the more polar your eluent should be. In TLC the plate and the eluent compete for the substance
being analyzed. Therefore if you want it to move further, use a solvent that it is more soluble in. And yes, over spotting will lead to spots
overlapping and tailing. I usually use a double open capillary tube to spot with.
On the note of abbreviation, if Me stands for methylene then methylene chloride should be abbreviated MeCl2, not Me2Cl2. Although, DCM is much
easier, and quite standard.
[Edited on 21-2-2009 by HydroCarbon]
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chemrox
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Yeah but its lousy shorthand and I won'y use it again. Anyway, I was reading some papers from the '70's that suggested that saturation of the chamber
cn affect Rf values greatly and that, and this was the kicker, that saturated chambers give poorer separations than unsaturated ones (Sherma, ed.,
1973). I have requestd the original papers because I have been using sheets of filter paper in my tank to insure saturation of the vapor phase. I
may want to change everything to storing the solvent system when not in use. Sorry about the CHCl2 thing ... mea maxima culpa OK?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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DJF90
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Shouldn't that be CH2Cl2...?
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HydroCarbon
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| Quote: | Originally posted by chemrox
Yeah but its lousy shorthand and I won'y use it again. Anyway, I was reading some papers from the '70's that suggested that saturation of the chamber
cn affect Rf values greatly and that, and this was the kicker, that saturated chambers give poorer separations than unsaturated ones (Sherma, ed.,
1973). I have requestd the original papers because I have been using sheets of filter paper in my tank to insure saturation of the vapor phase. I
may want to change everything to storing the solvent system when not in use. Sorry about the CHCl2 thing ... mea maxima culpa OK?
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Interesting, I've always done the filter paper thing too with the purpose of halting evaporation from the plate during the process. It would seem
that evaporation from the plate due to the chamber not being as saturated would slow the rise of the solvent, possibly allowing for better separation.
[Edited on 22-2-2009 by HydroCarbon]
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Arrhenius
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Iodine is good, but kindof slow. I would highly recommend using a basic solution of KMnO4. Just dip the plate, and heat it lightly with a heat gun.
This works well with any oxidizable analyte. Yes, TLC spotting solutions should be VERY dilute. Get some hexane, I promise it will work wonders for
you! Hexane:Ethyl Acetate is so widely used as a solvent system because it generally gives tight spots. DCM is not so great as a TLC solvent. If
you can't get spots to move in 100% EtoAC, try adding a very small amount of MeOH (too much and you'll have problems with the silica dissolving).
Good luck.
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Ebao-lu
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| Quote: | | My spots are too damned big. If I spotted less I'd have adequate separation to make sure the amine reactant was consumed. |
You can try to dilute your RM before spotting, maybe this could help? And what is the Rf of the higher spot, and is it amide or
amine? You can use a small amount of NH3 as well, but i need to enquire to be sure.. Still, it is strange that the method was previously used for
amines separation, and now it does not work.
ps: stop trolling chemrox for DCM! he already understood
[Edited on 22-2-2009 by Ebao-lu]
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chemrox
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Thanks for the solvent suggestions. I think I will move to hexane:EtOAc. I changed to EtOAc:MeOH (tr Et3N). The main improvements came from dilution
and prventing the vapor from saturating the tank. The latter resulted in higher Rfs and better separation. Complete separation was achieved and a
clear distinction of the spots was made. I'm happy to say that the reax went to completion with no reactant amine left. I've changed my TLC
procedure to storing the solvent in a bottle and placing in the tank just before the plate. I've been using glass plates. I cut them into strips
with a glass cutter for the one dimension runs. I also have a hook for hanging filter paper strips but large test tubes work OK for this too. Basic
KMnO4? I'll try it. NH4OH/KMnO4? NaOH/KMnO4? Either one? I can char then with con H2SO4 too. I2 takes about 10 min.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Arrhenius
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Maybe I've misunderstood, but you definitely want the vapor to saturate the tank. This is essential for repeatable TLC results & tight spots. If
you've had to add Et3N to get the amine spot to move up from the origin, adding hexane won't help you (decrease polarity = decrease Rf, you know
this). I use 9:1 EtOAc:MeOH (no triethylamine) with the amines I'm currently working with. I have also used ammonia saturated MeOH. It just takes
some time to find out what works.
I'll try to grab the recipe from work... it's something like 1M NaOH plus KMnO4 in water. The spots will be yellow/brown and your plate pink. Works
well with glass plates, and solution lasts a long time before going bad. Takes about 30seconds to visualize the plate. I've dipped & sprayed
H2SO4, but I would normally avoid it. It's a mess, and the spots are faint (though different colors, which is neat), and you generally have to load
the plate quite heavily. Of course UV is the easiest visualization if you've got UV dense analytes.
[Edited on 22-2-2009 by Arrhenius]
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Ebao-lu
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Arrhenius: thatks for nice method of spots viewing!
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chemrox
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@Arhenius: Yes- to the repeatability consideration. I pondered that and concluded there's no other way. Unless- everyone follows the same process
for setting up an unsaturated tank or at least publishes the proces along with the data. Even then...The damned thing is this; you do get higher Rf's
and better separation with unsaturated tanks. Oh well, I don't see it as a feasible option. Do you use a wick inside your tank?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Arrhenius
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I mean... realistically... Rf values will vary between your TLC plate and mine simply from the sorbent batch or whatever, but for your own purposes
having repeatable plates is good. If the Rf is too high, the proper thing to do is to decrease the polarity of your solvent by lowering the amount of
methanol, or if it's too high in 100% ethyl acetate, by adding hexanes. This is of course just simple TLC for checking reactions. There are lots of
fancy mixes with toluene, ether, acids, chloroform, etc. but these are really only appropriate for high performance separations for quantitative work,
or identification in complex mixtures. I would not generalize your statement that a non-saturated tank lowers Rf and improves saturation, as I'm sure
this is not always the case.
My plates are generally ~2cmx5cm for checking reaction progress or workup purity. I do generally fold a filter paper up and stick it on the side of
the chamber, and I keep a lid on the chamber and typically run up to a dozen plates in a sitting. So yes, a wick is a good habit, albeit not
essential for this small of a plate.
If you were running a 20cmx20cm plate, or 2D, you would find that the solvent front is uneven (sagged in the middle) if you don't saturate the tank.
[Edited on 23-2-2009 by Arrhenius]
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chemrox
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| Quote: | Originally posted by Arrhenius
I would not generalize your statement that a non-saturated tank lowers Rf and improves saturation, as I'm sure this is not always the case.
[Edited on 23-2-2009 by Arrhenius] |
Maybe- I experimented with it after reading an excerpt from a paper by Sherma in the Journal of Chromatography. I don't mean to wax authoritarian. I
haven't gotten the original paper and found out how many experiments the statement was based on or what compds were involved.
As to the solvent front. The first thing I noticed wth the unsaturated tanks was an uneven solvent front. I'm using 1-2 cm X 12 cm plates. I'm
cutting them from 12 X 12 plates. It's awfully hard glass and I have a lot of bad cuts. Sometimes I have the window shop cut up a couple of plates
for me.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Arrhenius
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In terms of synthetic work, a 12cm long plate is pretty huge. I don't know if you're having to buy them, but if so you can probably make your money
go a little further by using a small plate. Seriously, it probably sounds silly that I use a 2cmx5cm plate, but it works well 99% of the time. The
only time I've used larger plates is for a better assesment of purity, or for 2D analysis of raw extracts & radiotracers. Try that KMnO4 stain
out, it's quite a bit more sensitive than I2.
Another trick to get basic compounds to move on your plates is to soak the plate in EtOAc with a little bit of triethylamine, rather than putting the
TEA in your mobile phase. Just a thought. Sounds like you've got things figured out though.
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Daedalus
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TLC ELUENTS BY COMPOUND TYPE
Is anyone aware of a reference work on thin layer chromatography that contains a chart of appropriate eluent mixtures arranged by the class or
sub-class of compound to be eluted (i.e., what solvent mixture is best for amines, ketones, aldehydes, etc.)? I know that, with silica at least, the
more polar the compound, the more polar the eluent required to move it up the plate. Often times, however, it seems like much guess-work is involved
in selecting the ideal mixture to effect good spot separation between compounds of similar polarities. In my searches so far I have found only
general guidelines but nothing remotely comprehensive. Is anyone familiar with such a work, or must we go by "feel" in each specific situation?
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byko3y
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I have similar question. What confuses me the most is that mixtures of solvents are usually used instead of a single solvent, e.g.
acetonitrile-ethanol, chloroform-methanol-water. I wanto to empasize the last example, because it could be substituted with a pure methanol or
ethanol, but I'm sure there's the reason why a mixture works better than a single solvent. I've already spent some time searching the internet trying
to find some explanation, yet most of the authors have no idea about solvent selection, except general rules "more polar solvent for more polar
compound" and "guess and try". I'm pretty sure in a few days I will be able to find some answers, but maybe some of you already have one.
[Edited on 21-7-2016 by byko3y]
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Metacelsus
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You can easily adjust the ratio of solvents in a mixture if you want to tweak the retention factors. You can't do this when using a pure solvent.
In my experience, there aren't general rules beyond those you've mentioned. "Guess and try" is very much a part of selecting TLC solvents. Based on
the results of previous guesses, successive guesses can be improved.
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