goldberg
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very brittle coating on TLC
I have read bunch of materials about making own TLC plates(commertial ones are too expensive) and in theory and on videos it looks simple.
I have made a few attempts with very similar results that are not satisfactory.
I'm using microscope slides cleaned with surfactant, deionised water and acetone.
For coating i'm using Al2O3(polishing grade) with gypsum as binder. Amount of gypsum was 5% to 20% by weight. Before coating Al2O3 and gypsum was well
mixed with mortar and pestle. Then i have added water to make slurry, coated my plates and dried them in oven in 140C for 40min.
The problem is that coating layer is so brittle that i'm not able to mark them with pencil to know where they are spotted. Even gentle application of
sample with capillary tube damages coating.
Also visualisation by spraying with liquid reagent is very tricky, because it tends to make pieces of coating to move.
My second problem is that spots move very little if at all. I have tried with pyrogallol, hydroquione and aspirin. For eluent i have used ethyl
acetate, then i have tried adding acetone and ethanol to make it even more polar but without success. Even glacial acetic acid did not moved samples
very far away. Eozin have never moved even for millimeter.
Have i used too polar substance for test of my plates?
I would be gracefull for any hints what can be wrong.
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S.C. Wack
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The alumina is not the right size or kind. IDK if it would actually stick better with better alumina. One might have better luck moving less polar
substances.
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Bonee
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According to this patent you could try mixing some water glass to your sorbent layer to increase adhesion to the plate.
https://patents.google.com/patent/US3594217A/en
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Fulmen
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Wack is right, it's the wrong type. You need activated alumina, the abrasive quality is just hard sand. Perhaps kitty litter silica gel could work?
As for adhesion you could also try coating the glass with dilute water glass and then dry out before applying the stationary phase. Just a guess, I've
only used the commercial aluminum sheets.
We're not banging rocks together here. We know how to put a man back together.
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goldberg
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Thanks, wrong type of alumina explains much. Kitty litter silica gel seems to be a way to go.
The problem is how to effectively grind it. Mortar and pestle seems to be pretty labour intensive or i am wrong?
I will try with water glass.
But if i have alpha Al2O3 (corundum) instead of gamma Al2O3 then it should have little porosity. And this would cause very high retention factor
because sample would interact very little with stationary phase. And my case is opposite: too strong interaction with stationary phase. Or i have
missed something?
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Fulmen
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A coffee mill should do the trick, kitty litter is pretty friable in my limited experience.
As for the eluent I can't offer much help other than to try a few more alternatives. Try something really polar (like water) and something really
non-polar (heptane). Most of my TLC-work was done with methanol/chloroform in varying ratios.
We're not banging rocks together here. We know how to put a man back together.
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unionised
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Are you trying to use gypsum as a binder, rather than adding plaster of Paris to the alumina so that, once it sets, you have gypsum?
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Fulmen
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I naturally assumed plaster as I see little point in adding (hydrated) gypsum. But good call, not all languages make any clear distinction between the
two forms so it's easy to get lost in translation.
BTW, what are you using for visualization? The only methods I can remember other than UV was sulfuric acid + heat gun. Chars mostly anything organic.
[Edited on 30-3-21 by Fulmen]
We're not banging rocks together here. We know how to put a man back together.
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goldberg
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I'm using basic solution of KMnO4 for visualisation.
By gypsum i mean what you mix with water and wait to harden.
First i'm mixing Al2O3 with gipsum and then i'm adding water to allow it to harden.
Also how about TiO2 from paint store? Has anybody experience with it as TLC coating?
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S.C. Wack
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The hemihydrate, and it's not organic chemistry...TLC adsorbent of any kind under 600 mesh is probably not that great...it's not easy to grind things
finer than that without a ball mill.
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