My UV lamp just broke down so I can't visualize any bands to scrape. Since Iodine forms reversible complexes with most organic compounds, could I use it safely without destroying my compounds?
Hi,
Yes you can use iodine. Many years ago (in the 60's) we made our own TLC plates by dipping microscope slides into a Silica mixture in CHCl3, then letting them dry. Spotting the reaction mix then running in a solvent worked fine, and we visualized the plates in a jar with iodine crystals, getting various shades of brown, and sometimes even blue. If you can sacrifice the plate by cutting as suggested it should work fine.
Bill
Hi again,
Even better, run an analytical plate (i.e., rectangular 2 inches wide) made by the same manufacturer, in the same vessel with the preparative plate (8x8 inches), but subject only the analytical plate to the iodine. The spots you get on the analytical plate should be a good indication as to where to scrape the prep plate.
Bill
Hi,
On the other hand, it wouldn't be hard to find out: simply develop a test prep plate and a test analytical plate in the same solvent, then, after drying, in the same jar with iodine.
If the spots correlate, great.
Bill
Thanks William, thanks Thomas, thanks Hemayet. I have considered your contributions carefully and think I will use the Iodine on the prep plates and check the results from the spectroscopic analyses. I intend to use the harvest for nmr and I am out of options.
@Hemayat: Please could you give me more details? Have you used Iodine for prep TLC? What were your results like? Please help, I am stranded here!
Probably not for prep work, because the iodine might react with the products. It would be easy to check however. If possible, use 1 prep plate for iodine location of bands, and a second for product recovery: the Rf's will match. As suggested, an analytical plate might be a different type of silica, so you might break off a piece from a prep plate to use as an analytical plate (use a glass cutter), and the Rf's will match.
Bill
1. Expose to iodine analytical plate, cut the bands, extract, and spot again to compare with the original compound. If no change, go ahead with iodine method of detecting the preparative layers.
2. If it is reacting, cut 1cm wide strip of the preparative plate from one or both edges after development - expose this strip(s) to iodine- now cut the bands putting the strip(s) in their original position. Even you can spray with harsh reagent like sulfuric acid etc on the side strips! You have not sprayed the main mid-part of the prep plate!
@Bapuji: method 1.worked. RF values are still the same. I will run IR to verify and get back to you. thanks a million!
You can use iodine ,no harm to compound. Mark the Rf Value and kept plate in air to evaporate the iodine then scrap your band on prep TLC
Sure, use iodine. It is non destructive. Will not derivatise your compounds. Expose plate to air to vapourise off the iodine.
UV lumps can be charged with two types of bulbs - with 365nm and 254nm wave-length. Some devices are equipped with both types of bulbs. Usually laboratory catalogs offer them quite expensive. I have done in another way. I have bought UV money checker with magnifying glass and two long-wavelength lamps - they are much cheaper than laboratory instruments. Then I bought a shortwave bulb and replaced in the instrument.
Two-wave instrument detects almost all.
No iodine.
No H2SO4.
No contamination.
Only pure product :-)
To avoid missing of UV-invisible substance (after column chromatography), "glass-test" is suitable. A small drop of "invisible fraction" place to polished glass - and then watch the reflection of light after drying.This funny test is really fast and sensitive.
Enjoy!
most of the underivatized steroids, triterpenoids, their glycosides, many lower terpenoids etc are not seen under UV- under both wavelengths.
@Bapuji Maringanti
You are right, this list of UV undetectable compounds need in addition of perfluorocarbons.
Iodine stain is advisable for the fluorescent compounds only
I will go with the second point suggested by "Bapuji". Normally, i cover the prep Tlc plate leaving 1 spot each on either end. Then i sprinkle iodine absorbed on silica on the two edges. After 1 min (roughly) I tilt the plate to remove the silica. The two edges serves as the reference. I scrape the middle spots against the reference (edges).
I feel that a reference piolet TLC is carried out using iodine as stainer or chromate/sulphuric acid....then from the relative Rf values one can select the region in the prep. TLC plate to separaate the desired one.
I think that the staining with iodine for preparative TLC is not usable in all cases.
Iodine may react with organics, which contain unsaturated or reactive bindings in an electrophilic additional reaction. In the other hand a shifting or rearrangement of complex organics molecules may be initiated.
For this reason we always run a second TLC plate, which we stained with iodine or other coupling developers to get the right Rf index in our preparative chromatography.
1. Comparing two different plates for preparative purpose is always risky even when we take all precautions.
2. Yes Iodine reacts with a good number of compounds- i some cases mild oxidation also takes place. Only when Iodine does not react we may use. In the case of complicated mixtures, it is difficult to say if all of them remain unreacted!
3.Cutting the edge(s) of aluminum plates and spraying/exposing to iodine is a good method. Here also, if the bands do not move in straight lines, again we land ourselves in trouble.
4. If the compounds are highly hydrophobic, a gentle spray of distilled water would yield opaque bands which need to be cut at the earliest and processed.
Iodine will add to olefins and will also oxidize low valent sulfur compounds. I would not recommend it to stain a prep TLC plate.
Hi!
Revelate just only the side part of the preparative that would not contaminate the desired product,
best regards Patrícia Rijo
In my opinion for the same solvent system, the relative Rf values for a given mixture is the same both for pilot and prep. TLC. So using the relative positions we can have an idea of separating the desired compound provided we know which band corresponds to the compound.
Iodine staining is not advisable in Prep-TLC as iodine react with the separated compounds appeared as spots/bands. The iodine vapours chemically attach to analytes on the TLC plate, thus making the samples unrecoverable for future use. However, you may use separate prep-tlc plate for iodine location of spots/bands, and a second plate of same specification for product recovery and develop both the both under same conditions. Then after matching the Rf's of separated spots/bands you can identify and scrape the desired spots/bands.
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