The iodine stain is by far the easiest stain to prepare and use. The classic way is to simply have a chamber with some iodine crystals that you put your TLC plate into. The iodine vapours will react with many functional groups but is particularly good with unsaturated compounds (e.g. alkenes and alkynes) to give reddish brown spots (see TLC plate below). Another way to prepare the iodine stain is to mix silica gel with iodine crystals (see pic to the right). Stick the TLC plate into the red silica and wait a few minutes to develop your plate. This approach is a bit gentler and in my experience gives a better result. If you want to keep your TLC result for later remember to take a picture of your plate or circle the spots as they fade relatively fast. D!
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thanks for the informative post. For recording TLC data, this (http://medchemblog.blogspot.com/2010/01/simple-method-of-recording-tlc-data.html) method can be a good option.
@Med Chem: I checked your link but I am not a fan of this method. In fact I specifically tell my students not to do this. TLC silica, the stuff we spot on the TLCs and the stains we use are toxic. Don't put it in your lab book regardless of how you encapsulate it. In addition spots will fade and/or change colour over time. Plates will eventually decompose after 5-10 years making them completely useless. And finally, if you are very thorough and record all your TLC data like this it will make your lab book "explode" because the TLC plates are thick.
I agree that drawing is also not ideal. It's difficult to make an accurate copy of the plate like this. The easiest solution in my opinion is to photocopy the plate, cut it out and glue/tape it in your lab book. Alternatively, use your mobile and take a picture. This especially makes sense with e-lab books becoming more and more common. D!
We use a UV box with built in camera, takes photos under UV or visible light, ideal for eln.
I used to use 2.5x8 glass plates in wide mouth jars as developing chambers. I used one of the chambers with iodine crystals and silica gel as you have. What I liked about this was after pushing the plates into the silica gel several times, it became compacted. Just turning the jar end for end loosened the silica gel again.
I was lazy and this was sort of pre-digital cameras, I did draw a representation of my plate in my notebook. My glass plate was a virtual ruler and a modest size so it didn't take up too much writing space. This was very fast and while not as good as a picture, better than nothing. Because it was lifesize, I liked it better than what others had done. I did tape some things into my notebook, but for heavy taping, it made those notebooks twice as wide in their open edge than bound edge. I didn't like that either. A problem I had was lack of documentation so I felt the smaller the barrier to recording, the greater the chance of some record. Drawing it directly will be easy to do.
A good desktop scanner is not much more than $100. Put one in the lab somewhere and record your plates as they actually were, in full color and with the original annotations. All other methods are more cumbersome, more expensive, or invite data loss.
Okay, maybe I'm missing something, but it looks like sharpie on that TLC plate. I've always used pencil and was told never to use ink because it can run with the solvent. Full disclosure: I have done TLC maybe 5 times as I'm just starting grad school. Thank you!
A blunt pencil was used. D!
FYI -- in many cases, the iodine does not really "react" with the samples on the plate, but rather is dissolving into them. Since it has no permanent dipole, it prefers nonpolar media over polar surfaces. The increased local concentration allows you to see the iodine, but it appears brown within the silica gel instead of purple, just as it appears brown on the glass wool or other filler. That's why the stain fades quickly -- it's not the reverse of a chemical reaction, just the iodine returning to vapor by exiting its state of dissolution within the organic substance(s) on the plate.
@anonymous: Yes! You are absolutely 100% right. I regret how I have written the post. The colouration seen is mainly due to iodine dissolving into the organic matter which is the explanation for the colour fading again. Also this is the reason why compound that should be inert to iodine get colour. I think my point is that I believe it is particularly well suited for compounds with functional groups that have the possibility to react with iodine. I find it very useful for alkenes and alkynes for example. The TLC plate in the post is a separation of different alkynes. D!
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