[synthon]

I'm once again curious on a broad topic, but with a specific application.  I need to separate two compounds with very similar retention times (one which absorbs UV, one very weakly colored - ie, impossible to see until fractions concentrated)  Eluting with 2% MeOH in CH2Cl2, by flash chromatography gave the products as a mixture across 5 mL fractions, #123-150 (UV active product slightly ahead of weakly colored impurity). 

My first thought is to increase column length, decrease MeOH to 1% and not use an accelerated flow rate, but that will take forever and I'm impatient.  I suppose I could also try a different solvent system, but without being able to visualize the impurity, it will be hard to guage the likely success of any variation.  So I considered pHPLC, and prep TLC, (neither of which will see the impurity) and now I'm wondering about the relative resolution of these (and other) common chromatography methods.  Since I didn't find a comprehensive list using google or the search function, does anyone have any additions or edits to this short list?


pHPLC >> column chrom. > reverse phase column > flash chromatography > pTLC

[kriggy]

What is your compound? Maybe ion exchange chromatography? Or you can try visualize it by iodine: get some vessel (big enough so you can put your TLC plate into it) and fill it with some silica - about 2-3 cm height of the vesse - and drop few pieces of iodine into it. Then close it, shake it and wait until the iodine vapours fill the whole vessel (about 10 minutes should be enough). Then do your TLC and put the plate into the vessel and shake it again and keep it there for maybe 5 minutes. Then it will visualize your spots just like UV but it works with non-absorbing compounds (the plate will turn yellow and the spots will be still white most likely).

[synthon]

I've tried iodine and KMnO4 stain, and the impurity is either not visible or unresolved on TLC, at varying amounts of MeOH.  It sounds like a new solvent system is your recommendation?  I'll try adding toluene as has been recommended in other threads, since the product has an aryl system and the impurity doesn't.

Update---toluene FTW it looks like.  I added ~20% toluene to my solvent system and got the product to move @ Rf~0.4 compared to an Rf of ~0.1 for the impurity (which is apparently UV active, just incredibly similar Rf in previous eluent).  TLC attached if interested.  I've never had two spots overlap so perfectly with an Rf around 0.2, so it's great to see them this far apart!  Column #3 coming up...

[kriggy]

congrats  This TLC looks very nice. I never get TLCs like this when it matter