[DoctorDomo]

I've been struggling like mad trying to find a good mobile phase when I do a new reaction and I hate the way I waste dozens of TLC plates in the process. Firstly, I was wondering if you can reuse the same TLC plate (in my case I'm mainly using silica plates with alumina backing) when using non destructive visualization methods such as a UV chamber. I don't see why I couldn't just let the plate dry off, make note of where the previous spots are, then add new spots and rerun the TLC with the same plate. So long as the mobile phase doesn't dissolve the silica, I see no reason why this wouldn't work.

Secondly, can you recommend a good methodology for finding the right mobile phase. I've been gradually refining my methods, but I'd prefer to learn from other chemists rather than reinvent the wheel. I no longer waste time labeling every single TLC plate because until I find a proper mobile phase, the results are useless. Heres what I'm thinking. Lets say I spot my reaction mixture directly onto the TLC plate, and run it with 100% and note that the spot doesn't move at all. I could then add say a bit of isopropanol (I was gonna say acetonitrile, but I'm not sure if thats miscible with hexane) so that its now 5:95 isopropanol:hexane, then I put the same plate in the TLC chamber, let it run and see if it moves. I keep varying the ratio like that, using the same TLC and see if the spot eventually moves. Lately I've taken a liking to acetonitrile since it always moves spots that won't move, but it seems to move the less polar spots easily too, so the problem becomes separation of the spots. I tried mixing it with DCM, thinking that would slow down the more polar compounds but haven't had much luck.

In my case I'm dealing with compounds of moderate to low polarity, which have similar polarities to the starting materials, but oddly enough, the product will often stick to the baseline. A bit of acetonitrile solves that problem. 5:95 methanol:chloroform works too. I know the theory behind TLC and other chromatographic methods, and this would have me assume that products sticking to the baseline must be more polar, and require a more polar mobile phase and that the faster moving spots should be slowed down by the more polar mobile phase, but in my case, using 5% methanol or even 40:60 acetonitrile/DCM doesn't slow the starting materials down at all.

[DoctorDomo]

I answered my first question. Silica TLC plates can be reused. I just let all spots run to the top of the plate, then dry the plate, add some new spots and run the TLC again. Saves me from wasting mounds of TLC plates. As long as you don't use a mobile phase that dissolves the silica, you can reuse the plates as many times as you want. Unless you use a destructive staining technique of course.