I did something in the lab the other day that I hadn’t done in several years: run some preparative TLC plates. I had some small reactions that needed to be cleaned up, and the HPLC systems were all in use, so I thought “Why not?” (I wrote here about the decline of analytical TLC in general in some labs, and I think it's fair to say that the larger-scale prep version has seen an even steeper drop in use over the years).
Prep TLC, for those of you not in the business, is a pretty simple technique. You take a square glass plate that’s been coated with a dry layer of ground silica, a white slurry that for this application is about the grittiness of flour or ground sugar. You then take your mixture of gunk, dissolve it up in a small volume of solvent, and deposit it in a line across the bottom of the plate, an inch or so up from one side and parallel to it. Then you take a large glass container and add some solvent to the bottom of it, and put your plate in so that the streaked line of material is near the bottom. Here's one running.
The solvent soaks into the layer of silica, and after it gets up an inch or so it hits your line of stuff. As it continues to move up, soaking further and further up the glass plate, the different components of the mixture will be carried along at different rates. The compounds that stick to silica gel (for one reason or another) will lag behind, while the ones that don’t will move out into the lead. After an hour or so, the solvent line will be up near the top of the plate, and your mixture will now be spread out across it into a series of bands. (The TLC page at Wikipedia has some useful images of this). Up at the top, running with the solvent, will the the nonpolar stuff that didn’t have anything to slow it down. Right down near the bottom, not far up from your original streak, will be the most polar stuff, especially any basic amines – silica gel is mildly acidic, so the amines will stick to it very tightly indeed. And in between will be the other components, divided out according to how they balanced out the pull of the silica gel support with the attraction of the solvent moving them along. Sometimes you can see them as colored bands on the silica plate, but more often you shine a UV light on the whole plate to see them. The silica we use has an ingredient that makes it fluoresce green under ultraviolet, and our compounds usually show up as dark blue or purple bands against the green. It’s a color combination known to every working synthetic organic chemist.
You can see that picking different solvents for this process can change things a great deal. A weak solvent (like hexane) will allow almost everything to stick to the silica. (A compound has to be mighty greasy to be swept along by just hexane; I doubt if there’s a drug in the business that you’d be able to clean up that way). A standard mix is some proportion of ethyl acetate mixed with hexane. You can go up to straight ethyl acetate, or even further by mixing in methanol or the like. And if you’re desperate, you can go to most any solvent mixture you like – three-solvent brews, toluene, acetonitrile, acetone, whatever works.
So how do you get the things off? By the lowest-tech method you can imagine. You mark the position of the band (or bands) you want, and then take a metal spatula and scrape the silica there off the plate. You them dump that into a flask and stir it with a strong solvent, then filter off the silica and wash it some more to rinse your compound out.
This used to be much more of an everyday technique, but automated column chromatography (same principle, pumped through a tube) has taken over. But prep TLC still has its appeal. Done with skill, it can provide very clean compounds, with quite good recovery. In fact, its low cost and power have made it a favorite technique at places like WuXi, the outsourcing powerhouse in China. I've had several first-hand descriptions of their prep TLC room, with rows of plates being run, marked, and scraped in assembly-line fashion. It's the sort of thing you'd only do in a cheap-labor market, because of the unavoidable hand work involved, but it is effective.
I don't know where WuXi gets its plates, but if you make your own, it's an even cheaper technique (discounting labor costs, naturally). You take up the silica gel powder in water, make a thick, well-mixed slurry out of it, and spread it across a square of glass, shaking and tapping it to get the air bubbles out. Back when I was doing summer undergraduate work, I poured a number of these things, although it's certainly nothing I've had experience with since the first Reagan administration. For all I know, that's how WuXi does it now. Perhaps they've found a low-cost supplier of their own, but the idea of a cheap supplier for a Chinese outsourcing company is an interesting one all by itself. . .