I have fielded many questions about chromatography and one I get is just how to calculate the depth of the column to use. This tip is unorthodox and likely not in chemistry texts but it has suited me very well. Once done you really do not need to do it again.
DCVC works by ramping up the polarity of the mobile phase in a continuous gradient. Each addition to the mobile phase is made more polar and so as it washes the compound through the stationary phase then each addition of say 20 ml of solvent is vacuumed through the gel until the gel is dryish. So the question is just how far is the optimal distance for a DCVC column. Here is how you nail it down for a compound you will be running.
Pack a DCVC column extra high like 7 inches or maybe a bit higher. Then run your DCVC. By the third fraction the column will be saturated and running but you will see that the bands of color that begin to seoarate into clearly defined bands of color will begin to diffuse into the excess of silica gel and solvent. Once this happens measure the distance between the top of the silica gel and the bottom of the first band of color which should be above the as yet unused silica in the column.
The next time you run DCVC then pack your column the measured height plus a little to fudge factor this ballpark method. Distance is key and more distance does not mean better seperations with DCVC. Once you have distance nailed then it becomes a lot of fun changing up solvent ratios to see how different things separate. For more compound you need a wider column but never a longer one. In my columns the tightest seperation happens at about 3 ½ inches of silica gel 60 running a solvent system of hexane and ethyl acetate. The ethyl acetate is added in 5% increments. I have to date only ran standard phase DCVC but I suspect a reverse phase column would work just fine.
This photo below shows a DCVC run with a slighly taller than needed silica gel column because I am limited by the glassware. Finding a perfect sized column is tough but a little bigger works too. This example shows the Celite 545 on top of the column that held the compound for the run. Dry loading is hands down a better method for the nicest separations but wet loading to accomodate more in one load does work too.
