[Babcock_Hall]

In the 1978 paper on flash chromatography the authors wrote, "Slower flows clearly give poorer resolution with ethyl acetate/petroleum ether mixtures."  This seems counterintuitive in that I usually associate slower flow rates with better resolution in liquid chromatography.  I did find a set of guidelines on classical chromatography that stated "For every column there is an optimal flow rate. Adjust the stopcock to control the flow. If the flow rate is too slow, diffusion processes will lead to band widening (Fig. 1, a). If it is too fast, there is not enough time for equilibration and the compound will be forced down the column, leaving a long tail behind (Fig. 1, c)."

I find both statements a little counterintuitive, especially the former one.  If slower flows give poorer resolution, it suggests to me that diffusion is the culprit.  How can diffusion be a serious issue in flash chromatography?

[phth]

It could be due to particle size creating a flattening of the van deemter plot.  At slow flow rates diffusion up the column is possible leading to band broadening.  I have also heard that Et₂O is a poor solvent choice, and that also part of the C term .  The C term is the part of the function that give it the upward slope.

[kriggy]

Im not sure but we were told that Van Deemter eq. is used only in GC. LC / HPLC (thus I could say also a regular column) are run at maximal veocity because the parameter Cu doesnt change that much with increased flow

[BobfromNC]

If the flash is run slowly, the molecules can diffuse via Brownian motion to make the bands mix (blur/flatten/broaden, whichever word sounds best).  If you go slow enough, that can undo the separation, but is not usually a problem unless you go really slowly.   I think the main issue is that with smaller particle sizes, the flow is very slow without some pressure, plus you tend to get more voids in the column without some pressure which is bad for the separation as well.  That is why Waters used to put radial pressure on their older columns in their HPLCs, but now stainless steel and higher pressures have made that unnecessary.    But using an ISCO for example, you can see voids in the column forming if the flow is too low, at least with some solvents (CHCl3 shows it well).