Mixture is eluted too early from alumina in flash column chromatography

I have recently tried to purify my reaction mixture by column chromatography. Because my reaction mixture had poor resolution in silica-based TLC, I used neutral alumina-based TLC plate. After I determined the elution condition, I purchased the neutral alumina at Sigma Aldrich, and its particle size is 40~163 μm. Initially I tried 2 cm i.d. column and almost ~20 cm of packing with ~0.15g of crude sample. But yield was very poor.


I was advised to use smaller i.d. column and less amount of packing, so I tried using 1 cm i.d. column and 10 cm of packing with ~0.35g of crude sample. Because my major product is liquid at room temperature and I was advised that loaded sample should have ≤1 cm of height in column, I only used 200 μL of dichloromethane to dissolve the sample. I carefully loaded the sample above the top of the alumina, let it be absorbed into alumina, and added some sand. Additionally I washed the sample vial and wall of the column with a few drops of dichloromethane, and let it be absorbed into alumina too. I added fresh dichloromethane, and started elution. To carry out flash column chromatography, I applied Ar gas upon sample loading. But a few seconds later, the mixture was eluted too early without separation with very vivid UV absorption when I spotted it to the TLC plate, although the starting material moves to near the solvent front and the product hardly moves from the starting line when I checked the TLC with 100% dichloromethane. Because there was a little amount of starting material in the sample, the product was only eluted as time goes by.


But I don't understand why the mixture was eluted instantly. What was the problem? Some people suggested:


* Don't apply the pressure immediately. Try to apply the pressure after
you observe the separation of the mixture.
* Other people said that they don't think that applying the pressure upon the sample loading could be the problem. Instead, the smaller the i.d. of the column, the smaller should the particle size of the alumina. But '40~163 μm' of particle size is too broad distribition, so it might not hold the sample effectively. If you can't buy more smaller-size alumina, try again with 2 cm i.d. column to increase the amount of alumina while maintaining same height of packing.


Can these two be the reason of pre-elution? If not, what was the problem?


