Guide to Flash Chromatography Column (Chapter 2)
4. Determine the appropriate ratio of silica gel to the compound. For simple separation, the ratio of the two is usually 30~50:1 (weight ratio); but for the more difficult separation, the ratio is as high as 120:1.
5. Choose a suitable flash separation column. The amount of silica you need determines the size of the separation column. Whether to use a short and thick or long and thin separation column.
6. Select the appropriate test tube for collection. This is also a good opportunity to consult experienced colleagues. But there is also a simple method: divide the volume of silica gel by 4, and then select a test tube that can hold this volume. (200mL silica gel corresponds to 50mL component).
7. Once you have selected the separation column, you need to block the bottom end of the piston to avoid the loss of silica gel. Usually, it can be done with a small ball of cotton or glass wool plus a long stick or glass rod.
8. Pack the flash separation column in the fume hood. Considering the use of a large number of volatile solvents and the health hazards of dry silica gel, it is not allowed to operate the flash column outside the fume hood. Check to make sure that the flash column is completely vertical. The inclined column is not conducive to separation.
9. Close the piston and add a few inches of eluent.
10. Use the funnel to add some sand (dry and washed) to the flash separation column. The purpose is to spread a thin layer of sand (no more than 1cm) on the plug, so as to prevent silica gel from falling into the collection bottle.
11. Measure the appropriate amount of silica gel. The safest way is to measure it in a fume hood. The density of silica gel is about 0.5 g/mL, so it can be measured directly in an Erlenmeyer flask (100g=200mL). Do not let the volume of silica gel exceed 1/3 of the flask, because we have to add solvent to it.
12. Add at least 1.5 times the volume of solvent to the newly measured silica gel, make it into a slurry, vigorously shake and stir it to make it fully mixed, and remove the gas in the silica gel (the presence of bubbles will make the flash separation column, the efficiency is greatly reduced).
13. Use the powder funnel to carefully and slowly move the slurry into the flash separation column, taking care not to damage the sand layer below. Pay attention to stop and shake the slurry from time to time during the grouting process to ensure that the silica gel is evenly mixed. After grouting, rinse the flask several times with the eluent and add the remaining solvent silica gel mixture to the separation column.
14. Use a dropper and eluent to rinse the silica gel stuck on the top edge of the flash column into the solvent layer.
15. When all the silica gel has been washed away from the flash column wall, open the piston and pressurize the column with compressed air. The silica gel in the column will be compressed to about half of its original height. Check to make sure that the top section of the column is flat. If it is not, it must be stirred again and then settled down. Under pressure, add excess eluent and tap the column gently with a pencil tip or rubber stopper. This will make the silica particles packed more tightly.
Collect all the eluate from the flash column and reuse it after adding the compound.
Note: Remember not to let the solvent level be lower than the filling layer.