Guide to Flash Chromatography Column (Chapter 1)
Flash chromatography column (Flash Column) is a fast and (usually) easy method to separate complex mixtures. In some experiments, we need to perform a relatively large amount of flash chromatography column to separate about 1g of material.
The principle of chromatography column and thin layer chromatography is the same, but it can be used for the separation of preparative substances. Because we use compressed air to push the solvent through the column, it is called flash chromatography column. This not only makes the separation effect better but also shortens the column time.
Preparation and operation of flash chromatography column:
1. Determine the weight of the dry, solvent-free mixture be separated.
2. Use thin layer chromatography to select the solvent system so that the value of Rf is between 0.2 and 0.3, but if the mixture is complex, this may be unrealistic. In more complicated cases, you may need to use gradient elution. Simply put, it is to continuously increase the polarity of the solvent during the purification and elution process. However, in thin layer chromatography, you must determine which solvent system Will make the different spots in the range of Rf 0.2 ~ 0.3.
3. Determine the method used to load the sample onto the column. You have three choices: clean sample method, solution method, or silica gel adsorption method.
a.Net sample method: If the sample is a non-viscous oil, it is easiest to use the net sample method. You can use a long dropper filter to introduce the liquid into the column, and then rinse with a predetermined solvent system to wash all the components into the chromatography column.
b.Solution method: The clean sample method may sometimes cause the separation column to break. Therefore, for liquids and solids, the more common method is to dissolve the sample in a solvent and then add the solution to the separation column.
The most ideal state is that the Rf of all components in the mixture in the solvent system (usually pentane or hexane) is zero. This is difficult to achieve in most cases, so you can choose a solvent that moves only one compound in the mixture, or you can simply use the eluent of your choice. Remember: the latter two options are risky for difficult separation and purification.
c.Silica gel adsorption method: The last technique is to deposit (adsorb) compounds onto silica gel, which is useful for some liquids and all solids.
Note: Silica gel is acidic, so this step will destroy some acid-sensitive compounds, which usually need to be regenerated on the silica column.
First, dissolve the mixture in methylene chloride in a round bottom flask and add silica gel (the mass of silica gel is about twice the mass of the compound). The solution was concentrated on a rotary evaporator.
Note: Silica gel is a very fine powder and can easily be sucked into the rotary evaporator.
Plug the connector or pump protection device with glass wool to prevent solids from being sucked into the pump. Fast turning can also avoid this problem. When the solids are basically dry (when most of the solids fall off the wall of the container, the solids are already dry), remove the flask from the rotary evaporator, and then use a vacuum pump to exhaust the solvent (assuming there are no volatile substances in the mixture).
Note: Use glass wool to plug the vacuum pump connector, otherwise you may find silica gel (and your compound) enter the vacuum tube and deposit there. Once it is completely dry (no more bubbles in the solid), remove the flask from the vacuum system and scrape the solid from the wall with a clean spatula. Now, you can simply use the powder funnel to add this solid to the top of the separation column, and then rinse with the eluent (1.5mL each time).