Introduction to Flash Column Chromatography
Flash column chromatography is a fast and usually easy method to separate complex mixtures. The principle of flash column chromatography 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 column chromatography. This not only makes the separation effect better but also shortens the column passing time.
Development of Flash Column
Flash chromatography is a kind of fast chromatographic separation mode, which separates substances by using optimized pre-packaged medium and low-pressure columns. Flash chromatography, which was developed in 1978, is considered to separate by medium and low pressure. And it is a fast and inexpensive technology compared with traditional chromatography, which is widely used in drug research, sample purification, and purification of natural products currently. The characteristics of low-cost and simplicity result in its irreplaceable in the separation and purification.
The most used common filler for flash columns is silica gel. But why silica gel is chosen to be the filler? One of the reasons is the limited choice of fillers using for the chromatographic columns at that time. And another reason is other bonds, such as C8 and C18, are too expensive.
As a consequence, most methods of flash chromatographic application in the literature are developed using silica gel as the separation substrate. Flash chromatography usually is a normal-phase liquid chromatography technology. The size of the sample is usually in the milligram level to a hundred-gram. The flow rate is 10 mL/min to 300 mL/min. And organic solvents with low polarities, such as hexane and ethyl acetate, are used as the mobile phases.
Preparation and operation of flash column chromatography
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 Rf value is between 0.2 and 0.3, but if the mixture is complex, this may not be realistic. In more complicated situations, gradient elution may be required.
Simply put, the polarity of the solvent is continuously increased during purification and elution. This technique is described in more detail later. However, in TLC analysis, you must determine which solvent system will make the different spots within the range of 0.2 to 0.3.
3) Determine the method used to apply the sample to the column. You have three choices: net sample method, solution method, or silica gel adsorption method.
a. Net sample method:
If the sample is a non-sticky oil, it is easiest to use the net sample method. You can use a long dropper filter to introduce liquid into the column and then rinse with a predetermined solvent system to wash all the components into the 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 all components of the mixture have an Rf of 0 in the solvent system (usually pentane or hexane).
This is difficult to achieve in most cases, so choose a solvent that only moves one compound in the mixture, or you can simply use the eluent of your choice. Remember: the latter two options are risky for more difficult separations and purifications.
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. They usually need to be regenerated on the silica gel column.
First, dissolve the mixture in dichloromethane 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 be easily drawn into the rotary evaporator.
Use glass wool to plug the connector or the pump’s protective device to prevent solids from being drawn into the pump. Quick rotation can also avoid this problem. When the solids are substantially dry (when most solids fall off the wall of the container, indicating that the solids have dried), remove the flask from the rotary evaporator and use a vacuum pump to exhaust the solvent (assuming there are no volatile substances in the mixture).
Note: Plug the vacuum pump connector with glass wool, 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 off the wall with a clean spatula. Now, you can simply use a powder funnel to add this part of the solid to the top of the separation column, and then rinse with eluent (1.5mL each time).
Hawach provides Spherical C8 Flash Columns, Spherical C18 Flash Columns, Spherical SCX Flash Column, Spherical Phenyl Flash Column, and other flash columns for your choice.