About The Advantages, Components, Operation And Safety Of Flash Column
In general, the column chromatography process is usually accompanied by lateral diffusion. This diffusion will cause a large decrease in resolution. Therefore, when the column is taken, the column chromatography time should be shortened as much as possible, especially to avoid column overnight. The flash column is to reduce this lateral diffusion with the fastest possible flow rate without sacrificing exchange efficiency (adsorption and desorption).
Advantages of flash column
Fast column chromatography has at least the following advantages:
1. Time is greatly saved, from packing to completion (excluding concentration), generally only 30 minutes (the whole preparative thin layer chromatography takes about 4 hours);
2. The compound has a short residence time on the column and a reduced possibility of chemical change, which is suitable for the separation of unstable compounds;
3. It is suitable for separation of 20-100mg samples
4. It is also suitable for separating large quantities of samples and can load 10-25g at a time. Stainless steel cylinders are even used in the factory and can be loaded 100-200g at a time. For example, if 10g of crude product is separated, a glass column with a diameter of 8.0 cm can be used, and the height of the silica gel is only 16 cm, that is, the filling height: diameter, only 2:1. the rest will contain eluent, the amount of eluent is roughly the same as that of conventional column chromatography.
The fast column consists of a “chunky” pressure-resistant glass column, an air-conditioning valve, a compressed air source, and a set of tubes that can move freely. It is equipped with 4 columns with diameters of 2.5, 4.5, 6.0, 10 cm and heights of 30, 45, 60 and 80 cm respectively, which can meet the requirements of general experiments and can separate 20mg-20g crude products.
Compressed air can be supplied by a low-pressure air compressor, plus a tank for buffering air pressure and smooth airflow. The pressure can be controlled by a regulating valve. This control valve is fully closed, always allowing some of the air to escape and adjust to ensure the safety of the entire system.
Since the eluent flows down continuously, the automatic fraction collector is no longer suitable, and a tube assembled on the test tube can be pushed by hand to collect the fraction. Usually, it takes a few dozen seconds to collect a test tube.
Choice of eluent
The whole process is not much different from conventional column chromatography. The first is the choice of an eluent. The solvent system obtained by the TLC test can be directly applied to the rapid system, and the Rf value of the main product is preferably 0.2-0.3.
Packing of the column
The second is the packing of the column. The quality of the packing has a great influence on the separation effect. The ideal column has a white sintered glass piece on the bottom where the silicon limb can be directly added. It can also be blocked with a small amount of cotton at the bottom exit, and evenly pour a 1 cm thick sea sand to create a flat layer, and then fill the required amount of silica gel. The filling height is generally 1.5-4 times the diameter of the column.
Then, a 1 cm thick sea sand layer was evenly poured on the top of the silica gel. After loading, you can wash the column. Slowly add the prepared eluent along the column wall and try not to impulsive sea sand as much as possible. The air is then quickly removed by pressure. The liquid left is only about 1mm above the sea sand layer, and then it can be the same. The sample can be placed in a small amount of organic solvent, or it can be mixed into silica gel and added to the column, followed by elution.
Carefully add a small amount of eluent along the column wall and pressurize it to enter the sea sand layer and stop. The formal elution can be performed after repeating the operation once. The speed varies depending on the diameter of the column, but it always flows out like water instead of a drop.
The success of flash chromatography depends primarily on the selection of the appropriate eluent and the correct packing. If the first separated product is still mixed with other components, it can be separated after the concentration and recovery of the eluate. The time of the second total is still far less than the time of conventional column chromatography.
Especially in the initial test of this method, be sure to wear protective glasses or protective masks. As long as you do not use excessive pressure and do not use the damaged glass column, there is no accident because the column resistance is small. Some experimenters encountered a situation where the isolate was precipitated in the column due to stepwise purification in the column to block the column. However, no accident occurred because excess compressed air was automatically adjusted to escape from the valve.