Development of Flash Column Method
The flash column method is an alternative to long column chromatography developed in 1978.
The method aims to separate the components from the mixture, thereby purifying it. It is established from the existing long-column chromatography technology, which is time-consuming and often unsatisfactory. In short, the flash column method allows the sample to pass through a column filled with gel, and the gel separates the sample.
Still, the founder of the flash column and his colleagues have been using medium pressure chromatography and short column chromatography to replace long column chromatography. They decided to combine the two to overcome the shortcomings of long column chromatography, which is time-consuming and low recovery.
The gel originally used to pack the column was silica gel, which is still widely used. They used air pressure to push the solvent through the silica column, compressing the column. The sample is then applied and the same (can be different, but usually the same) solvent is used to pass the sample through the column. Then collect the purified components or fractions, the whole process takes about 5-10 minutes. Usually, a small part appears first and most eluted last. The collected fractions were subjected to original analysis by thin layer chromatography (TLC) plates.
Since its introduction, the silica gel flash column has been widely used in organic chemistry. However, guidelines are often loose, driven by personal experience, or not translated well when settings are changed. However, some things are still true. Increasing the sample size will cause the resolution to decrease.
Compared with the HPLC column, the resolution of the flash column is already at an intermediate level, but it is sufficient for sufficient separation, so the increase in the number will only worsen the situation. Second, the optimal flow rate depends on the length and width of the column and the nature of the gel.
This is due to the number of plates available, for example, longer and narrower columns will provide more theoretical plates, thereby affecting the flow rate. Finally, the resolution is affected by the stationary phase. If the stationary phase or gel arranged on the column is more uniform and has a smaller particle size, it provides better resolution. The smaller the particle size, the larger the surface area, and the higher the resolution.
Manipulating all these factors to optimize the purity or recovery of the components can be quite complicated because they interact, but they have different effects when tested independently. For example, the mobile phase selectivity has the greatest impact on the resolution, but it depends on the column capacity. Then, this will be affected by the chosen solvent.
Therefore, before starting any real experiment, you need to test or calculate the settings used in the flash column. If you have data from TLC, you can calculate the best settings without testing any flash column settings. Low TLC delay factor (Rf) provides better separation. Using such data, the amount of solvent required can be calculated because they are inversely proportional to each other, and both are related to the column void volume (the solvent in the column before the sample is loaded).
In the original method, it was very important to prevent the column from drying out. However, a more user-friendly alternative has since been developed, called dry column flash chromatography. This method has been adopted by first-time students, while still producing good results comparable to the quality of TLC analysis.
The principle is the same, but the column contains dry silica gel. The powdered xerogel is filled into the column by suction, and finally, a uniform bed of about 1 cm for solvent and sample is obtained. The column was also eluted by suction and dried after each fraction.