Advantages of fast column chromatography with dry method

Fast dry column chromatography combines the speed and separation efficiency of fast column chromatography. It USES cheaper TLC-grade silica gel, is easy to operate, and requires no special instruments. The risk of explosion of glass column during pressure elution is avoided by dry column rapid chromatography with decompression elution. In addition, the column is eluted with a predetermined amount of solvent, which is drained until eluent is added. These characteristics make dry column chromatography especially suitable for gradient elution. In fact, this is the preferred way to develop such columns. As the name implies, gradient elution USES solvent elution that increases the polarity gradually. At the beginning, the non-polar elution system is used to remove the weak polar components, and then the polar elution is increased to separate the polar compounds, which can save a lot of time without losing the good separation effect. 

Don't forget, if you allow air into the column, this is the only instance. In fact, compared to all the classical chromatographic principles, the whole process is like flying, and it works at least as well as standard techniques, at a much lower cost. It is particularly useful for separating large samples of up to 50 grams. Inexperienced people should not try this kind of post. This technique, used by this author, has been in common use in many LABS for a long time.

 1. The device

The equipment required is simple: a cylindrical porous funnel for decompression pumping is connected to a round-bottomed flask through a tapered socket transfer head and another socket on the side is connected to a water pump (Figure 1). The size of the sample to be purified determines the size of the funnel used and the volume of the eluent received. See Table 1 for specific recommendations.

If you master this method, a sand core glass funnel and some 50 ml or a 100 ml glass bottle is the same, the author used this method a pillar for a long time, faster than combi flash, but the method of solvent polarity sensitive demand is higher, you have to be very familiar with the polarity of your sample, the solvent elution effect also have to be very familiar with, can make good use of this method.

 

2. Selection of elution system 

Generally no matter what, first with the minimum polar solvent rinse one or two bottles of solvent, the selected elution system should be TLC target product Rf value of 0.5 or so solvent. Although there is no solvent that is particularly unsuitable for this technique, the various combinations of hexane, ether, ethyl acetate, and methanol are sufficient for most separations. Because of decompression conditions, some of the collected solvents will evaporate and may cool the liquid receiving unit, so that atmospheric water will condense on the unit. This does not affect the separation effect, but if the column length is too long, some of the water will still get 

into the collected eluent. Replacing petroleum ethers with weak volatile heptane may help.

 

3. Packing column 

This column USES silica gel grade TLC without gypsum adhesive. This is cheaper than the silica gel used for rapid column chromatography and is sufficiently active for a variety of dry columns. For each size of funnel, the recommended amount of silica gel is sufficient to leave sufficient space at the top to add eluent after column loading and pumping. Silica gel can be weighed according to the amount recommended in the table, but more simply, fill the funnel with silica gel directly, and the space left at the top can be used to add solvent after the silica gel is fully pumped. During compaction, there is a tendency, especially in large columns, for the silica gel to detach from the edges of the funnel or to form cracks that may not be visible on the column. For good column loading, grind and compacted the surface layer, especially the edge, with a glass plug. Do not worry about the condition of the column surface, with the scraper around the funnel repeated tapping is easy to smooth. When the column is fully packed, the column is preeluted with a small polar solvent in the elution system. If the column is properly mounted, the solvent front will appear to be a straight horizontal line going down. Keep the silicone surface covered with solvent until the solvent enters the receiving bottle, and then allow the silicone to be drained. Remember to check for any abnormalities behind and in front of the column. If the solvent front is not horizontal, drain the solvent, recompact it, and preelute it again. Never use a poorly mounted column for separation. A compacted silicone surface is strong enough to add solvent, so no protective sand layer is needed.

4. Sampling and elution

The sample is dissolved with a minimal amount of preeluting solvent, and is evenly added to the silicone surface while the column is being pumped. Rinse the sample solution bottle and add the lotion to the column until all samples have been transferred. If the sample does not dissolve well in a preeluting solvent, dissolve it quickly in a combination eluent with the least polarity. Using the sample solvent, the gradient elution is initiated by following the recommended volume of the solution in the table (smaller for those that are difficult to separate). Allow the column to drain, transfer the first part of the infusion to a test tube or other convenient container, and clean the flask and funnel handle. When the column is drained, prepare the next elution solution, adding 5% polarity. Repeat the elution procedure. This gradient elution continues until eluted with a pure large polar solvent. Continue this wash if necessary. When the target product is eluted from the column, it is advantageous to stop the gradient elution and elute several more parts with the same proportion of solvent. During the separation process, the receiving fluid should be analyzed using TLC. As a general guideline, the elution time of the target product from the column is 0.5 Rf value of TLC of the target product in the eluting solvent. When more than 100 mg of sample is purified and the target product is eluted from the column, the lower part of the funnel foams. If the product is solid, it may crystallize on the funnel stem or on the receiving bottle, especially if large amounts of samples are separated. Be sure to thoroughly clean the receiver and funnel handle between the two receivers, and check that the solids do not block elution. Typically, a low level of product lateral diffusion bands often means that the eluting components of the pure compound are relatively small, reducing the number of cross-contamination shares. If there is no

polymer in the rough sample, the sample will be easily recycled.

 

5. Silicone treatment 

After elution, the silica gel is drained and transferred to the silica gel residual cabinet. Usually flip the column and knock, and all the silicone comes out. Be careful not to get silica gel dust into the lab air.