Medium-pressure column packings

Low to medium pressure column operation, select design pressure drop of 0.40 to 0.60 in. water/ft of packing height, although some towers will operate... 

The newer LC techniques of flash column chromatography and short path or medium pressure column chromatography though offering resolution approaching that of thin layer are still comparatively slow and expensive in terms of packing and solvents. The principal advantage of these techniques is in their preparative capability. 

The term medium-pressure liquid chromatography (MPLC) covers a wide range of column diameters, different granulometry packing materials, different pressures, and a number of... 

The submitters purified the product by medium pressure liquid chromatography on a 60-cm x 5-cm column packed with 230-400 mesh silica gel 60 purchased from E. Merck. Ethyl acetate was used as eluant at a flow rate of 4 0 mL per min. Fractions (20 mL) were collected and analyzed by thin layer cHromatography. 

An early concern with the HPLC technique was the use of high pressures to achieve high flow rates of the mobile phase through a column packed with microparticulate silica. Recent improvements in column design and operating procedures, however, allow the purification of proteins at modest pressures (e.g., 500 psi) and flow rates (30-60 ml/h). Since it has been reported that C 3-alkyl chains are compatible with catalytic activity of adsorbed and eluted proteins, but larger alkyl substituents may cause denaturation (26), the use of reversed-phase columns of medium polarity, e.g., —C Hy-phenyl, when combined with a judicious choice of organic modifier and salt concentrations (e.g., isopropanol and phosphate) at pH... 

The purity of the product was determined to be 100% by medium pressure EC (silica gel packed column, eluent 5 1 n-hexane ethyl acetate). 

Flash chromatography is a quick preparation technique that is, in effect, a hybrid between medium pressure and short column chromatography. It uses a short, fat column (e.g., 1-5 cm i.d. x 45 cm) packed with silica gel and filled with solvent. Compressed air is used to compress and remove the air from the solvent which then elutes quickly. The sample is then added and the column filled again. Pressure is adjusted to achieve a separation in 5-10 minutes. It is a fast and inexpensive method for the preparative separation of mixtures requiring only moderate resolution. Use of 40-63 pm sihca gel and a pressure driven flow rate of 2.0 in/min are essential for successful separation [30]. 

Another approach to preparative liquid chromatography is to use medium pressure system (upto 300 psi, max), with shatter-proof glass columns and Merck silica gel 60 (230-400 mesh). These column can be dry packed with 100 g silica gel and will separate 5 g of solute in 2-3 hours. Solvent used does not need to be of analytical grade so long as it is distilled. 

Another means of resolution is to use a chiral material in a physical separation. Currently, many resolutions are done using medium- or high-pressure chromatography with chiral column-packing materials. Resolution by chromatography depends upon differential adsorption of the enantiomers by the chiral stationary phase. Differential adsorption occurs because of the different fit of the two enantiomers to the chiral adsorbent. Figure 2.7 shows such a separation. Topic 2.1 provides additional detail on several types of chiral stationary phases. 

Figure 11.3. General procedure for dry packing medium-pressure, preparative-scale, liquid chromatography columns. (From ref. [6] Academic Press).

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