Adsorbents for columns

Alumina and silica gel are the most commonly used adsorbents for column chromatography, just as they are for TLC. The quality of these adsorbents is high in that they have uniform particle size and high specific area. The higher the specific area, the faster the equilibrium of the solute between the mobile and solid phases is established and the narrower the bands. High specific areas on the order of several hundred m /g are common for good grades of alumina and silica gel. 

Fig. 3 Comparison of the structural differences between adsorbents for column and paper chromatography (above) and those for TLC (below). (Stahl [674])...

Adsorption. Adsorption (qv) is an effective means of lowering the concentration of dissolved organics in effluent. Activated carbon is the most widely used and effective adsorbent for dyes (4) and, it has been extensively studied in the waste treatment of the different classes of dyes, ie, acid, direct, basic, reactive, disperse, etc (5—22). Commercial activated carbon can be prepared from lignite and bituminous coal, wood, pulp mill residue, coconut shell, and blood and have a surface area ranging from 500—1400 m /g (23). The feasibiUty of adsorption on carbon for the removal of dissolved organic pollutants has been demonstrated by adsorption isotherms (24) (see Carbon, activated carbon). Several pilot-plant and commercial-scale systems using activated carbon adsorption columns have been developed (25—27). 

Column Chromatography. The substances to be purified are usually placed on the top of the column and the solvent is run down the column. Fractions are collected and checked for compounds using TLC (UV and/or other means of visualisation). The adsorbent for chromatography can be packed dry and solvents to be used for chromatography are used to equilibrate the adsorbent by flushing the column several times until equilibration is achieved. Alternatively, the column containing the adsorbent is packed wet (slurry method) and pressure is applied at the top of the column until the column is well packed (i.e. the adsorbent is settled). 

Phosphoproteins (various). Purified by adsorbing onto an iminodiacetic acid substituted agarose column to which was bound ferric ions. This chelate complex acted as a selective immobilised metal affinity adsorbent for phosphoproteins. [Muszyfiska et al. Biochemistry 25 6850 1986.]... 

The submitters mixed active anhydrous silica gel with water (12% v>/w) and stored it in a sealed container for at least 24 hours prior to use. A ratio of 60-80 g. of silica gel per gram of crude product was used for column chromatographic separations, and a column was chosen that would give a 10 1 height diameter ratio of adsorbent. Columns were wet-packed with distilled petroleum ether (b.p. 60-68c), and after the crude product had been applied a step-gradient was run rapidly through 2% vjv ether in petroleum ether, 5% ether, and 10% ether. The column was then eluted with 20% vjv ether in petroleum ether until the bromohydrin acetate was obtained. 

Figure 6.29. The matrix element 4d expresses how the d band of the metal couples with the s or p level of the atomic adsorbate, for the three transition metal series. Note how the matrix element increases when movingto the left. It also increases when moving down a column, due to the larger geometrical extent of...

In place of silica gel, Florisil is also used as the adsorbent in column chromatography. Purification of chlornitrofen using a Florisil column is as follows after installing a column packed with 10 g of Florisil suspended in n-hexane, the sample solution is added continuously to the column and the initial eluate is discarded. A 100-mL volume of diethyl ether-n-hexane (1 19, v/v) is charged to the Horisil column and the eluate is discarded. Chlornitrofen is eluted with 30 mL of this mixture and the eluate is concentrated to dryness before the addition of acetone for GC analysis. ... 

Since these liquid phases on the adsorbant are, eventually, liquids, you can boil them. And that s why there are temperature limits for columns. It is not the best to heat a column past the recommended temperature, boiling the liquid phase right off the adsorbant and right out of the instrument. 

All such processes suffer one disadvantage in that the capacity of the adsorbent for the adsorbate in question is limited. The adsorbent has to be removed at intervals from the process and regenerated, that is, restored to its original condition. For this reason, the adsorption unit was considered in early industrial applications to be more difficult to integrate with a continuous process than, say, a distillation column. Furthermore, it was difficult to manufacture adsorbents which had identical adsorptive properties from batch to batch. The design of a commercial adsorber and its operation had to be sufficiently flexible to cope with such variations. 

Figure 19.9. Chromatogram for stepwise elution of bovine serum albumin on a Vistec diethyl aminoethyl cellulose ion-exchanger, using stepwise increases in sodium chloride concentration in the mobile phase to achieve selective desorption. Proteins 1, serum fraction not adsorbed by column (includes y-gobuhn) 2,3,...

Slowly put sand over the cotton until you have at least one centimeter. Next, very slowly add the adsorbent (alumina, silica). Adsorbents liberate heat, possibly causing the eluent to boil, ruining the column. Add the adsorbent slowly. Use about 25 g of adsorbent for every 1 g of mixture you want to separate. When the alumina settles, add another 1 cm of sand to the top. During the entire procedure the level of the eluent must be higher than any solid material placed in the column,... 

In a chromatographic column a tube is filled with a powder of a solid that acts as an adsorbent for the molecules to be identified. As shown in Figure 12-19, a pulse of the gas mixture is injected into the tube in a flowing stream of an inert carrier gas (which does not adsorb on the solid) such as He or H2, and the products leaving the column are detected using a thermal conductivity or flame ionization detector and recorded as a signal versus time in the chromatogram. Each species is identified by its residence time on a particular column, and the amount of that species is proportional to the area under the peak. 

---