Controlled Pore Glass

The method used for the preparation of CPG was originally invented by Hood and Nordberg (21). They discovered that by heat treating a certain sodium borosilicate glass composition, the glass separated into two intermingled and continuous glassy phases. One phase, rich in boric oxide, was soluble in acids; the other phase was high in silica and stable toward acid solutions. This process was used by the inventors to prepare nonporous silica particles by first removing the boron phase with an acid solution to obtain porous particles and then revitrifying this material by heating to 900 °C or above to yield transparent homogenous nonporous particles having a composition of greater than 95% silica.

The porous glass material obtained by this method typically had an average pore diameter of about 4 to 5 nm. A procedure for enlarging the pore size to approximately 20 nm was invented by Chapman and Elmer (22). This was carried out by impregnating a porous glass body with an aqueous solution of a weakly reactive fluorine-containing compound, reacting the compound in situ with a mineral acid to release hydrofluoric acid at a temperature sufficient to dissolve a portion of the glass body and washing the body to remove the soluble constituents. Ammonium bifluoride and nitric acid at 95 °C were applied to a porous glass disc to enlarge the pore size from 4 to 24 nm.

The process for making porous glass was modified by Haller (23) to produce the material with a range of controllable pore sizes. This was achieved by varying the temperature and duration of the heat treatment to obtain porous glass with pore sizes from 12 to 250 nm. Larger pore sizes were also obtained by the addition of a sodium hydroxide leaching process.

Manufacturing Process

CPG can be commercially obtained from a number of sources, and the manufacturing processes are all based on the process invented by Haller (23). The composition of the starting metal oxides mixture and the exact conditions used for the heat treatment and leaching of the glass, which influence the quality and physical properties of the porous glass, are the propriety technology of the manufacturing companies. An outline of the general procedure used for preparing CPG is shown in Figure 1.

CPG is manufactured in a range of particle sizes, 40 to 200im, and different pore sizes, 7 to 300 nm (Table 3). The manufacturing batch sizes are generally much smaller than silica particles, possibly because of the heat treatment process. The largest batch size commercially available is in the region of 50 L (20 kg).

Physical Characteristics

Most commercially available CPG granules are irregularly shaped and highly porous (Figs. 2 and 3). CPG has a narrow pore size distribution, with 80 to 90% of the pores showing a deviation less than ±10% from the nominal pore diameter (Fig. 4). CPG has a very high surface area that varies according to the pore diameter and pore volume. The surface area increases with increasing pore volume and decreases with increasing pore diameter, as is shown by the data in Table 3. CPG is a rigid insoluble matrix, compatible with most reagents used in the purification of biological molecules. Because CPG is essentially pure silica, it generally exhibits a high degree of solubility in alkali solutions. The solubility of CPG is a function of temperature, time, and pH of the solution.

The surface of native CPG is slightly hydrophobic and contains acidic hydroxyl groups that could denature bio-

Figure 1. Process for the preparation of porous glass.

Table 3. Properties of Controlled Pore Glass (CPG, Inc.)

Nominal

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