Analytical Chemistry

in aqueous media and in such solvents as ethylene glycol and dimethylformamide. Bio-Gel (a
copolymer of acrylamide and N,N'-methylenebisacryl amide) and Sephadex (dextran cross-linked with
epichlorhydrin) are two commercially available gels made in bead form. Cross-linking produces a
three-dimensional network which renders them insoluble. By varying the degree of cross-linking, the
range of pore sizes can be controlled so as to produce beads that will fractionate samples over different
molecular weight ranges (Table 4.19). The gels are chemically stable over a pH range of 2–11 but are
attacked by strong acids, bases and oxidizing agents. Sephadex may contain a small number of carboxyl
groups which act as ion-exchange or adsorption sites, but the effect on retention volumes is only slight.
Gels based on agarose have exceptionally large pores which facilitate separations in molecular weight
ranges up to 2 × 108. They are not cross-linked and are more chemically and thermally unstable than
other hydrophilic gels.

Table 4.19 Fractionation ranges of some commercial gels

Hydrophilic gels

Sephadex G- 10

Fractionation range^

Up to 700

G- 50 500 – 10 000

G- 100 4000 – 150 000

G- 200 5000 – 800 000

Hydrophobic gels

μ-Styragel (polystyrene/DVB)

Mean pore size in Ångstroms

100

Fractionation range

Up to 700

500 0.05 × 104 – 1 × 104

104 1 ×^104 – 20 × 104

106 5 × 106 – 10 × 106

Rigid gels

μ-Bondagel (silica) 125 0.2 ×^104 –^5 ×^104

500 0.2 ×^105 – 5 × 105

1000 5 × 105 – 20 × 105

Zorbax PSM- 60 60 0.01 × 104 – 4 × 104

PSM- 500 350 0.1 × 105 – 5 × 105

PSM- 1000 750 0.03 × 106 – 2 × 106

Hydrophobic gels are made by cross-linking polystyrene with divinyl benzene and thus resemble ion-
exchange resins but without the ionic groups. As a result they can absorb relatively non-polar solvents
such as tetrahydrofuran, benzene, chloroform and cyclohexanone with ease but not aqueous or other
highly polar liquids. Dextran-based gels can be made hydrophobic by acylation or alkylation of the
hydroxyl groups. The fractionation ranges of some commercially available hydrophobic gels are also
given in Table 4.19.

Porous glasses and silica gels with controlled pore sizes which are rigid