Immobilized pH gradients in ultrathin-layer (100 μm) gels: Avoiding capillarity by a 'squeezing-sealing mold' technique

Several procedures for casting 100 μm ultrathin immobilized pH gradients are described. When acrylate/glass molding cassettes (Pascali et al., Electrophoresis 1987, 8, 371–373) are used two main problems are encountered: (i) a tendency of polymerization solutions, at the beginning of delivery, to spread across the glass surfaces with troublesome effects on the gradient stratification, and (ii) the raising of steep menisci at both extremities of the pH intervals, originating from capillarity phenomena and resulting in nonuniform gradients with bowed electrophoretic patterns. The first shortcoming was acceptably solved by increasing the density of sucrose gradients, and pouring them into prewarmed molding cassettes. The detrimental effect of menisci could be overcome by using a ‘squeezing-sealing mold’ technique. A molding cassette was endowed with a continuous, squared spacer frame, the upper side being open by inserting a wedged clip. A slight excess of polymerization solution was first dispensed into the cassette and squeezed away on removal of the clip. By completely excluding air from the molding cassette, uniform and well reproducible ultrathin gels could be cast. A major advantage of ultrathin immobilized pH gradient gels is the drastically shorter focusing time.