|Explanation of Gels Poured in Two Steps:
Gels are poured in two steps in order to get a flat, even interface between the resolving gel and the stacking gel. First, the resolving gels is poured and allowed to polymerize over a 1 hour period. Next, the top of the gel is rinsed and a low concentration stacking gel is poured on top of the resolving gel and allowed to polymerize. A resolving gel that has a low concentration of acrylamide at the top, such as 4% acrylamide, would provide an uneven or wavy interface, if it were not allowed to polymerize first. The flat interface between the stacking gel and resolving gel provides better separations across the gel. The additional labor required to pour the gels in two steps is reflected by the extra charge.
A higher concentration resolving gel, such as at least 6%, can accept a stacking gel which can be applied without waiting for the resolving gel to polymerize first. Those gels do not require a two step process and are therefore priced lower.