The procedures involved in isoelectric focusing that entail the use of an electric current can be performed with the gel side of the IPG strip facing either up or down in a tray and electrode assembly in which the electrodes are mounted on an insert from which the inner ends of the electrodes extend into the troughs to reside either below the IPG strips or above them. Slots in one part (either the insert or the tray) mate with walls or partitions in the other part to secure the two parts together, and the heights of the electrodes within the troughs of the tray can be adjusted by the height of the insert in the tray or by a resilient mounting of the electrode to the insert.