The invention relates to a cost effective and industrially advantageous process for the selective methylation of a hydroxy group at the 6 position of erythromycin A derivative which comprised methylating the erythromycin A derivative with a methylating agent in a mixture of toluene and a ploar aprotic solvent. The jaw members are movable from a first position in spaced relation relative to one another to a subsequent position wherein the jaw members cooperate to grasp tissue therebetween. A port or vent is located within the chamber or gap between the two layers to provide an intermediate path for gas or liquid to flow through one layer of filter or membrane and out the vent or port for integrity testing. The first layer so-exposed is then removed to expose the region of the conductive layer thereunder.