an electrode assembly for capillary electrophoresis comprises a manifold , a connector a sheath , and a seal . a capillary tube passes through the manifold , the connector , the sheath , and the seal , stopping just beyond the end of the sheath . the sheath is fillable with water or another fluid that cools the capillary tube in the vicinity of the electrode , thereby preventing degradation of a sample due to heat . the sheath may be metal or plastic with a metal sleeve electrode on its exterior . the sheath is sufficiently strong to penetrate a rubber or other pierceable cap on a vial . the manifold and connector incorporate an air path so that when the electrode is fully inserted into a vial , the contents of the vial are at atmospheric pressure .