a capillary electrophoresis apparatus and method are disclosed which utilize a meltable plug in an end of the capillary tube to selectively pass small ionic contaminants electrophoretically and retain macromolecular analytes against one end of the plug until the plug is melted . when this plug is melted , the analytes and the contaminants pass through unimpeded . this permits separation of the analytes from the contaminants during electrophoretic separation and enhances instrument resolution .