with a solution containing between 2 and 30 trimethoxysilylpropyl polyethyleneimine rinsing the inner surface of the capillary tube and treating the inner surface with a cross linker to cross link the polymer groups the detailed steps for preparing a coated capillary tube according to the invention are described below in example i trimethoxysilylpropyl polyethyleneimine pei silane is a linear polyethyleneimine attached to trimethoxy silyl propyl groups pei silane was dissolved in methanol to obtain a 5 v v solution of pei silane in methanol by vortexing for 3 min in a glass vial a capillary tube was first treated with methanol then water and then with 1 0 n sodium hydroxide and 1 0 n hydrochloric acid the capillary was rinsed with deionized water and afterward with methanol the pei silane solution was pumped through the capillary for 12 16 h at room temperature after this the capillary was rinsed briefly with methanol next a 10 v v solution of 1 4 butanediol diglicidylether budge in 1 4 dioxane was pumped through the capillary for 4 6 h at room temperature to cross link those polymer chains after this nitrogen was passed through the capillary while heating it for 1 0 h at 80 u00b0 c the capillary was then rinsed thoroughly with methanol the cross linking of the polymer chains on the capillary surface enhances the stability of the coating the trimethoxysilylpropyl polyethyleneimine forms a covalent bond between the polymer and the silica capillary surface via trimethoxy silane groups attached to the polymer amine groups present in the polymer electrostatically bind to the surface.