a method for separating electrically charged species contained in a solution by an isotachophoresis method applied in an electrophoresis device , the isotachophoresis method being coupled to isotopic measurement using a mass spectrometer . the method notably comprises a step of stopping the voltage applied to the terminal of the electrophoresis capillary and transient application of a counter - pressure making it possible to utilize the length of the capillary several times and extend the separation distance artificially .