The system has inlets (10,20) for liquid mobile phases (65,75) and a rotary valve switching unit (110) to produce two different fluid connecting states. A controllable device (96) applies local magnetic field to consecutive sections (97a,97b) of fluid connecting line (95) provided between entry and exit ports (91,92) of extractor (90). An independent claim is included for detecting purified analyte from complex liquid sample material.