The cell-sorption of paramagnetic ions of Mn2+ and Cr3+ onto a Chlorella vulgaris (C. vulgaris) cell-immobilized micro-column was significantly improved in the presence of an external magnetic field generated in a finite solenoid, by placing the micro-column in the center of the solenoid in a sequential injection system. Magnetic field creates an opposite drift velocity on the hydrated paramagnetic ions against the flow of the sample zone, retards the moving velocity of the metal ions and provides extra contacting time with the cells on the micro-column and offers more chances for the paramagnetic ions to interact with the various functional groups or binding sites on the cell surface, which significantly facilitates cell-sorption of the paramagnetic ions. The sorption efficiencies of Mn2+ and Cr3+ at the 20 microg L(-1) level were improved from 45 to 80% and 60 to 90%, respectively, in a magnetic field of 240 mT. The system was applied for the separation/preconcentration of ultra-trace level of manganese. The presence of an external magnetic field significantly alleviated the interfering effects from coexisting metal ions. Within a liner range of 0.025-0.5 microg L(-1) and a sampling volume of 500 microL, an enrichment factor of 21.2, a limit of detection of 0.008 microg L(-1), along with a sampling frequency of 20 h(-1) was attained, achieving a precision of 2.1% R.S.D. (0.2 microg L(-1)). Manganese contents in a certified reference material of riverine water and a snow water were analyzed.