L-cysteine functionalized multi-walled carbon nanotubes (MWCNTs-cysteine) are synthesized and characterized by XPS, FT-IR, XRD, and TEM. The capability of MWCNTs-cysteine for selective separation and preconcentration of heavy metal ions are statically and dynamically evaluated with Cd2+ as a model heavy metal ion. Unlike MWCNTs, the sorption of Cd2+ onto MWCNTs-cysteine is not influenced by ionic strength in a wide range. The MWCNTs-cysteine is demonstrated to be good column packings for on-line microcolumn separation and preconcentration of Cd2+. Effective preconcentration of Cd2+ on the MWCNTs-cysteine packed microcolumn is achieved in a pH range of 5.5 to 8.0. The retained Cd2+ is efficiently eluted with 0.5 mol L-1 HCl for on-line flame atomic absorption spectrometric determination. The MWCNTs-cysteine exhibit fairly fast kinetics for the adsorption of Cd2+, and offer up to 1600-fold improvement of the tolerable concentrations of co-existing metal ions over the MWCNTs for on-line solid-phase extraction of Cd2+. With a preconcentration time of 60 s at a sample loading flow rate of 5.0 mL min(-1), an enhancement factor of 33 and a sample throughput of 36 h(-1) along with a detection limit (3s) of 0.28 mu g L-1 are obtained. The precision (RSD) for 11 replicate measurements is 1.6% at the 10 mu g L-1 level. The developed method using the MWCNTs-cysteine as sorbent is successfully applied to determination of trace cadmium in a variety of biological and environmental materials.