A chemically modified silica (dialkyldithiocarbamate, DDTC) has been developed which strongly binds copper(II) ion. This metallic surface has been shown to be an effective means for the preconcentration of complexing analytes in natural media (e.g., amino acids chosen as model compounds). The Si-DDTC is more stable than other dithiocarbamate functions previously tested, and no copper or silane leaching occurred with this surface. The possibility of on-line coupling of the enrichment precolumn packed with the copper-loaded silica with an RP-18 analytical column allowed the direct liquid chromatographic analysis of some aqueous media containing amino acids at the 10⁻⁷−10⁻⁹M level and of some complex matrices as urine or blood serum. The total analysis procedure takes not more than 40 min. The method minimizes manipulation errors, contamination, losses and the total analysis time, and enhances the selectivity.