A chemically modified silica (bisdithiocarbamate) strongly binds copper and has a loading capacity of 0.5 mmol g^-1 for the copper ion. This new metal-loaded silica, packed in a short stainless-steel pre-column, is highly selective for the enrichment of two amino acids chosen as model compounds. On-line coupling of this precolumn with an analytical RP-18 column allowed the separation and detection of l-tryptophan and l-tyrosine at parts per billion levels in aqueous media with good accuracy. Separation was achieved by ion-pair chromatography and the solutes were detected by fluorescence. The ions generally present in natural aqueous media did not interfere.