In this work, an on-line system for preconcentration and determination of copper, iron, nickel, and zinc at mu g L-1 level by flame atomic absorption spectrometry (FAAS) has been developed. Amberlite XAD-4 functionalized with 3,4-dihydroxybenzoic acid packed in a minicolumn was used as metal sorbent. The retained metals can be quickly eluted from sorbent material, with the eluent stream consisting of hydrochloric acid solution, directly to the nebulizer burner system of the FAAS. Analytical parameters were evaluated and the results demonstrated that all studied metals can be determined, using borate buffer to adjust the sample pH at 8.0. The results showed that the proposed method is simple and rapid. The limits of detection were estimated as 2.3, 5.0, 7.8, and 0.1 mu g L-1 for copper, iron, nickel, and zinc, respectively, using a preconcentration time of 60 s and a sample flow rate of 5.5 mL min(-1) . Enrichment factors of 22, 15, 12, and 54 and coefficients of variations of 3.5, 4.4, 4.4, and 3.2% were obtained in the determination of copper, iron, nickel, and zinc, respectively. The system presented an analytical throughput of 10 samples per hour and was successfully applied in the determination of metals in ethanol fuel.