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The suitability of 1-nitroso-2-naphthol(NN) as a complexing agent for on-line preconcen tration of cobalt eluted on the C18 microcolumn by means of the FI-FAAS system was tested.Various parameters affecting the complex formation and its elution were optimized. A 2.3 × 10-3 mol/L reagent solution and the aqueous sample solution acidified with 0. 1 % (volume fraction) nitric acid were on-line mixed (6.4 mL/ min) on a reaction coil set at (65 ±1) ℃ and flowed through the microcolumn for 30 s. The pH of the mixed solution was adjusted to 3-4 with HNO3(1 mol/L) or NaOH(1 mol/L). The adsorbed complexes in the microcolumn were eluted into the nebulizer of FAAS in 10 s with ethanol acidified with 1 % HNO3(3.0 mL/min). A good precision(1.6 % for 100 μg/L Co( Ⅱ ), n=10), a high enrichment factor 17.2, with detection limit (3σ) 3.2 μg/L, and sample throughput (90 h-1) were obtained. The method was applied to the certified reference materials(CRMs), NBS-362 and NBS-364, for the determination of cobalt and the results were in good agreement with the certified values.