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A rapid and sensitive method for the species-selective determination of chromlum(III) and chromlum(VI) In water samples by flame atomic absorption spectrometry using online preconcentration on a microcolumn packed with activated alumina (acidic form) has been developed. Sequential species-selective sorption was possible by using the Clark-Lubs buffer systems with pH 7 for Cr(III) and pH 2 for Cr-(VI). The preconcentrated species were eluted directly from the column to the nebulizer-burner system using 1.0 mol/L nitric acid and 0.5 mol/L ammonia for Cr(III) and Cr(VI), respectively. The retention efficiency was better than 80%for Cr(III) and better than 90 % for Cr(VI), giving a sensitivity enhancement of 25 for a 3-mL sample loading. The effect of concomitant species was Investigated, and satisfactory recovery of 90-106% could be obtained from natural water samples. Linear calibration for both species was established over the concentration range 10-200 pg/L• with detection limits (3 s) of 1.0 and 0.8 pg/L• for Cr(III) and Cr(VI), respectively.

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