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A novel reversed phase ion-pair micro HPLC system with on-line fluorescence detection has been developed systematically and studied for the determination of cadmium in its bio-available fractions of soil samples. In this system, a micro ODS column of 1.0 mm i.d.×150 mm length and a mobile phase containing 6 mmol L-1 8-hydroxyquinoline 5-sulphonic acid (HQS), 3 mmol L-1 cetyltrimethylammonium bromide (CTMABr), 10 mmol L-1 acetic acid-acetate buffer (pH 5.1) as well as 50% acetonitrile at 50 μL min-1 flow rate were employed to determine cadmium with a 2 μL flow cell through its fluorescence at 518 nm under 338 nm excitation. Furthermore, the composition of Cd-HQS chelate formed on the column was confirmed to be [Cd(HQS)2]2- through a log–log plot method, and then combined with the ion-pair reagent by the electrostatic force under the chromatographic condition proposed. With such a method, the detection limit of cadmium was 8.48 ng mL-1 (3σ) with 1 μL sample injection, and the linear range for the determination of cadmium was 30–800 ng mL-1 (R2=0.992). This method has been successfully applied to determination of cadmium in its bio-available fractions of BCR-483 and soil samples without interference from other coexistent metal ions. The RSD (n=6) was less than 7.3%. The results were in agreement with the indicative value for BCR-483 and those for the soil samples obtained by ICP-MS with a pretreatment of bis(1,1,3,3-tetramethylbutyl) phosphinic acid extraction. [ABSTRACT FROM AUTHOR]