Objective To establish a method of an online preconcentration-fiber optical sensing spectrophotometry for the determination of the content of hexavalent chromiumCr(VI) in soil.

Methods A flow injection analysis system comprising a self-made flow cell and a fiber optical spectrometer was used to determine the content of Cr(VI) in soil samples, after on-line Cr(VI) preconcentration through a polytetrafluoroethylene filter under alkaline conditions, with cerium nitrate as the precipitant, nitric acid as the eluent, and 1, 5-diphenylcarbazide as the chromogenic reagent. The effects of the precipitant, eluent, flow rate, preconcentration time, chromogenic reagent concentration and reaction tube length were examined.

Results The linear regression equation of Cr(VI) concentrations was: y=0.010 1x-0.000 2. The linear range of Cr(VI) determined under the optimal experimental conditions was 5.0 to 25.0 μg/L. The molar concentration of Cr(VI) and the absorbance showed a good linear relationship, with a correlation coefficient of 0.999 5. The detection limit was 0.4 mg/kg, with relative standard deviation 0.69%, and recovery 95.24-104.59%.

Conclusion The method established in this study is a simple and direct approach to measuring the content of Cr(VI) in soil through on-line sample preconcentration, avoiding the influence of confounding factors caused by pre-processing.