Solid phase extraction coupled with a liquid waveguide capillary cell for simultaneous redox speciation analysis of dissolved iron in estuarine and coastal waters

A portable automatic flow injection (FI) based system incorporating on-line C18 solid phase extraction (SPE) cartridges and a 2 m long liquid waveguide capillary cell (LWCC) is established for simultaneous redox speciation analysis of dissolved iron in estuarine and coastal waters. Utilization of the SPE preconcentration and the LWCC enhanced the sensitivity of the ferrozine method for Fe(II) analysis. The Fe(II)-ferrozine complex was formed and extracted onto a C18 cartridge, and eluted with an HCl-ethanol solution for spectrophotometric detection with an LWCC. The determination of total Fe(II + III) was realized in a parallel flow channel after the reduction of Fe(III) to Fe(II) with ascorbic acid. The optimal combination of the pre-eluting solution and eluent was investigated to eliminate the Schlieren effect. The parameters of the FI-SPE-LWCC system were optimized based on a univariate experimental design. The effect of salinity on the method sensitivity was low enough to apply the system in both estuarine and coastal waters without adjustment. The limit of detection was 0.056 nmol L-1 for Fe(II) and 0.096 nmol L-1 for Fe(II + III). A linear determination range of 0.5-50 nmol L-1 iron was obtained with a sample loading volume of 5 mL and a sample throughput of 6 h(-1). The system was successfully applied in situ in Wuyuan Bay, Xiamen, for the continuous monitoring of dissolved iron species for 20 h.