A shipboard method for catalytic kinetic spectrophotometric determination of trace Cu(II) concentrations in seawater using reverse flow injection analysis coupled with a long path length liquid waveguide capillary cell

By using reverse flow injection analysis (rFIA) and a 1.0 m liquid waveguide capillary cell (LWCC), an automatic and sensitive catalytic kinetic spectrophotometric method for on-line monitoring of Cu(II) concentrations in estuarine and coastal waters was established. Cu(II) detection was based on its catalytic effect on glutathione (GSH) oxidation by potassium ferricyanide under acidic conditions. The change in absorbance of potassium ferricyanide at 420 nm was used to monitor the reaction spectrophotometrically. Experimental parameters were optimized using a univariate experimental design approach and it was ensured that the method was free of interference from co-occurring ions such as Fe(II), Fe(III), Mn, Zn, Cd, Al, Cr, Ni and Co. The proposed method was shown to have high sensitivity with a detection limit of 0.15 nmol L-1 in a pure water matrix and 0.23 nmol L-1 in a seawater matrix. Linearity was achieved within the range of 0.45-15 nmol L-1 in a pure water matrix and 0.69-10 nmol L(-1 )in a seawater matrix using a 200 cm reaction coil, while the upper limits can be extended to 40 and 25 nmol L-1, respectively, by using a shorter (100 cm) reaction coil. A high level of Cu(II) recovery was maintained, ranging between 95.2% and 101%. The relative standard deviations (RSDs) of blanks and samples spiked with 5 nmol L-1 Cu(II) standard were 1.52% (n = 13) and 1.10% (n = 13), respectively. The method was confirmed to be free of carry-over effect and a sample throughput of 26 h-1 was achieved. The method was successfully applied to underway and on-board determination of Cu(II) concentrations from a surface transect in the Jiulong Estuary and a vertical profile in the Yangtze Estuary, China, respectively.