Toward a versatile flow technique: Development and application of reverse flow dual-injection analysis (rFDIA) for determining dissolved iron redox species and soluble reactive phosphorus in seawater.

A versatile flow analyzer that extended the features of reverse flow injection analysis (rFIA) was developed in this study and named reverse flow dual-injection analysis (rFDIA). Compared with typical rFIA, the analyzer requires less reagent and is more environmentally friendly, which has two injection valves and two reagent loops for the accurate and successive injection of two reagents. With a 2-m long liquid waveguide capillary cell (LWCC) and a spectrophotometer, the analyzer was applied to underway determination of dissolved iron redox species in estuarine and coastal waters. Detection limits of 0.18 and 0.20 nmol L were achieved for Fe(II) and Fe(II + III), respectively and a linear dynamic range of 0.5-450 nmol L was obtained for both Fe(II) and Fe(II + III). The sample throughput for the simultaneous measurement of Fe(II) and Fe(II + III) was 12 h, and each analysis consumed only 8 mL sample, 520 μL ferrozine solution, and 260 μL ascorbic acid solution. The analyzer was also used to measure nanomolar amounts of soluble reactive phosphorus (SRP) in seawater. The detection limit and the linear dynamic range for the SRP assay were 0.5 nmol L and 1.5-850 nmol L. For SRP determination, the sample throughput was 20 h, and each analysis required 9 mL of sample, 130 μL of mixed reagent solution and 260 μL of ascorbic acid. The analytical results were reproducible, with a relative standard deviation of 1.4% (2.5 nmol L, n = 10), 2.1% (2.5 nmol L, n = 10), and 2.1% (10 nmol L, n = 11) for Fe(II), Fe(II + III), and SRP, respectively.