Flow injection analysis method with on-line resin separation for determining trace Fe(II) and Fe(II plus III) using catalytic spectrophotometry

A simple and sensitive flow injection analysis method using catalytic spectrometry was developed to measure sub-nanomolar levels of Fe(II) and Fe(II + III) in seawater using N,N-dimethyl-p-phenylenediamine dihydrochloride (DPD). Even without a preconcentration step, the minimum detection limit was 43 pmol/L, which was comparable to the limit of methods using a preconcentration step. DPD solutions were prepared using sodium sulfite as an antioxidant to increase the stability and extend the shelf-life of the reagent. The linear determination range was 6.0 nmol/L, and the relative standard deviation for sample measurements in 0.61 nmol/L Fe(II + III) was 4.4% (n = 14). The salinity and seawater matrix had a negligible impact on the results. The analytical results of the certified reference seawater NASS-5 (3.81 +/- 0.21 nmol/L) agreed with the certified value (3.71 +/- 0.63 nmol/L). The accuracy was further verified by measuring GEOTRACES Atlantic Ocean reference seawater samples. By adopting a nitrilotriacetic acid (NTA) Superflow chelating resin, the method was used for the on-line separation of the Fe(III)/Fe(II + III) redox pair. The method was used to assay dissolved Fe (<0.45 mu m) in samples collected from the South East Asia Time-Series station in the South China Sea.