Off-line chromatographic assessment of Fe(II) in seawater

The present paper presents a model to account for waiting time in off-line Fe(II) assessment. The model comprises Fe(II) oxidation in a reducing environment (∼1 × 10−5 M Na2SO3 in filtered seawater) and binding to column-associated ferrozine, for use with ferrozine preloaded SepPak® C18 cartridges. The model is essentially based on mathematical treatment of transport in micro-vessels and uses known rate factors for the oxidation and reduction of Fe. In off-line chromatographic Fe(II) assessment, the model was shown to account for variances in Fe(II) recoveries ranging from 10 to 54%, and for waiting times ranging from 2 to 80 min. The presented data shows that waiting time resulted in underestimation up to a factor 10 as measured by direct recovery counting of loaded Fe(II). As excess amounts of ferrozine were used for these experiments, this underestimation will be mainly caused by the oxidation of ferrous iron during this waiting time. The data also suggests that time-modelling may account for all effects, thus permitting off-line counting of Fe(II) without loss of data quality.