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

The speciation of iron in seawater is receiving much attention worldwide, and several methods have been developed to measure its various chemical species. Although probably the most important in algal iron accumulation, Fe(II) is very unstable in seawater, is rapidly oxidised to Fe(III), thus the time between collection of the samples and the actual Fe(II) assessment may have significant impact on the obtained results. Especially for kinetic analysis, where radiotracer methods ask for off-line counting, waiting times should be taken into account. 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 ( approximately 1x10(-5)M Na(2)SO(3) in filtered seawater) and binding to column-associated ferrozine, for use with ferrozine preloaded SepPak((R)) C(18) 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 80min. 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.