Determination of iron(II) in acidified seawater samples by luminol chemiluminescence

The redox speciation of iron has been proposed to influence the biological uptake of iron, yet little is known of its true redox partitioning in the open ocean. This is due to its extremely low concentration and to the tendency for the reduced species, Fe(II), to undergo rapid oxidation before analyses can be undertaken. Although several methods have been developed to measure Fe(II), many require extensive sample preparation and delicate instrumentation which limit their utility in the field. Also, because most lack sufficient sensitivity to make direct measurements, a preconcentration step is often required, introducing the potential for significant artifacts. For these reasons, the existing open ocean Fe(II) dataset has been both limited in scope and internally inconsistent. We recently used a highly sensitive chemiluminescent method to measure Fe( II) in 1452 samples collected at 121 stations in the Pacific Ocean. The method allowed for rapid analyses to be made with minimal sample preparation and no preconcentration. To overcome the logistical problems arising from rapid oxidation of Fe(II), we modified this method to include a modest pH adjustment of the sample, which greatly increases the half-life of Fe(II). Herein, details of the method are presented and analytical performance is assessed under actual field conditions. Potential interferences are evaluated and minor artifacts arising from sample acidification are discussed.