Extran ma01

All solutions were prepared using purified water from a Millipore system (Milli-Q water, MQW). To clean the low-density polyethylene (LDPE) bottles, surfactants (Extran MA01, Merck) and hydrochloric acid (HCl; Guaranteed Reagent, Wako Chemicals) were used. Ultrapure-grade aqueous ammonia, HCl (Tamapure AA-100, Tama Chemicals), and luminol sodium salt (Sigma-Aldrich) were purchased commercially. We dissolved 0.737 g of luminol sodium salts in 250 mL of MQW to prepare a 14.8 mM luminol solution. The solution containing luminol (0.74 mM), aqueous ammonia, HCl, and MQW was heated to 60 °C for over 15 h [37] (link) and left in the dark until the temperature reached room temperature. Standard stock solutions of Fe(II) (10 mM) were prepared by dissolving ferrous ammonium sulfate hexahydrate (Wako Chemicals) in a 0.1 M HCl solution before the research cruise and stored in a refrigerator until use. Standard working solutions (1 µM) were prepared by diluting the stock solutions with 0.1 M HCl before each use. All solutions were prepared in LDPE bottles (Nalge).

The Teflon tubes and fittings were cleaned using the method adapted for the analysis of Fe in seawater [52, (link)53] (link). LDPE bottles were soaked in 5% surfactant (Extran MA01, Merck) for 1 d and then 3 M HCl for 1 d. These bottles were filled with 0.1 M HCl and then heated in a microwave oven at 60-70 °C and left overnight at room temperature. The same procedure was repeated using MQW. Polyethylene bags for the reagents were filled with 0.1 M HCl overnight, and then MQW overnight. Pharmed tubes for the peristaltic pump were filled with 1% surfactant (Extran MA02, Merck), 0.1 M HCl, and MQW for 1 d, respectively. Teflon tubes in the flow system were filled with 0.1 M HCl and MQW for 1 d each.