Determination of size-dependent metal distribution in dissolved organic matter by SEC-UV/VIS-ICP-MS with special focus on changes in seawater

Abstract

Iron is an essential micronutrient for all marine organisms, but it is also a growth limiting factor as the iron concentrations in the open ocean are below 1 nmol/L in sea water iron is almost entirely bound to organic ligands of the dissolved organic matter fraction, which are mostly of unknown structure. The input from rivers was traditionally considered as less important due to estuarine sedimentation processes of the mainly colloidal iron particles. However, recent studies have shown that this removal is not complete and riverine input may represent an important iron source in the open ocean. In this context, iron transport by land-derived natural organic matter (NOM), and dissolved organic matter (DOM) have been identified as carrier mechanisms for riverine iron. The aim of this work is to characterize complexes containing iron and other metals in waters simulating estuarine conditions in order to help understand which role iron-DOM compounds play in the open ocean. A method based on size-exclusion chromatography (SEC) with sequential UV/VIS and ICP-MS detection was developed for investigation of DOM size distribution and for assessment of the size-dependent metal distribution in NOM-rich surface water. Furthermore, sample matrix experiments were also performed revealing a dependence of DOM size distribution upon seawater concentration and different compounds present in seawater. Finally, efforts toward determination of DOM size with standardization with typical SEC standards indicate that only relative comparisons are possible with this approach, and that the sample matrix composition strongly influences obtained results.

Keywords: Humic substances; Iron; Natural organic matter; Ocean; SEC-UV/VIS-ICP-MS.

Figure 1

Chromatograms of Suwannee River NOM in 50% artificial seawater (conditions given in Tables 1 and 2). Three DOM fractions can be distinguished in the SEC‐UV/VIS chromatogram. The chromatograms obtained by SEC‐ICP‐MS signals clearly show that iron is predominantly bound to the largest size fraction, whereas zinc has a high affinity to the low molecular weight fraction (fraction 3).

Figure 2

Influence of selected seawater constituents on DOM size distribution. The depicted UV/VIS chromatograms indicate that the conformation of DOM is severely altered in dependence of salt added. Solid line: selected seawater constituent, dashed line: Suwannee River DOM in 50% seawater.

Figure 3

Comparison of Suwannee River DOM to polystyrene sulfonate (PSS) standards. The results obtained in the aqueous phase (distilled water) suggest aggregation due to hydrophobic effects of both DOM and PSS molecules leading to larger molecular structures. The chromatograms obtained for both DOM and PSS dissolved in 50% seawater indicate a wide size range. Dashed line: Suwannee River DOM, solid line: PSS.