De Salvo, Kimber Marie (2018-08). Using Flow Field-Flow Fractionation Coupled to Inductively Coupled Plasma Mass Spectrometry to Study the Physicochemical Speciation of Colloidal Iron in Seawater. Master's Thesis.
Iron is a critical micronutrient that marine phytoplankton need to perform photosynthesis that produces about half of the world's oxygen. Colloidal iron (0.003-0.2 ?m) comprises a significant portion of the oceanic bioavailable dissolved iron pool (<0.2 ?m). The colloidal iron phase may have different scavenging residence times and bioavailability to phytoplankton based on its physicochemical speciation, so it is imperative to understand the size distribution and composition of marine colloidal iron as a function of size. To unveil the colloidal spectrum in low salinity regimes (rivers and estuaries), past studies have coupled Flow-Field Flow Fractionation (FlFFF) and UV Visible Spectrometry (UVvis), with Inductively Coupled Plasma Mass Spectrometry (ICPMS) to quantify the size distribution of organic and metal colloids. However, this method is challenged by low detection limits at full salinity because of salt effects and low sample volumes, inhibiting the ability to determine the colloidal iron size spectrum in oceanic environments. This thesis research pioneers a new method that overcomes these hurdles by fraction collecting size-separated FlFFF aliquots and analyzing iron using an offline, low-volume pre-concentration method. This new approach couples FlFFF-UVvis-ICPMS with online Multiple Angle Laser Light Scattering (MALLS) and offline fluorescent Excitation Emission Matrices (EEMs) to calculate sphericity and determine associated fluorescent dissolved organic matter (FDOM) to help to identify the compositional characteristics across the colloidal size spectrum. We first test this method for iron blanks, reproducibility, and sampling artifacts. Results of this new method in the Damariscotta River estuary and Maine shelf waters to describe the physicochemical speciation of marine colloidal iron size continuum. Our results suggest that there are both organically-bound and inorganic iron colloids in coastal Maine waters, and the iron distribution was not tightly correlated with organics or overall colloid abundance with compositionally distinct size fractions where: 0.25-1.5nm hydrodynamic radius is iron- and organic-rich, 1.5-3.5nm is organic, 2.5-5nm is non-spherical with low concentrations of organic iron, 5-9nm found only at Estuarine Station 2 15m is iron-poor, 9-12nm is organic and abundant, 12-15nm, found at both station's chlorophyll maximum, is iron-rich and inorganic, 15-20nm is organic-rich.