Supor PES filters (25 mm or 47 mm diameters, 0.45 µm pore size) were acid-washed in 10% HCl and soaked repeatedly in fresh baths of ultrahigh purity (UHP) water, until the pH of the resulting rinse water was circumneutral. Particulate samples were collected directly from the GO-Flo sampling bottles (pressurized (<8 psi) with filtered air), drawing ~7 L of seawater over the filters loaded into Swinnex or Advantec cartridges, and stored frozen in new polystyrene petri dishes.

Before processing and analysis, filters were divided into halves: one half for Total digestion (total element concentrations) and one half for Labile digestion (using the Berger et al. (2008) method). Digest solutions (Total and Labile) included a 10 ppb In spike to correct for instrumental drift.

Total digestion: Filters were digested in 15-mL PFA digestion vials (Savillex), extensively cleaned using HNO₃, HCl, and trace HF acids (trace metal grade or better). Digestions were performed on enclosed Teflon-coated hotplates ("flowboxes") under double HEPA filtered air (effectively Class-1), inside a Class-1000 clean room. The total digestion procedure involved a two-step digestion, modified from Ohnemus et al. (2014). During the first digestion step, filter halves were submerged in 4 mL of a mixture of 4 M each of HNO₃, HCl, and HF acids (all Optima or double-distilled). Samples were capped and heated for 3 hours at 100-120°C in the flowbox. Each digest solution was then carefully decanted into a second PFA vial, the original vial and filter rinsed twice with 1.5 mL of ultrahigh purity (UHP) water, and the UHP water rinses transferred to the second vial while retaining the filter in the original vial. The digest solution in the second vial was then taken to dryness (uncapped) in the flowbox, by heating overnight at 100-120°C. During the second digestion step, 20 µL of concentrated H₂SO₄(Optima), 20 µL of H₂O₂ (Optima), and 2 mL of 8 M HNO₃ Optima or double-distilled) were added to the digest residue to breakdown any PES filter fragments, and the vials heated (capped) at 150°C for 30 minutes. The vials were uncapped and taken to dryness at 210-230°C. The final digest residue was redissolved in 4 mL of 0.32 M HNO₃(Optima or double-distilled) and stored in acid-washed 5 mL cryovials until analysis.

Labile digestion: Filters were digested in rigorously cleaned 22-mL PFA digestions vials (Savillex). All digestion steps were performed in a Class-100 clean room using standard clean techniques. Filters were digested in a solution of 25% Optima-grade acetic acid and 0.02 M hydroxylamine hydrochloride following the protocol of Berger et al. (2008). One milliliter of this solution was added to the filter stored in a 1.7-mL polypropylene vial. The solution was heated to 95°C in a water bath for 10 minutes and then allowed to cool to room temperature. The filter was in contact with the acetic acid leach for a total of two hours. The filter was removed to an acid-cleaned PFA bomb and the acetic acid/hydroxylamine leachate was centrifuged at 14,000 rpm for 10 minutes to sediment all particles. Without disturbing particles on the bottom of the tube, approximately 0.8 mL of leachate was transferred into a 7-mL PFA digestion vial. Optima-grade HNO₃ was added (100 µL) to the 7-mL digestion vial, which was then heated uncapped at 110ºC to near dryness. Vial contents were redissolved with 2% HNO₃ (Optima grade).

Process blanks and reference materials were digested alongside samples for both chemical treatments. All samples were analyzed at NHMFL/FSU by HR-ICP-MS in both Low and Medium Resolution modes. Concentrations were quantified by multi-element external calibration (seven standards, up to 500 ppb), and drift corrected using a 10 ppb In spike added to each sample.