AF4-ICPMS with the 300 Da Membrane To Resolve Metal-Bearing "Colloids" < 1 kDa: Optimization, Fractogram Deconvolution, and Advanced Quality Control.

The smallest colloids exert a disproportionately large influence on colloidal systems owing to their greater surface area; however, the challenges of working in the smaller size range have limited most field-flow fractionation-ICPMS analyses to sizes > ca. 1 kDa. We discuss considerations and present solutions for overcoming these challenges, including high pressures associated with using the 300-Da membrane, calibration in this small size range, accounting for drifting LODs and separation conditions during membrane aging, and optimizing the compromise between resolution and recovery. Necessary flow program ranges for observing pressure limits are discussed, and calibration is conducted using a combination of bromophenol blue and polystyrene size standards. The impact of membrane drift on size is demonstrated and effectively corrected by routine calibration. Separation conditions are optimized by monitoring the recovery and resolution of several trace metals. A precise, high-resolution separation is achieved using fractogram deconvolution to fully resolve overlapping peaks. Method effectiveness and precision are demonstrated through triplicate analyses of three natural water samples: Mp = 2.89 ± 0.04, 3.20 ± 0.03 and 3.50 ± 0.12 kDa for DOM-associated Fe in the three samples (±95% CI). A primarily inorganic Fe fraction with Mp = 14.7 ± 0.5 kDa was also resolved from the DOM-associated fraction. Quality control methods and considerations for optimizing flow conditions are detailed in the Supporting Information as a guide for researchers seeking to analyze colloids in this smallest size range using AF4-ICPMS with the 300-Da membrane.