Optimization of flow field-flow fractionation for the characterization of natural colloids

The methodological approach used to robustly optimize the characterization of the polydisperse colloidal phase of drain water samples is presented. The approach is based on asymmetric flow field-flow fractionation coupled to online ultraviolet/visible spectrophotometry, multi-angle light scattering, and inductively coupled plasma mass spectrometry. Operating factors such as the amount of sample injected and the ratio between main-flow and cross-flow rates were considered. The evaluation of the injection and fractionation steps was performed considering the polydispersity index and the contribution to the polydispersity of the plate height, the recovery, the retention ratio and the size range of the fractionated colloids. This approach allows the polydispersity of natural colloid samples to be taken into consideration to achieve the most efficient and representative fractionation. In addition to the size characterization, elemental analysis was also evaluated using the recovery, precision, and limits of detection and quantification relative to a trace element of interest (copper) in drain water. To complete this investigation, the potential application of the methodology was assessed using several independent drain water samples from different soils. The contribution of the polydispersity to the plate height ranges from 4.8 to 8.9 cm with a mean precision of 6%. The mean colloidal recovery was 81 ± 3 %, and the mean retention ratio was 0.043-0.062. The limits of detection and quantification for copper were 0.6 and 1.8 μg L(-1), respectively.