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Jahresbericht 2014 - Annual Report 2014

Asymmetric Flow Field-Flow Fractionation (AF4) is a fi eld-driven separation and sizing technique that reminds of chromatography with a laminar fl ow in place of a stationary phase. Recently, AF4 has become a method of choice for the detection of nanoparticles in complex matrices. Today, recovery ratios for nanoparticles in AF4 are below the recovery ratios reached for macromolecules. In this project, we aim to improve the particle recovery by identifying and suppressing the main particle loss mechanisms. During AF4, a liquid “crossfl ow” moves perpendicular to the main fl ow direction in a thin slit-like channel and drives the analytes towards an ultrafi ltration (UF) membrane. Particles are separated by size. Due to diff erences in the diff usion coeffi cient the particles accumulate in distinct regions of the main fl ow, which have diff erent fl ow velocity. Hence, particles of diff erent size leave the channel at diff erent times and can be characterized downstream by att ached detectors. AF4 has no stationary phase, but unwanted particle-surface interactions still occur and can lead to particle loss and biased results. Materi- als and process conditions (Fig. 1) infl uence the degree of particle adsorption on internal surfaces, such as the tubing, valves, de- tector windows, and in particular the UF membrane. Th e closely related agglomeriation of nanoparticles, which occurs easily in nano- particle dispersions, depends sensitively on surfaces and concen- trations, and is very likely to aff ect adsorption phenomena. We investigated the particle loss during AF4 using gold nanoparti- cles modifi ed with diff erent ligands as a well-defi ned model system. Preliminary results suggest that adsorption on the membrane is an important loss mechanism (Fig. 2). In an ongoing systematic study of the particle loss mechanisms, we assess eff ects of diff erent particle types, sizes and measurement conditions. NANOPARTICLES IN FLOW FIELD-FLOW FRACTIONATION: INTERACTIONS AND LOSSES A.-R. JOCHEM, G. N. ANKAH, T. KRAUS 53JAHRESBERICHT 2014 / ANNUAL REPORT 2014 Fig. 1: Major parameters infl uencing particles losses due to NP-surface interactions. Fig. 2: Fractograms of Au-citrate nanoparticles with increasing crossfl ow rate. PARTICLE CORE Morphology Composition Hamaker constant Size SURFACES Surface charge Roughness Composition Porosity PARTICLE SURFACE Surface charge Surface area Functionality Hydrophobicity PROCESS CONDITIONS Ionoc strength pH Temperature Crossfl ow density ADSORPTION Nanoparticle-surface interactions parabolic liquid fl ow AGGLOMERATION STRUCTURE FORMATION

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