Da Young Kang

Different elution modes and field programming in gravitational field-flow fractionation: Effect of channel angle

Gravitational field-flow fractionation (GrFFF) has been shown to be useful for separation and characterization of various types of micrometer-sized particles. It has been recognized however that GrFFF is less versatile than other members of FFF because the external field (Earths gravity) in GrFFF is relatively weak and is not tunable (constant), which makes the force acting on the particles constant. A few approaches have been suggested to control the force acting on particles in GrFFF. They include (1) changing the angle between the Earths gravitational field and the longitudinal axis of the channel, and (2) the use of carrier liquid having different densities. In the hyperlayer mode of GrFFF, the hydrodynamic lift force (HLF) also act on particles. The existence of HLF allows other means of changing the force acting on the particles in GrFFF. They include (1) the flow rate programming, or (2) the use of channels having non-constant cross-section. In this study, with polystyrene latex beads used as model particles, the channel angle was varied to study its effect on elution parameters (such as selectivity, band broadening and resolution) in the steric or in the hyperlayer mode of GrFFF. In addition, the effects of the channel thickness and the flow rate on the elution parameters were also investigated. It was found that, in the steric mode, the resolution decreases as the flow rate increases due to increased zone broadening despite of the increase in the selectivity. At a constant volumetric flow rate, both the zone broadening and the selectivity increase as the channel thickness increases, resulting in the net increase in the resolution. It was also found that the retention time decreases as the channel angle increases in both up- and down-flow positions. The zone broadening tends to increase almost linearly with the channel angle, while no particular trends were found in selectivity. As a result, the resolution decreases as the channel angle increases.

Effect of carrier fluid viscosity on retention time and resolution in gravitational field-flow fractionation.

Gravitational field-flow fractionation (GrFFF) is a useful technique for fast separation of micrometer-sized particles. Different sized particles are carried at different velocities by a flow of fluid along an unobstructed thin channel, resulting in a size-based separation. They are confined to thin focused layers in the channel thickness where force due to gravity is exactly opposed by hydrodynamic lift forces (HLF). It has been reported that the HLF are a function of various parameters including the flow rate (or shear rate), the size of the particles, and the density and viscosity of the liquid. The dependence of HLF on these parameters offers a means of altering the equilibrium transverse positions of the particles in GrFFF, and hence their elution times. In this study, the effect of the viscosity of the carrier fluid on the elution behavior (retention, zone broadening, and resolution) of micrometer-sized particles in GrFFF was investigated using polystyrene (PS) latex beads as model particles. In order to change the carrier liquid viscosity without affecting its density, various amounts of (hydroxypropyl) methyl cellulose (HPMC) were added to the aqueous carrier liquid. It was found that particles migrate at faster rates as the carrier viscosity is increased, which confirms the dependence of HLF on viscosity. At the same time, particle size selectivity decreased but peak shape and symmetry for the more strongly retained particles improved. As a result, separation was improved in terms of both the separation time and resolution with increase of carrier viscosity. A theoretical model for plate height in GrFFF is also presented, and its predictions are compared to experimentally measured values.

Effect of Reaction Parameters on Size Distribution of Emulsion-Polymerized Polystyrene Latex Beads Studied by Gravitational Flow-Flow Fractionation (GrFFF)

Abstract A gravitational field-flow fractionation (GrFFF) was employed to study the influence of various reaction parameters in emulsion polymerization on size distribution of polymeric latex beads. Micron-sized polystyrene (PS) latex beads were synthesized by conventional emulsion polymerization. The reaction parameters such as the stirring rate (rpm), reaction time, concentration of azobisisobutyronitrile (AIBN) and polyvinylpyrrolidone (PVP), were systematically varied. Then the PS beads were analyzed by GrFFF for size determination. It was found that the mean diameter and the size distribution of the PS latex beads change as some of the reaction parameters change. No clear trends were observed in the mean diameter and the coefficient of variation (CV) with the stirring rate in the range of 60 to 300 rpm, or with the reaction time in the range of 13 to 24 hrs. A clear trend of increase in the mean diameter with the AIBN concentration was observed, suggesting the particle size can be controlled by varying the AIBN concentration without affecting the size distribution much. It was also found that the mean diameter tends to decrease as the PVP concentration increases while the CV tends to increase. Results obtained from GrFFF were compared with those obtained from photon-correlation spectroscopy (PCS) and optical microscopy (OM).