Electrophoretic mobility measurement by laser Doppler velocimetry and capillary electrophoresis of micrometric fluorescent polystyrene beads

Many studies have been made and techniques developed to measure the mobility of particles and molecules by laser Doppler velocimetry and capillary electrophoresis. We propose here to evaluate and compare these two measurement techniques for their ability to characterize various fluorescent polystyrene beads as a function of the buffer pH. The repeatability of electrophoretic mobility determination by the two techniques in buffer at different pHs (neutral to alkaline) was first examined and compared. The accuracy of the determination was then evaluated. A wide range of beads which varied in their size (diameters ranging from 270 to 1000 nm), surface functional groups (NH2, COOH, and neutral), and the presence or absence of surfactants or incorporated dye molecules were investigated in order to perform a comprehensive study. The results indicated that apart from large amino beads (with a diameter over 800 nm), capillary electrophoresis generally gave better or similar relative standard deviations for most polystyrene beads, which could be attributed to a stronger adsorption of these beads onto the silica capillary surface in CE. Beads with neutral pH were more difficult to measure accurately with both methods. We also concluded that capillary electrophoresis measurements are not accurate for amino beads in the pH range of this study. However, both methods were capable of distinguishing polystyrene beads with different sizes or surface groups. We found that dye molecules introduced in beads did not alter their electrophoretic mobility values. Taken together, the data and discussion provide a guide to choose the right technique to characterize any given set of functional particles precisely and with the highest accuracy.