Electroosmotic flow measurement

Kim Ml, Kim HJ, and Kihm KD 2001 Micro-scale PIV for electroosmotic flow measurement. Proceedings of PSFVIP-3, March 18-21, 2001, Maui, Hawaii, USA. 

McKillop and associates have examined the electrophoretic separation of alkylpyridines by CZE. Separations were carried out using either 50-pm or 75-pm inner diameter capillaries, with a total length of 57 cm and a length of 50 cm from the point of injection to the detector. The run buffer was a pH 2.5 lithium phosphate buffer. Separations were achieved using an applied voltage of 15 kV. The electroosmotic flow velocity, as measured using a neutral marker, was found to be 6.398 X 10 cm s k The diffusion coefficient,... 

FIGURE 11.32 Flow profiles in microchannels, (a) A pressure gradient, - AP, along a channel generates a parabolic or Poiseuille flow profile in the channel. The velocity of the flow varies across the entire cross-sectional area of the channel. On the right is an experimental measurement of the distortion of a volume of fluid in a Poiseuille flow. The frames show the state of the volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule, (b) In electroosmotic flow in a channel, motion is induced by an applied electric field E. The flow speed only varies within the so-called Debye screening layer, of thickness D. On the right is an experimental measurement of the distortion of a volume of fluid in an electroosmotic flow. The frames show the state of the fluorescent volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule [165], Source http //www.niherst.gov.tt/scipop/sci-bits/microfluidics.htm (see Plate 12 for color version). 

The low electroosmotic flow (EOF) of the PMMA chip material facilitated the rapid switching between analyses of explosive-related cations and anions using the same microchannel and run buffer (and without an EOF modifier) [29], This led to a rapid (<1 min) measurement of seven explosive-related cations and anions down to the low micromolar level. The presence of an 18-crown-6 ether modifier in the run buffer allowed separation of the peaks of the co-migrating ammonium and potassium ions. 

Typically, buffers in the region of pH 7-9 have been used in MEEKC. At these pH values the buffers generate a high electroosmotic flow (EOF). Extreme values of pH have been used in MEECK specifically to suppress solute ionization. For example, a pH of 1.2 of the buffer has been used to prevent the ionization of acids (30,31). To eliminate the ionization of basic compounds, a buffer at pH 12 has been used. These pH values were used in MEEKC to measure the solubility of ionic compounds (30). High-pH carbonate buffers (31) were applied in place of the standard borate or phosphate buffers. 

L. E. Locascio, C.E. Perso and C.S. Lee, Measurement of electroosmotic flow in plastic imprinted microfluid devices and the effect of protein adsorption on flow rate, J. Chromatogr. A, 857 (1999) 275-284. 

Pikal, M.J., and S. Shah. 1990. Transport mechanisms in iontophoresis. II. Electroosmotic flow and transference number measurement for hairless mouse skin. Pharm Res 1 (3) 213. 

The external electric field is in the direction of the pore axis. The particle is driven to move by the imposed electric field, the electroosmotic flow, and the Brownian force due to thermal fluctuation of the solvent molecules. Unlike the usual electroosmotic flow in an open slit, the fluid velocity profile is no longer uniform because a pressure gradient is built up due to the presence of the closed end. The probability of the particle position is obtained by solving the Fokker-Planck equation. The penetration depth is found to be dependent upon the Peclet number, which is a measure of significance of the convective electroosmotic flow relative to the Brownian diffusion, and the Damkohler number, which is a ratio of the characteristic diffusion-to-deposition times. 

Electroosmotic hold-up time (in capillary electromigration), teo — Time required for a liquid in a capillary to move due to -> electroosmosis through the effective length of the capillary, Leff. This time is usually measured as the -> migration time of a neutral compound, called an electroosmotic flow marker, which is assumed to have an -> electroosmotic mobility that is negligible compared to that of the analyte. 

The parameter normally measured in capillary electrophoresis is migration (retention) time, /. In a given CE system this parameter is inversely proportional to the electrophoretic mobility, pi. The pt (cm /V) is a normalized parameter allowing for comparison of data obtained in different CE systems. If a series of analytes are analyzed under the same conditions then the 1/r and pt are equivalent. There are only a few reports on QSRR analysis of CE data. This may suggest the unsuitability of routinely determined mobility parameters as the LEER descriptors of analyte behaviour. Probably the reproducibility of analyte migration times in CE is poor due mainly to the non-reproducible electroosmotic flow velocity 26. ... 

In most electroosmotic flows in microchannels, the flow rates are very small (e.g., 0.1 pL/min.) and the size of the microchannels is very small (e.g., 10 100 jm), it is extremely difficult to measure directly the flow rate or velocity of the electroosmotic flow in microchannels. To study liquid flow in microchannels, various microflow visualization methods have evolved. Micro particle image velocimetry (microPIV) is a method that was adapted from well-developed PIV techniques for flows in macro-sized systems [18-22]. In the microPIV technique, the fluid motion is inferred from the motion of sub-micron tracer particles. To eliminate the effect of Brownian motion, temporal or spatial averaging must be employed. Particle affinities for other particles, channel walls, and free surfaces must also be considered. In electrokinetic flows, the electrophoretic motion of the tracer particles (relative to the bulk flow) is an additional consideration that must be taken. These are the disadvantages of the microPIV technique. 

Some types of electrophoretic cells are stationary layer problem free , but in the other cells the electroosmotic flow can lead to erroneous results. The observed velocity of particles is a sum of the electroosmotic flow of the fluid and the velocity of particles with respect to the fluid. The latter is a function of the potential of the particles and the former is a function of the position in the cell cross section. Hydrodynamic calculations make it possible to find the stationary levels, i.e. the positions in the cell cross section where the electroosmotic flow equals zero. Certainly the position of stationary levels in commercial electrophoretic cells can be found in the user s manual, and there is no need to perform any calculations. The fastest method to determine the electrophoretic mobility is from the velocity at one stationary level, but such a procedure can lead to substantial errors. For example, when the cell position is adjusted at room temperature and then measurements taken... 

In microelectrophoresis the dispersed particles are viewed under a microscope and their electrophoretic velocity is measured at a location in the sample cell where the electric field gradient is known. This must be done at carefully selected planes within the cell because the cell walls become charged as well, causing electroosmotic flow of the bulk liquid inside the cell. 

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