Minimizing the number of voltage sources and fluid reservoirs for electrokinetic valving in microfluidic devices.

A microchip gated valve is demonstrated that uses a single voltage source and three fluid reservoirs. The fluidic valve is a cross intersection, and the channels are dimensioned to perform the appropriate voltage division, simplifying the voltage control hardware. A single voltage source is applied directly to the sample reservoir and through a high-voltage relay to the buffer reservoir, and the waste reservoir is grounded. The volume of sample dispensed is determined by the duration that the high-voltage relay is open. Volumetric reproducibility is demonstrated to be <0.5% relative standard deviation for volumes of ≥20 pL. The valve is tested for the minimum applied voltage necessary for leakage-free operation, i.e., sample diffusing from the cross intersection into the analysis channel. Moreover, appropriate channel dimensions are used to minimize the number of fluid reservoirs allowing effluent from the analysis and waste channels to be combined into a single reservoir.

Isotachophoresis at pH extremes: theory and experimental validation.

The evolution of an isotachophoresis (ITP) system in acidic or basic pH ranges can be quite different from that predicted by the existing theory. It was found theoretically and proved experimentally that the contribution of hydrogen or hydroxyl ion to conductivity of solution and/or its net charge changes the behavior of the ITP system, creating in the terminating electrolyte an additional zone close to the initial interfaces between electrolytes (leader and terminator). One boundary of the zone, being either sharp or dispersed, moves toward the leader; the other is always sharp and stationary and coincides with initial electrolytes' discontinuity. The latter can be registered in the presence of electroosmotic flow which delivers it to the detection point. In order to describe the dynamics of the ITP system at pH extremes an algorithm of analytical solution was developed, based on the revised Kohlrausch theory. Its predictions coincide well with computer simulations and experimental data. The results presented can help in a correct analysis of ITP data and explain some confusing phenomena which were considered to be artifacts.

Computer simulations of electrokinetic transport in microfabricated channel structures.

A mathematical model describing electrokinetically driven mass transport phenomena in microfabricated chip devices is presented in this paper. The model accounts for principal material transport mechanisms such as electrokinetic migration (electrophoresis and electroosmosis) and diffusion. A computer code that implements the model is capable of simulating transport of materials during electrokinetic manipulation in 2-D channel structures. The computer code allows arbitrary channel geometries with various boundary conditions for the electric field and the sample concentration. Two fundamental microfluidic chip elements, a cross and a mixing tee, are of particular interest. An electrokinetic focusing experiment using a cross structure and mixing in a tee structure are simulated. Simulations revealed an optimum focusing voltage for which the ratio of sample concentration to sample width is maximized. They also verified that the mixing tee provides very accurate dilution/mixing characteristics for both charged and neutral samples. Good agreement between simulated and experimental data verified the accuracy of the mathematical model.

On the solvent motion in electrophoretic systems.

A corrected model describing transport processes for multicomponent mixtures in electric field is proposed. This model is more consistent compared to the other models of this sort used for simulations of electrophoresis in the case of concentrated solutions. The main idea underlying the model is in accounting for the motion of a solvent. Usually the concentrations of solutes are considered small compared to that of a solvent and the equation describing its motion is not considered. This automatically leads to the violation of momentum balance and, hence, to the defects of the model itself. In the model presented in the paper by means of redefining the mass fluxes the balance of momentum is satisfied automatically while the equations governing the evolution of the system look more symmetric and simple. This model allows us to discover some fine effects in evolution of the mixture and clarify the essence of some conservation laws (conservation of the Kohlrausch function) arising in the simplified mathematical models describing electromigration phenomena.

'Tunable' positive and negative surface charges on a capillary wall: exploiting the Immobiline chemistry.

The Immobiline (weak acrylamido acids and bases) chemistry has been applied to the covalent attachment of a positively (or, if needed, negatively) charged layer onto the inner surface of the silica wall. In particular, the following basic Immobilines have been used: pK 6.2, pK 7.0, pK 8.5 and pK 9.3. In order to avoid pK changes, the charged Immobilines are mixed with neutral acrylamido derivatives (in particular the highly resistant and hydrophilic N-acryloyl aminoethoxyethanol) so as to form a co-polymer having a 1:5 molar ratio (charged to neutral). The mu(eo) vs. pH curves have a slope opposite to that of a naked capillary and fan out on the pH scale following the titration curves of the different weak bases. Such chemistry allows the covalent attachment of charged species having known pK values and offering controlled charged densities on the wall. However, with the atomic force microscope, it is found that such soft coatings (whether charged or neutral) do not seem to provide complete coverage of the surface: naked patches of fused silica are found interdispersed among the polymer-coated ones. A good solution is a hybrid bonded and dynamic coating, obtained by adding short chain linear polyacrylamides to the background electrolyte. Good separations of polycations [poly(L-histidine)] and of histones are reported up to pH 5.7.

On the measurements of electrophoretic mobilities by means of capillary isotachophoresis at a constant voltage.

A method for measuring electrophoretic mobilities by means of isotachophoresis (ITP) at a constant voltage as described by H. Carchon and E. Eggermont (Electrophoresis, 1982, 3, 263-274) is analyzed. An error made in this work, disregarding the pH shift arising at the initial discontinuity on the leader-terminator boundary, has been corrected. This method has been carefully studied and generalized for the presence of constant electroosmotic flow in a capillary. The limits of its applicability and the diffusionless ITP theory in general are discussed. A detailed study of the evolution of initial discontinuity (stationary boundary) showed some anomalies not reported previously, particularly non-monotonic concentration profiles in the vicinity of stationary boundaries. Moreover, in some cases, diffusion effects and the contribution of H+ ions can also strongly influence the behavior of moving boundaries. Computer modelling (confirmed by experimental data) showed that these effects could lead to the decay of the ITP train, despite the fact that the steady state diffusionless ITP theory predicts its stability.

Quantitative studies of different injection systems in capillary electrophoresis.

For a rigorous assessment of the precise amount of sample loaded, for quantitation purposes, different sample injection systems were evaluated with two commercially available units, Waters Quanta 4000 and Beckman P/ACE 2100. In the first system, sample introduction by hydrostatic means (i.e., placing the sample vial at some height, usually 10.1 cm, above the other capillary end) was evaluated. It was found that in this system there is a constant positive bias, i.e. the amount of sample loaded lies on a curve parallel and above the theoretically predicted loading curve. However, the excess of mass loaded was constant along the injection times explored (covering from 5 to 35 s) and, for a 75 microns capillary, was found to be of the order of +6 nL (above the expected injected value). Thus it is easy to correct for this sample bias. In the electrokinetic mode, a very good correlation between expected and predicted sample loads was obtained for both units. In the pressure system (by positive pressure from a nitrogen tank, Beckman unit) a substantial discrepancy was found between experimental and predicted values (13.5% overload). Since the manufacturer claims a constant pressure of 0.5 psi, i.e. 3447.5 Pa, it would appear that the injection pressure is higher than the given value.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer simulation of transient states in capillary zone electrophoresis and isotachophoresis.

Transient states in the evolution of electrophoretic systems comprising aqueous solutions of weak monovalent acids and bases are simulated. The mathematical model is based on the system of nonstationary partial differential equations, expressing the mass and charge conservation laws while assuming local chemical equilibrium. It was implemented using a high resolution finite-difference algorithm, which correctly predicted the behavior of the concentration, pH and conductivity fields at low computational expense. Both the regular and the irregular modes of separation in capillary zone electrophoresis and isotachophoresis are considered. It is shown that the results of separation, particularly zone order, strongly depend on pH distribution. Simulation data as well as simple analytical assessments may help to predict and correctly interpret the experimental results.