Jan C.T. Eijkel

A circular ac magnetohydrodynamic micropump for chromatographic applications

Abstract A circular ac magnetohydrodynamic (MHD) micropump for chromatographic purposes has been developed. The device consists of a glass–gold-laminate–glass sandwich structure, with the channel structure defined in the electroformed gold layer. Channels were 200xa0μm wide and 30xa0μm high. Experimental details on the manufacturing of the device and the optimisation of the setup are presented. Reversible flow of maximally 40xa0μmxa0s−1 has been obtained. The flow speed attained and the small channel height make the device in principle suitable to perform circular chromatography.

Measuring Donnan-related phenomena using a solid-state ion sensor and a concentration-step method

Measurements are performed with a device consisting of an ISFET pH-sensor in the middle of a Ag/AgCl electrode, on top of which a microporous composite membrane is deposited. A sudden change of the salt concentration in the bathing electrolyte causes a transient change in the electrical potential of these sensors when measured vs. a reference electrode in the bathing electrolyte. The potential transient is modulated by adsorption of protein to the membrane. To explain the measured transients, a model is presented for the measuring device describing the ion transport by the Nernst-Planck and Poisson equations, incorporating the different proton-dissociation reactions occurring in the system, and the sensor responses to their potential determining ions (the proton or the Cl− ion). A finite-difference solution method is presented to solve the resulting differential equations. n nMeasurements are performed before and after the adsorption of the model protein lysozyme to the membrane. Analysis of the measurement results indicates that the measured potential transient is caused by a change of the Donnan potential of the membrane, followed by a compensating change in the concentration of the potential determining ion. It is proven that no diffusion potential is generated. In addition, it is shown that an interlayer of electrolyte between membrane and measuring electrode will not influence the measured response. n nThe potential transients measured by the ISFET have a larger amplitude and a longer duration than the Ag/AgCl-measured transients. An analysis shows that this is caused by the buffering action of the proton-dissociating membrane groups. The longer duration results from the release of a large amount of protons from binding to fixed groups, while chloride ions are not bound. The larger amplitude can be explained by refining the Donnan model to account for the inhomogeneous charge distribution in the membrane. The proton-dissociating groups reside at the surface of the polystyrene beads, at which place the potential change on an ion step is larger than the average in the membrane pore solution. This surface-potential change can be measured by the pH-sensitive ISFET because a proton release occurs from the surface-bound groups into the membrane pores, changing the pore pH.

An atmospheric pressure dc glow discharge on a microchip and its application as a molecular emission detector

Scaling theory for direct current glow discharges predicts that normal discharges can exist at atmospheric pressure in microscale discharge tubes. The validity of this theory is demonstrated by the creation of an atmospheric helium plasma in a nanoliter-size discharge chamber on a microchip. It is shown that the microchip plasma can be successfully applied for molecular emission detection. It exhibits a low detection (10−14xa0gxa0s−1) limit for methane with a calibration curve that is linear over two decades. Simple instrumentation, small detector size and good sensitivity make the device highly suitable for integration in micro-analysis systems.

An AC electroosmotic micropump for circular chromatographic applications.

Flow rates of up to 50 microm s(-1) have been successfully achieved in a closed-loop channel using an AC electroosmotic pump. The AC electroosmotic pump is made of an interdigitated array of unequal width electrodes located at the bottom of a channel, with an AC voltage applied between the small and the large electrodes. The flow rate was found to increase linearly with the applied voltage and to decrease linearly with the applied frequency. The pump is expected to be suitable for circular chromatography for the following reasons: the driving forces are distributed over the channel length and the pumping direction is set by the direction of the interdigitated electrodes. Pumping in a closed-loop channel can be achieved by arranging the electrode pattern in a circle. In addition the inherent working principle of AC electroosmotic pumping enables the independent optimisation of the channel height or the flow velocity.

On-line monitoring of chromium(III) using a fast micromachined mixer/reactor and chemiluminescence detection

A fast micromachined mixer/reactor was employed in a ncontinuous flow injection set-up for the direct detection of nchromium(III) in aqueous samples by chemiluminescence (CL). With nthe device Cr(III) can be detected at a concentration of n10−7 M and a linear calibration curve was obtained from n10−6 to 10−4 M Cr(III). In the ncontinuous flow injection mode the system can perform fully automated ndetection with a reagent consumption of only 43.2 mL per day. In addition, na general theory is presented for the performance of diffusion-based nmixer/reactor devices. This is the first report of the use of a nmicromachined device for the quantitative detection of Cr(III) nwith CL. Simple instrumentation, easy operation and low reagent consumption nmake this system a potential candidate for the in situ monitoring nof Cr(III).

Miniaturization and chip technology. What can we expect

Miniaturization and chip technology play an important role for analytical chemistry instrumentation in the future. A brief theory of the relevant microfluidics with reasons for miniaturization is given.

Evaporation driven pumping for chromatography application.

A continuous transport process for liquids in micro-channels is reported. Flow was generated by evaporation at the channel end plus capillary forces. The micro-channels integrated into a two-glass-layer device were 110 microm wide, 28 microm deep and 4 or 10 cm long. A continuous liquid transport velocity of up to 2.25 mm s(-1) was observed for aqueous solutions. The flow velocity is shown to increase when an air stream is guided over the evaporation zone.

Towards an on-chip gas chromatograph: the development of a gas injector and a dc plasma emission detector

Work is reported on two components for a gas chromatograph-on-a-chip, namely an injector and a dc plasma emission detector. The injector uses two intersecting channels on a chip for injection. This geometry is commonly used in on-chip capillary electrophoresis, and is here for the first time applied for gas injection. Off-chip valves are employed to direct gas flows. Successful injection of a plug of air in helium is demonstrated by off-chip detection with a dc micro-plasma detector. The work on the dc plasma emission detector was aimed at general characterization of dc micro-plasmas and the detection of halogen atoms. Detection of bromine and chlorine is demonstrated from their atomic lines. A micro-plasma chip is coupled to a conventional gas chromatograph and specific detection of elements and molecular fragments in the eluted peaks is demonstrated. The detection limit for chlorine is shown to be about 8.10−10 g Cl s−1.

Micromachined heated chemical reactor for pre-column derivatisation

A micromachined heated chemical reactor is presented. The microreactor consists of a sandwich of glass and silicon chips and contains a reactor channel of 50 μl volume. On-chip heating resistors supply a power of 2 W, resulting in a heating rate of 2°C/s. On-chip measuring resistors enable feed-back control of the temperature. The reactor volume makes the device suitable for derivatisation purposes in HPLC. As an example the pre-column derivatisation of amino acids with NBD-F is demonstrated, requiring 2 min of heating at 60°C, followed by HPLC separation and fluorescence detection.

An ISFET-based dipstick device for protein detection using the ion-step method

A compact and easy-to-operate device for the detection of proteins in solution is described. The device consists of an ISFET pH-sensor in the middle of a Ag/AgCl electrode, on top of which a microporous composite membrane is deposited. The measurement proceeds by offering an ion step and measuring the transient change of the ISFET potential versus the Ag/AgCl electrode. The potential transient is modulated by adsorption of protein to the membrane by incubation in a protein-containing solution. Theory is developed to describe the ISFET potential transient versus the Ag/AgCl electrode starting from the ISFET and Ag/AgCl potential transients versus an external reference electrode. Conditions are formulated under which the Ag/AgCl electrode potential is sufficiently stable to be used as the reference. Both theory and preliminary measurements show that this condition is met under several characteristic measuring conditions. When the internal reference electrode is used, the ion step can be offered simply by manual transference (`dipping?) of the device between vessels of different salt concentration instead of by using a complex flow-through set-up. Measurements show a remarkable absence of noise in the potential signal of the dipstick device.