K.M. van Delft

Micro-evaporation electrolyte concentrator

The sensitivity of miniaturized chemical analysis systems depends most of the time on the obtainable detection limit. Concentrating the analyte prior to the detection system can enhance the detection limit. In this writing an analyte concentrator is presented that makes use of evaporation to increase the ion concentration of an electrolyte. The evaporation rate can be enhanced using forced convection. In order to control the evaporation rate a nitrogen flow is fed over a liquid channel covered with a hydrophobic vapor permeable membrane. Water vapor can pass through this membrane in contrast to water itself because of the hydrophobic nature of the membrane surface. An electrolyte conductivity detector is used to measure directly the concentration effect as a function of the nitrogen flow velocity. The influence of the convective nitrogen flow and the residence time of the analyte inside the concentrator are investigated in this paper. It is shown that the evaporation rate is enlarged with an increase in convective flow. The concentration effect is also enhanced when the residence time of the analyte inside the concentrator is increased. The higher concentration enhancement due to the longer residence time, however, results in an increase in water vapor present in the nitrogen flow. This results in a lower normalized evaporation rate when the available evaporation time is enlarged.

A Closed-Loop Calibration System for a Microdialysis-Based Lab-On-A-Chip

In this contribution a closed loop calibration facility for a microdialysis based lab-on-a-chip is presented. Each facility consists of a channel structure etched in silicon, closed by a Pyrex® cover comprising two small platinum electrodes. In the silicon structure the calibration liquid is stored in the meander shaped channel that starts in an electrolyte filled reservoir. By applying an electric current via both electrodes through the electrolyte in the reservoir, gas bubbles are generated by electrolysis, forcing the calibration liquid into the main carrier channel leading to the sensor area of the lab-on-a-chip. Above the electrolyte reservoir two additional interdigitated electrodes are present that are used to measure the gas bubble volume by measuring the impedance of the gas/liquid mixture in the reservoir. Since the volume of the gas bubbles is equal to the dispensed calibration liquid, its total volume can be controlled by a closed loop impedance measuring system. In this way precise plugs of 60nl calibration liquid have been dispensed into the main carrier channel.

Fluorocarbon coated micromachined gas sampling device

A thin micromachined alternative for commercially available hydrophobic microporous membranes used in diffusion gas samplers has been realized. The thin membrane is made of Si3N4 and is etched porous. Underneath the membrane a channel is created. The membrane is rendered hydrophobic with a fluorocarbon coating. Bubble point and contact angle measurements have been performed on the membrane.