The introduction of a parallel approach in analytical chemistry in the form of microtiter plate technology has been a significant contribution to the increase of the throughput and a substantial reduction of the cost per analysis. Currently in biotechnology much effort is spent on the unravelling of the genome of living cells and an even larger effort will be spent on the proteome. In addition, combinatorial chemistry is increasingly applied in many areas of (bio)chemistry. Rapid progress in these domains is crucially dependent on a highly parallel, high throughput approach that can only be achieved by a substantial miniaturisation of the existing technology. Scaling down the well diameter of the 5 mm wells in a classical 96 well microtiter plate to 200 muM as required for the fabrication of high density arrays implies that liquid volumes become even more reduced, from microliters to picoliters. Special techniques are needed to accurately dispense such extremely small volumes of reagent solutions (enzymes, antibodies, etc.) on specified spots of a suitable substrate. Currently available techniques are based on direct contact or piezo-electric dispensing. Direct contact methods require time-consuming washing and drying steps when dispensing multiple reagents. Disadvantages of piezo-electric dispensing are the difficulty in handling viscous liquids and liquids containing surfactants. Furthermore, the reagents are exposed to high shear forces, and cross contamination is likely to occur due to splashing [1]. Finally, the relatively large dispensing heads require a spacing in the order of 3 mm and therefore do not allow for parallel dispensing of spots in a straight forward manner, which limits its applicability. Electrospraying in the stable cone-jet mode is used by us as a powerful alternative technique to dispense arrays of micrometer sized spots. Optimal conditions for dispensing were achieved by combining precision liquid delivery with precision positioning using motion controlled high precision actuators. Flow rates down to 50 pL.s(-1) were achieved with a satisfactory accuracy and reproducibility (CV < 7%). Furthermore, it is possible to confine the diameter of the electrosprayed spots to 200 <mu>M, and parallel dispensing is proven to be possible as is shown here with a set-up consisting of three needles, allowing for the production of high-density arrays.
