The effects of an additional electric field established near the capillary nozzle on the performance of the electrospray were evaluated through experiments and simulations. Applying the additional electric field, a circular guard plate was connected to a capillary nozzle. The stable cone-jet voltage, droplet size, spray plume angle, and spray current were measured for three types of nozzles. An electrospray nozzle with a nonconducting guard plate (nozzle I), and electrospray nozzles having a conducting guard plate (nozzles II, III) were employed. For the electrospray with the conducting guard plate, the stable cone-jet mode is initiated at a higher voltage and maintained in a wider range of applied voltage than that of the electrospray with the nonconducting guard plate, because the conducting guard plate reduces the electric field near the cone-tip and prevents some electric disturbances. In addition, as the capillary tip to the conducting guard plate distance became smaller, the effect of the guard plate on the spray characteristics appeared more clearly. For the motion of spray droplets, the spray angle became smaller for the case employing the conducting guard plate. The spray plume angles of nozzles II and III were about 17degrees and 24degrees, respectively, while the spray plume angle of nozzle I was about 68degrees. On the other hand, if only the cone-jet mode is initiated, there is not a big difference between all three types of nozzles for the droplet size and the spray current. Therefore, a conducting guard plate could be appropriate for some applications which need a narrow spray plume angle, high velocity droplets, and a wide operating voltage range while maintaining the original advantages of the cone-jet mode. (C) 2004 Elsevier Ltd. All rights reserved.