We have developed an instrument that can simultaneously detect positive and negative scanning ion images of the same location on the same sample. It consists of a primary ion irradiating column, sample chamber, detection system for secondary ions and data processing and imaging system. The scanning ion probe uses a gallium liquid metal ion source with a sealed-cone-type reservoir. Positive and negative secondary ions sputtered from the same point on a sample by the scanned gallium ion beam are simultaneously extracted and converted into electrons at two separate ion-electron converters. The electrons are then accelerated toward scintillators to generate light signals, which are then detected by means of photomultipliers. The output signals of the negative and positive ion detector are then simultaneously processed by the data acquisition system in synchronization with the primary ion scanning signal. (C) 2004 Elsevier B.V. All rights reserved.