Contactless Conductivity Detector for Microchip Capillary Electrophoresis: Fast Measurements of Explosives and Explosive Residues

Abstract

INTRODUCTION Improvised explosives have been used in recent terrorist bombing activities. These home-made explosives generates significant amount of inorganic ions, such as ammonium, methylammonium, potassium, sodium, perchlorate, chloride or nitrate. Such explosives and explosive residues are frequently analyzed using ion-chromatography (IC) or capillary electrophoresis (CE) systems. However, these laboratory-based methods are not field portable, nor can they analyze relevant samples within extremely short (<30 sec) time scales. Therefore, fast-responding field-deployable analytical systems are desired for monitoring ionic (“low-energy”) explosives at the sample source. Lab-on-a-chip technology offers great possibility for obtaining the desired forensic information in a faster, simpler, and cheaper manner compared to traditional laboratory-based instruments. This paper describes a microchip capillary electrophoresis with an integrated contactless conductivity detection system in connection to a low-energy explosives detection. The new contactless conductivity microchip detector is based on placing two planar sensing aluminum-film electrodes on the outer side of a microchip (without contacting the solution) and measuring the impedance of the solution in the separation channel. The contactless route obviates problems (i.e., fouling, unwanted reactions) associated with the electrode-solution contact, offers isolation of the detection system from high separation fields, and greatly simplifies the detector fabrication. Relevant experimental variables, such as the frequency and amplitude of the applied ac voltage were examined and optimized. The detector performance was illustrated by the separation of low-explosive ionic components, such as the potassium, sodium, ammonium and methylammonium cations and the nitrate, perchlorate and chloride anions.