Chromatographic detection of nitroaromatic and nitramine compounds by electrochemical reduction combined with photoluminescence following electron transfer

The oxidizing agent tris(bipyridyl)ruthenium(III), or Ru-(bpy)(3)3+, is used as a postcolumn reagent for the detection of nitroaromatic and nitramine explosive compounds. After separation, the explosives are reduced electrochemically to oxidizable products such as hydroxlamines and nitrosamines, and these products react readily with Ru-(bpy)(3)3+ and Ru(bpy)(3)2+. The photoluminescence from the latter is used for detection. A porous carbon electrode was used for on-line analyte reduction following chromatography. Another porous carbon electrode was used to generate the nonluminescent Ru(bpy)(3)3+ from Ru(bpy)(3)3+ on-line at high efficiency. The two streams were combined, and the Ru(bpy)(3)2+ produced by oxidation of the reduced analytes was detected by laser illumination and light detection. Reductive hydrodynamic voltammograms of nitrobenzene, 2,4,6-trinitrotoluene, and hexahydro-1,3,5-trinitro-1,3,5-triazine indicated that a potential of - 1500 mV vs Ag/AgCl was sufficient to achieve a maximum signal from the reduced analytes. HPLC with a water/acetonitrile gradient on a C-18 reversed-phase column was then used to determine these three compounds plus the four additional examples, 1,3,5,7-tetrazocine, 2,4-dinitrotoluene; 2,6-dinitrotoluene, and 4-nitrotoluene. For both hydrodynamic voltammetry and HPLC detection, the photoluminescence following electron-transfer signal was calibrated using the one-electron standards ferrocene and ferrocenecarboxylic acid. Detection limits were in the low-nanomolar range for 20-microL injections of nonpreconcentrated nitro compounds.