Separation and detection of explosives on a microchip using micellar electrokinetic chromatography and indirect laser-induced fluorescence

A new approach for sensitive detection on a microfabricated chip is present ed. Indirect laser-induced-fluorescence (IDLIF) was used to detect explosiv e compounds after separation by micellar electrokinetic chromatography (MEK C), The detection setup was used in an epifluorescence configuration with e xcitation provided by a near-IR diode laser operating at 750 nm, To achieve indirect detection, a low concentration of a dye (5 mu M Cy7) was added to the running buffer as a visualizing agent. Using this methodology, a sampl e containing 14 explosives (EPA 8330 mixture) was examined. Concentrations of 1 ppm of trinitrobenzene (TNB), trinitrotoluene (TNT), dinitrobenzene (D NB), tetryl, and 2,4-dinitrotoluene (2,4-DNT) could be detected with S/N ra tios between 3 and 10, Analyses showing 10 peaks, with plate numbers on the order of 60 000, were completed within 60 s using a 65 mm long separation channel. The three isomers of nitrotoluene (2-, 3-, and 4-nitrotoluene) wer e not resolved. Additionally, the two nitramines (HMX and RDX) could only b e detected at much higher concentrations, likely due to the low fluorescenc e quenching efficiencies of these compounds. The analysis method was also u sed to separate and detect nitroaromatic compounds in extracts from spiked soil samples. The presence of 1 ppm (1 mu g of analyte/1 g of soil) of TNB, DNB, TNT, tetryl, 2,4-DNT, 2,6-DNT, 2-NH2-4,6-DNT, and 4-NH2-2,6-DNT could readily be detected. In the interest of increasing the sensitivity of the analysis, various on-chip injection schemes were evaluated. It was found th at a 250 mu m double-T injector gave a 35% increase in peak signal compared to a straight-cross injector, which is less than expected based on injecte d volume.