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 presented. Indirect laser-induced-fluorescence (IDLIF) was used to detect explosive compounds after separation by micellar electrokinetic chromatography (MEKC). The detection setup was used in an epifluorescence configuration with excitation provided by a near-IR diode laser operating at 750 nm. To achieve indirect detection, a low concentration of a dye (5 microM Cy7) was added to the running buffer as a visualizing agent. Using this methodology, a sample containing 14 explosives (EPA 8330 mixture) was examined. Concentrations of 1 ppm of trinitrobenzene (TNB), trinitrotoluene (TNT), dinitrobenzene (DNB), tetryl, and 2,4-dinitrotoluene (2,4-DNT) could be detected with S/N ratios between 3 and 10. Analyses showing 10 peaks, with plate numbers on the order of 60000, were completed within 60 s using a 65 mm long separation channel. The three isomers of nitrotoluene (2-, 3-, and 4-nitrotoluene) were not resolved. Additionally, the two nitramines (HMX and RDX) could only be detected at much higher concentrations, likely due to the low fluorescence quenching efficiencies of these compounds. The analysis method was also used to separate and detect nitroaromatic compounds in extracts from spiked soil samples. The presence of 1 ppm (1 microg 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 that a 250 microm double-T injector gave a 35% increase in peak signal compared to a straight-cross injector, which is less than expected based on injected volume.