Nanoporous Silica Preconcentrator for Vapor-Phase DMNB, a Detection Taggant for Explosives

The detection of trace amounts of explosives in the vapor phase is of great importance. Preconcentration of the analyte is a useful technique to lower the detection limit of existing sensors. A nanoporous silica (pSiO₂) substrate was evaluated as a preconcentrator for gas-phase 2,3-dimethyl-2,3-dinitrobutane (DMNB), a volatile detection taggant added by law to plastic explosives. After collection in pSiO₂, the DMNB vapor was thermally desorbed at 70 °C into a gas chromatography-mass spectrometry sorbent tube. This was analyzed for the total mass of DMNB collected in pSiO₂. The loading time and loading temperature of pSiO₂ were varied systematically between 15 and 60 min and 5-20 °C, respectively. The preconcentrator's performance was compared to that of a nonporous substrate of the same material as a control. The collection efficiency of pSiO₂ was calculated as approximately 20% of the total DMNB that passed over it in 30 min, at a concentration of 0.5 ppm in N₂ carrier gas. It had enhancement factors compared to the nonporous substrate of 12 and 16 for 0.5 and 4.1 ppm DMNB, respectively, under the same conditions. No advantage was found with cooling pSiO₂ below room temperature during the loading phase, which removes any need for a cooling system to aid preconcentration. The low desorption temperature of 70 °C is an advantage over other preconcentration systems, although a higher temperature could decrease the desorption time.