Sensitive Detection of Black Powder by a Stand-Alone Ion Mobility Spectrometer with an Embedded Titration Region

Sensitive detection of black powder (BP) by stand-alone ion mobility spectrometry (IMS) is full of challenges. In conventional air-based IMS, overlap between the reactant ion O₂^–(H₂O)_n peak and the sulfur ion peak occurs severely; and common doping methods, providing alternative reactant ion Cl^–(H₂O)_n, would hinder the formation of ionic sulfur allotropes. In this work, an ion mobility spectrometer embedded with a titration region (TR-IMS) downstream from the ionization region was developed for selective and sensitive detection of sulfur in BP with CH₂Cl₂ as the titration reagent. Sulfur ions were produced via reactions between sulfur molecules and O₂^–(H₂O)_n ions in the ionization region, and the remaining O₂^–(H₂O)_n ions that entered the titration region were converted to Cl^–(H₂O)_n ions, which avoided the peak overlap as well as the negative effect of CH₂Cl₂ on sulfur ions. The limit of detection for sulfur was measured to be 5 pg. Furthermore, it was demonstrated that this TR-IMS was qualified for detecting less than 5 ng of BP and other nitro-organic explosives.