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.