Establishing a universal swabbing and clean-up protocol for the combined recovery of organic and inorganic explosive residues.

A single-step solvent extraction and a solid-phase extraction (SPE) clean-up procedure was developed and optimised in order to establish a universal sampling and clean-up protocol for the combined recovery of organic and inorganic explosive residues. Mixtures of three common swabbing solvents (acetone, acetonitrile and methanol) with water, in various ratios, were assessed for the extraction of four target organic explosives [pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and triacetone triperoxide (TATP)] and two inorganic anions (chlorate and nitrate) from alcohol wipes that were used as a swabbing medium. An efficient, single-step extraction of both organic and inorganic compounds from the wipes was achieved using 60% v/v methanol/water. To develop a clean-up procedure, four commercially available SPE cartridges (Oasis HLB, Isolute(®) C18, Bond-Elut(®) ENV and ABS ELUT Nexus) and an in-house packed XAD-7 cartridge were firstly evaluated for their retention capacity toward three organic explosives (PETN, TNT and RDX) in a mixture of methanol and water. A SPE technique was then developed and optimised from the short-listed sorbents with four representative organic explosives (including TATP). The Nexus cartridge was found to provide a suitable sorbent for extract clean-up following swab extraction with 60% v/v methanol/water. By incorporating the optimised clean-up procedure with the application of a polyester-based alcohol wipe as a sampling medium, a universal swabbing protocol for the combined recovery of both organic and inorganic explosive residues was established. The feasibility of the proposed protocol was assessed by collection and quantitation of the residue from a mixture of TNT, PETN and chlorate deposited on a laminate test surface.