Tandem Ion Mobility Spectrometry for the Detection of Traces of Explosives in Cargo at Concentrations of Parts Per Quadrillion

A tandem ion mobility spectrometer (IMS2) built from two differential mobility analyzers (DMAs) is coupled at ambient pressure with a thermal fragmenter placed in between, such that the precursor ions selected in the first DMA are thermally decomposed at ambient pressure in the fragmenter and the product ions generated are filtered in the second DMA. A thermal desorber and a multicapillary gas chromatography (GC) column are coupled to a secondary electrospray (SESI) ion source, so the adsorption sampling filters are thermally desorbed and the liberated vapors are separated in the GC column, prior to their ionization and mobility/mobility classification. The new fragmenter allows the fragmentation of the five explosives studied: RDX, PETN, NG, EGDN, and TNT. The background of the analyzer is evaluated for the five explosives using air samples of 500 L volume. An atmospheric background of only 2.5 pg (5 ppq) is found for TNT, being somewhat higher for the rest of explosives studied. The architecture GC-IMS2 is compared with GC-IMS obtaining a 100-fold increase of sensitivity in the first configuration, confirming the high selectivity provided by the fragmentation cell and the second IMS stage for the product ion mobility analysis. The analyzer is tested also with real explosives hidden in cargo pallets achieving successful detection of four (EGDN, NG, TNT, and PETN) out of five explosives.