Explosive vapor detection technology in the atmosphere at concentrations of parts per quadrillion and below

Resumen

This doctoral thesis is aimed at the development of new analytical instrumentation for the detection of trace vapor species in the atmosphere at concentrations of parts per quadrillion (ppq = 1x10-15 atmospheres of partial pressure) and below. The work is structured around three overall objectives: • To optimize the analytical performance of the planar differential mobility analyzer (DMA) and the secondary electrospray ionization source (SESI), so as to bring their performance closer to the ideal one. • To develop an explosive vapor detector, based on DMA-triple quadrupole mass spectrometry technology (DMA-MS/MS), capable of detecting explosive vapors at sub-ppq concentrations. • To develop an explosive vapor detector based on analyzers and detectors far simpler and more economical than mass spectrometry, yet still capable of detecting explosive vapors at concentrations of a few ppq. The SESI ionization source was redesigned to achieve successful desolvation of the electrospray cloud, preventing droplets and neutral vapors from reaching the DMA. This improvement yielded ideal tail-free mobility peaks with a tailing ratio (TR) much larger than previously observed (100-1000 times better), reaching the theoretical limit ~105 (Gaussian peaks). Furthermore, the ionization efficiency was also improved, delivering an unprecedented value for TNT of one ion out of 140 neutral molecules. The planar DMA was adapted to minimize vapor emissions which, combined with the desolvation in the SESI ion source, allowed eliminating a previously existing mobility peak tailing problem phenomenon. The laminarization in the DMA was also improved, allowing to achieve resolutions of 110, practically matching the theoretical limit of the planar DMA at the operating conditions. The improvements in the DMA and the SESI ion source significantly enhanced the analytical performance of the planar DMA, enabling the development of an explosive vapor detector, based on DMA-MS/MS technology with unprecedented sensitivity, capable of detecting explosive vapors at sub-ppq concentrations. The method of detection was patented and, through the analysis of hundreds of blank and loaded air samples, has demonstrated the capacity to detect explosive vapors at unmatched concentrations of only 0.01 ppq for RDX and 0.1 ppq for TNT. Despite the unique performance of the DMA-MS/MS technology, the use of mass spectrometry involves a higher cost (also volume and complexity) of the analyzer, limiting the number of potential customers. This fact motivated the development of a radically new technology, called tandem differential mobility analysis with ambient pressure ion fragmentation in between (henceforth DMA-F-DMA), which emulates the operation principle of the triple quadrupole mass spectrometer, at a substantially reduced level of complexity and price thanks to the operation at ambient pressure instead of under vacuum conditions. The traditional approach of ion mobility spectrometry has been used in airport security for several decades; however, its limited selectivity has limited its operativity, precluding vapor detection of most explosives. The radically new method presented here has the capacity to detect explosive vapors at concentrations of a few ppq. A first DMA-F-DMA prototype was used as a proof of concept to demonstrate the fragmentation feasibility of five explosives (RDX, PETN, NG, EGDN and TNT) at ambient pressure by thermal means exclusively. A second prototype was built, including a new fragmenter with better fragmentation ability and combining multicapillary gas chromatography in series with the DMA-F-DMA technology, which improved selectivity. The analyzer background was evaluated for the five explosives studied, using air samples of 500 L volume. An atmospheric background of only 5 ppq (2.5 pg in 500 L) was found for TNT, being somewhat higher for the rest of the explosives studied. The GC-DMA-F-DMA technology was also tested with commercial explosives hidden in cargo pallets at the Spain’s National Institute of Aerospace Technology (INTA), achieving successful detection of four (EGDN, NG, TNT and PETN) out of the five explosives. The DMA-F-DMA development has been funded by the United Kingdom Government by three consecutive projects (EffeX, EffeX II and HITEX) and also by the Horizon 2020 Research and Innovation Programme (COSMIC), highlighting the potential of this technology. The tandem mobility analysis with intermediate ion fragmentation at ambient pressure has recently been demonstrated (results published in 2019) by only one other research group worldwide, illustrating the degree of novelty of this technology.