Chemical weapons are still a threat, although banned
All lethal chemical weapons were banned by the Chemical Weapons Convention of 1997, which has been signed by nearly every country in the world. However, the recent use of chemical weapons in Syria shows that they remain a threat. The use of such weapons by terrorist groups is also possible and could conceivably cause large-scale loss of life. Initial detection of chemical agents in the field is often carried out by ion mobility spectrometry, with confirmation by portable GC-MS. However, less volatile nerve agents, such as VX and related compounds, are not easy to detect by the latter method. INFICON’s Hapsite ER instrument is able to detect a wide range of volatile nerve agents and other chemical weapons by GC-MS, but cannot detect VX, which is strongly retained on the sampling probe, column and hydrophobic membrane. The Japanese researchers introduced a pre-column treatment step in which silver fluoride converted compounds such as VX to more volatile fluorine compounds (i.e. F replacing SR on the phosphorus in the case of VX and its analogues), which are readily detected by GC-MS. This is known as a VX-G conversion, since the products are EtGB in the case of VX and iBuGB in the case of Russian VX (RVX, which possesses an isobutyl group rather than the ethyl group found in VX).
VX and analogues converted to fluoride derivatives for GC-MS
The nerve agents were taken into the Hapsite ER instrument and reacted in a VX-G conversion tube containing a silver fluoride pad (a mixture of silver nitrate and potassium fluoride). Each tube had either a filter pad after the silver fluoride pad or else two filter pads forming a sandwich around the silver fluoride pad. The tubes were heated using a hot air gun. The volatiles were taken through the Hapsite ER’s Tri-Bed concentrator, which uses carbon-based sorbents to concentrate vapours. GC was carried out using the Hapsite ER system, which was fitted with a DB-1 capillary column run from 60 to 200 °C in a series of ramps. Bromopentafluorobenzene was used as an internal standard. The Hapsite ER’s quadrupole mass spectrometer with a membrane inlet and electron ionisation (EI) in positive mode gave mass spectra for the fluorine-containing species, which were confirmed by comparison with spectra from the NIST spectral library. Total ion chromatograms (TIC) and extracted ion chromatograms (EIC or XIC) were run. The authors noted that all work was carried out in a fume cupboard with protective clothing also being worn. In addition to VX and RVX, the fluoridation system also worked with GA (Tabun); in this case the cyanide group of the GA molecule was partially replaced by fluorine to give GAF, although unreacted GA was also detected by the system. The recovery ratios for VX varied, with the average overall recovery ratio being determined as 6.1%. The equivalent figure for RVX was 5.9%. The LOD (limit of detection) for both VX and RVX was estimated as 0.3 mg m-3, which is sufficiently low for effective field identification of these compounds. It was noted, however, that petrol (gasoline) vapour interfered with the detection process.
VX and RVX nerve agents can now be readily identified
The use of the conversion tube extends the efficacy of the Hapsite ER GC-MS system to the relatively non-volatile nerve agents VX and RVX. This extends the range of the apparatus, which now can detect a very wide range of chemical warfare agents. Further work is needed to reduce interference from other volatile substances, such as petrol vapour.
See also: Chemical Weapons and/or their Precursors