Small and perfectly formed: Miniature mass spectrometer for field detection of chemical warfare agents

Onsite toxic gas monitoring

There are many types of industrial facility which would benefit from continuous monitoring of the air quality, to ensure both plant safety and public safety. Ideally, the instrumentation deployed for this purpose should be able to run automatically in situ and be capable of remote control of the operating conditions and relaying the data remotely to a monitoring station.

Currently employed techniques that have been successfully rolled out in the public sector include ion mobility spectrometry, flame spectrometry, electrochemical detection and surface acoustic wave detection. On their own they are effective but their specificity can be markedly improved if they are combined with a technique such as mass spectrometry which yields supplementary information.

One research group that has been working on miniaturising mass spectrometers so that they are suitable for onsite monitoring is headed by Graham Cooks at Purdue University in West Lafayette. Perhaps better known for their work on ambient mass spectrometry, this group has also developed a series of miniature mass spectrometers with increasingly improved performance.

Now, they have published data measured on a commercial miniature mass spectrometer designed for continuous indoor air monitoring and detection for building and facilities protection. The ChemSense 600 instrument is 44.5 x 48.3 x 26.4 cm in size and has typical response times ranging from less than a few seconds to one minute for chemicals in the vapour phase.

It incorporates a cylindrical ion trap that can perform single stage or tandem mass spectrometry and is operated under glow discharge electron ionisation. An onboard computer is fitted for data collection and distribution via an ethernet connection.

Cooks and colleagues Jonell Smith and Robert Noll tested the device for the monitoring of chemical warfare agent simulants in air and published their results in Rapid Communications in Mass Spectrometry.

Chemical warfare agent simulants in miniature

Gaseous samples of four compounds were drawn into the mass spectrometer by a pump and trapped on an adsorbent bed in one of two adsorption tubes. One tube was desorbed by heating while fresh sample was being introduced to the second tube.

The total trapping-desorption cycle for each tube was 90 seconds, 45 seconds trapping in one tube while the other tube was desorbed and analysed. The ions were scanned over the mass range m/z 40-200.

The analytes and the chemical warfare agents they simulate were 2-chloroethyl ethyl sulphide (CEES) and methyl salicylate (MeS) (both simulating mustard gas, also known as HD), diethyl malonate (DEM) (HD, tabun, sarin, soman, VX) and dimethyl methylphosphonate (DMMP) (tabun, sarin, soman).

The molecular radical cation was detected for CEES and MeS whereas the protonated molecule was observed for DEM and DMMP, even though all spectra were recorded under identical conditions. The difference was attributed to the relative proton affinities of the compounds.

The mass spectra of the simulants were characterised and one ion from each was selected for constructing a calibration curve and subsequent quantitation. The detection limits ranged from 0.25-5 ppb and the calibration curves were reproducible, although not linear over the full range up to 2000 ppb.

The instrument sensitivity and selectivity were deemed to be good by nature of the receiver operating characteristic (ROC) curves as defined by the Department of Defense.

So, the instrument is capable of detecting and measuring the four chemical warfare agent simulants one by one. They were also analysed in an approximate 1:1:1:1 mixture, and their tandem mass spectra were sufficiently different for unambiguous identification.

Cooks also analysed some simulants in the presence of likely environmental interferents. Bleach is used to decontaminate surfaces following exposure to chemical weapons but the analysis of CEES was still possible in the presence of bleach headspace vapours. At 70 ppb CEES, the simulant peaks dominated the spectrum.

It was slightly more problematical for lower concentrations of CEES mixed with diesel fuel exhaust. At 35 ppb CEES, peaks at m/z 57 and 92 were observed and attributed to a butyl fragment and toluene, respectively, but an MS/MS spectrum would still be able to identify CEES.

The results illustrate the ability of the miniature mass spectrometer to detect and measure chemical warfare agents rapidly and at low concentrations either alone or in mixtures with each other, as might occur during a chemical accident. They were also detected during simulated urban conditions in the presence of bleach and diesel exhaust.

The performance and portability suggest that this instrument will be suitable for facility air monitoring for toxic chemical vapours.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.