Detection of Chemical Warfare Agents on Surfaces Relevant to Homeland Security by Direct Analysis in Real-Time Spectrometry

Unlike other analytical methods that necessitate that the surface be sprayed with electrically charged solvents, or that require solvent extraction, DART leaves the sample surface undisturbed.

Analysis of low-volatility, condensed-phase chemicals on surfaces has been an extremely difficult and long-standing objective in environmental monitoring. When target analytes possess picotorr vapor pressures, the problem of monitoring becomes formidable. As such, any noncontact sampling at atmospheric pressure that does not require solvents or wipes would be a technological breakthrough. In addition, the absence of sample preparation would allow extremely rapid analysis in time-critical situations. Today, such a technology is now available. It is known as Direct Analysis in Real Time1 or DART™ (JEOL USA, Inc., Peabody, MA).

The U.S. Army has been testing and developing DART since 2002 for the detection of chemical warfare agents on surfaces. Fast, safe, and accurate detection of chemical agents is critical for protection, security, and decision-making. Sample preparation is seldom a requirement with DART, since the contaminated surface is simply analyzed directly using a plume of gaseous Rydberg atoms. Unlike other analytical methods that necessitate that the surface be sprayed with electrically charged solvents,2 or that require solvent extraction, DART leaves the sample surface undisturbed. This is a forensically worthy advantage. Recent findings of chemical warfare agent detection on militarily relevant surfaces in Homeland Security is a new approach to Warfighter safety and counter-terrorism.

Experimental

The DART source

The use of Rydberg atoms as a replacement for the radioactive source in chemical agent monitors was conceived by Dr. James A. Laramée in 2001. A working prototype was developed by Drs. Cody and Laramée, which was publicly disclosed on April 14, 2003 in two patent applications as the first ambient mass spectrometric method.3

DART/AccuTOF™ system

The experimental apparatus consisted of three practically identical DART/AccuTOF (JEOL USA) systems, which were used in unison to give multiple checks. The DART source consists of a tube divided into three chambers through which a gas, typically helium, flows. A needle in the first chamber is electrified to a few kilovolts to form a plasma that consists of helium cations, electrons, and excited-state species, which are the working reagent in the DART method. The second and third chambers contain electrodes to remove most of the ionic species from the gas stream. Detailed operating parameters have been described elsewhere.1 The gas flow and electrode potentials have a broad range of operating values that are not critical to the information fidelity of the experiment. Many combinations of settings gave similar sensitivity.

Mass spectra were recorded on a modified AccuTOF time-of-flight mass spectrometer (model JMS-100TLC) operating at 6000 mass resolution (FWHM definition). The sampling orifice was set to 60 V relative to ground in order to attract the analyte ions into the spectrometer. Higher-orifice voltages were used when fragmentation of the molecular cation was desired.

Chemicals

Chemical Agent Standard Reference Materials (CASARM) were used: O-ethyl N,N-dimethylphosphoramidocyanidate (GA), O-isopropyl methylphosphonofluoridate (GB), bis-(2-chloroethyl) sulfide (HD), and O-ethyl S-2-diisopropylaminoethyl methylphosphonothiolate (VX). The Chemical Weapons Convention allows development, production, acquisition, stockpile, or possession of these chemicals and their transfer, directly or indirectly, only to treaty declared facilities and private facilities with bailment agreements.4

Results and discussion

A meaningful chemical warfare agent detector must fulfill three criteria in order to be useful. First, the technology must be able to detect actual agents and not just simulants; although simulants are popular in academic laboratories, they give meaningless evaluations of detection capability. Second, trace-level concentrations must be detectable regardless of their physical state. Third, chemical warfare agents must be detectable regardless of the nature of the sample surface that holds them. The DART technology was tested with these constraints in mind.

Exact mass measurements of the major peaks in the mass spectrum were made in order to assign correct and unambiguous elemental compositions. The average mass accuracy of the measurements was ±0.00004 Da. By way of contrast, the instrument manufacturer claims a mass accuracy of ±0.002 Da and recently reported peaks in agreement with predicted values of less than or equal to 0.007 Da with even larger deviations observed at masses below 200 and above 600 Da.5 This instrument behavior has not been observed on any of the authors’ three DART/AccuTOF systems, which they attribute to careful operation and setup, both of which appear to be essential for obtaining meaningful exact mass measurements.

Figure 1 - High-fidelity mass spectra were obtained by DART for 800 ng of VX on aluminum, concrete, and a bird feather. VX appearsas the MH+ molecular cation at m/z 268.150 Da. Other militarily relevant surfaces were examined with equal success.

VX was spiked onto various sample surfaces collected from an urban environment. Eight hundred nanograms of VX (in isopropyl alcohol) was applied to 20 surfaces and analyzed. None of the surfaces tested were treated or cleaned up in any way. These included aluminum, roadway asphalt, automobile tail light, birch tree bark, cardboard, two types of concrete (foreign and domestic), glass, holly leaf, bird feather, waxed paper cup, paper towel, clothing fabric, stainless steel, rusted steel, plastic, tire tread rubber, roofing tile, and a Viton® (DuPont Performance Elastomers L.L.C., Wilmington, DE) O-ring. Difficult sample surfaces were readily analyzed, such as porous concrete, electrical conductors, and biological samples. The protonated molecular cation at m/z 268.150 appeared within seconds and with little fragmentation (Figure 1). Only a small peak at m/z 128.144 corresponding to the diisopropylaminoethyl cation fragment (C8H18N) was seen. This soft ionization is an advantage of the DART ion source over other methods for the detection and identification of chemical warfare agents, since the mass spectra remain uncluttered by extraneous peaks. Ion intensities were typically greater than several hundred thousand counts, with signal-to-noise ratios in excess of 100,000.

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