Chemical Warfare Agents (CWA) are a class of Weapons of Mass Destruction (WMD) designed to kill and injure humans through exposure to extremely toxic chemicals. In addition to CWA many chemicals used by industry, often referred to as Toxic Industrial Chemicals (TICS) can potentially be used by terrorists as weapons.
Despite the fact that numerous treaties such as the Geneva Protocol (1925) and the Chemical Weapons Convention (1997) have been signed over the past century, these highly toxic compounds are still produced, stockpiled and used by both state and non-state actors. Recent uses of CWA include the use of chlorine gas and sarin in Syria, as well as assassination attempts in the United Kingdom and Malaysia. A 1995 attack by the Aum Shinrikyo cult on the Tokyo subway system using sarin in 1995 resulted in the deaths of 13 and over 6,300 were sickened.
Chemical warfare agents are generally grouped into four classes based on their effect on victims:
Blister Agents, which cause severe skin, eye and mucosal pain and irritation including chemical burns and blisters on those exposed to them;
Nerve Agents, which disrupt the message transfer of nerves;
Blood Agents, which are absorbed into the blood resulting in poisoning;
Pulmonary Agents, which damage an exposed victim's ability to breathe, leading to suffocation.
Chemical Warfare Agents can result in death or severe injury after even short exposures at very low concentration levels. Table 1 lists the Acute Exposure Guideline Levels for several CWA. The AEGL2 (10 minutes) indicates the concentration that results in irreversible or other serious, long-lasting effects or impaired ability to escape. The AEGL3 indicates the concentration at which life-threatening effects or death occur.
The high toxicity of CWA requires the ability to detect these gases quickly at low concentrations in order to be able to enact a response plan to minimize damage. A typical response to a chemical detection may include an evacuation of the impacted area. Smart buildings have been proposed and built that incorporate detection systems integrated with building air management systems, allowing building management to suspend air intake, switch to an alternate air inlet or reverse air flow to pump the contaminated air out of the building[2, 3].
The general requirements for a chemical agent detector used to protect critical infrastructure include:
Ability to operate 24/7/365. Many key government buildings do not shut down.
Low false alarm rate – This is required to build confidence in the detector. Evacuating a building for a false alarm can create severe disruption and panic and will incur large expense.
Minimal routine maintenance
Selectivity – The ability to identify required compounds even in the presence of potentially interfering species. High selectivity also will help first responders more effectively treat any victims and can help in attribution of the attack.
Sensitivity – The ability to detect compounds at or below the levels hazardous to health.
Ease of use – Technicians and security personnel must be able to operate and immediately understand the results.
CWA detectors have been developed based on several technologies including electrochemical sensors (EC), ion mobility spectroscopy (IMS), gas chromatography-mass spectroscopy (GC-MS). Infrared spectroscopy, specifically Fourier Transform InfraRed Spectroscopy (FTIR) is a mature, nondestructive technique that can meet the performance specifications for this type of detector.
Figure 1 highlights the need for high selectivity because of the overlapping spectral features of the gases shown. Ten conventional chemical warfare agents are shown, along with two common atmospheric interferent gases. Each different compound results in a unique spectrum. The spectrum of each chemical, even chemically and structurally similar compounds, is unique in comparison to other compounds. In this figure the x-axis is wavenumber (cm-1) and the y-axis is absorbance. The actual wavenumbers have been excluded intentionally and the names of the specific agents have been suppressed based on customer requests.
The MKS AIRGARD® Air Analyzer is an advanced CWA and TIC detector based on an FTIR spectrometer. The AIRGARD is a complete self-contained system that is typically wall-mounted and only needs power and a communication connection to local command systems. Experts determine the optimum location in buildings for the system based on the flow of personnel and air flow modeling or experiments. Systems may be installed to measure the air circulated through a building's ventilation or HVAC system, allowing operators to stop or divert air flow if detection occurs. Figure 2 shows the MKS AIRGARD® analyzer.
To meet the demanding performance requirements, each critical subsystem in the AIRGARD air detector was optimized as part of the design process.
FTIR – The heart of the system is a high-performance interferometer. Trade-off studies were performed between resolution, scan time and other critical specifications. The current system performs a complete analysis and reporting cycle in 9 seconds.
Gas Cell – A pathlength of 10 Meters was selected to optimize the optical throughput and detection limits. A low volume (400 cubic centimeters) allows for quick turn-over based on a flow rate of 10 liters per minute.
Background Approach – The system uses a patented rolling background approach that allows it to use air as the background. Screening rejects any air containing any level of threat agents, and an exponentially weighted moving average is used to calculate a background scan based on a specified time window. This results in a background that is always current, allowing the system to achieve an optimum signal to noise ratio.
Detector – A high-performance, Mercury Cadmium Telluride (MCT) detector is run at cryogenic temperature by use of a Stirling Engine cooler.
Algorithm – The analysis algorithm includes identification and quantification of gases, as well as the use of several confidence “gates” based on a t-value to reduce the occurrence of false alarms. A proprietary algorithm is used to effectively match the spectra recorded against a library of known compounds including common chemical warfare agents and toxic industrial chemicals. Statistical tools are used to quantify the confidence of the detections and if the alarm criteria are met, an alarm is sent to the command center.
Library – A critical component of the system is a gas library containing reference spectra of both threat agents as well as interfering gases expected to be present at times. Typical libraries include from 31 to 70 threat agents and over 300 interferent gases. The inclusion of a broad library of interfering gases allows the algorithm to correct the analysis spectrum for their presence. Threat agents may be set to include many of the 2012 CTRA (Chemical Terrorism Risk Assessment) list which was generated by the CSAC (DHS Chemical Security Analysis Center). The library can also be updated as new threat agents are identified.
Self-Diagnostics – During each analysis cycle extensive diagnostics are performed and if any performance parameters are out of specified ranges, warnings are issued or analysis results are suppressed.
Figure 3 shows changes in the spectra based on routine atmospheric changes. The use of a rolling background allows the system to reduce these effects, allowing the system to detect critical gases at lower levels.
Before adoption, the AIRGARD underwent extensive testing sponsored by the Department of Defense (DoD) as well as the Department of Homeland Security (DHS) in a variety of environments including office buildings, a mail sorting room, a subway system and a convention center. Testing using live agents was performed at Battelle Memorial Institute Laboratories and the Edgewood Chemical and Biological Laboratories. During the latest round of DHS-sponsored tests, known as ARFCAM (Autonomous Rapid Facility Chemical Agent Monitor), an AIRGARD was exposed to IDLH (Immediate Danger to Life and Health) levels of 8 TICs under 10 varied conditions of temperature, humidity and background interferents. All agents were properly detected and the AIRGARD has continued to demonstrate the lowest false alarm rate for stationary detectors. The initial AIRGARD air analyzer library has been deployed extensively, protecting critical infrastructure facilities. A more recent expanded library which detects 70 threat agents has been tested for the past 2 years and has demonstrated a false alarm rate of less than one false alarm per year.
After successful use in the field the AIRGARD air analyzer was awarded Safety Act Designation as an approved technology by the Department of Homeland Security.
The AIRGARD® air analyzer has been deployed for several years protecting multiple critical infrastructure locations. The combination of optimized subsystems allows the system to successfully identify and quantify a wide range of chemical warfare agent and toxic industrial chemical threats. The ability to update the reference library without any required hardware changes will allow the system to be adapted to protect against new threats.
- Charles M. Phillips and Huwei Tan “FTIR gas analysis with improved sensitivity and selectivity for CWA and TIC detection”, Proc. SPIE 7665, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XI, 766515 (5 May 2010); https://doi.org/10.1117/12.850910
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