Monitoring for CWAs and TICs in Homeland Security Applications Charles M. Phillips MKS Instruments Inc., Andover, MA 01810 INTRODUCTION Chemical Warfare Agents (CWAs) and/or Toxic Industrial Chemicals (TICs) are prominent in the list of weapons of mass destruction (WMDs) identified by the Department of Homeland Security. CWAs are of obvious concern since they are specifically designed for lethality, typically at very low ambient concentrations. TICs, although usually less toxic than CWAs, are of equal concern to Homeland Security. TICs possess significant levels of toxicity, and they can be obtained relatively easily and in large enough quantities to represent a threat. TICs are shipped throughout the country in large tank cars whose routes crisscross our cities and suburbs by both rail and road. If one of these tank cars were to be hijacked and its contents released with malevolent planning in an urban environment, large numbers of civilian casualties could occur. It is therefore necessary to develop the capability to continuously monitor for CWAs and TICs, and to detect them at ambient concentrations much less than toxic levels. Such detection capability will help to avoid a situation in which the first sign of exposure to a chemical agent is the outbreak of visible symptoms of poisoning since, by then, it may be far too late to avoid widespread death and/or injury. Specifically, security for populated buildings needs to provide for continuous chemical monitoring of the air handling system for these buildings or, at the very least, throughout the occupied areas of the buildings. Detector technologies for CWAs and TICs have, of necessity, been adapted from analytical chemistry techniques. Unfortunately, most chemical analytical approaches are not well suited to Homeland Security (HS) applications, since conventional techniques that have the ability to continuously monitor for CWAs and TICs with reasonable cost and portability do not, in general, have the selectivity necessary to avoid frequent false alarms. In HS situations, CWAs and TICs must be monitored in the presence of interferents such as common solvents and cleansers and techniques with limited selectivity cause a high level of false alarms. In addition, these techniques tend to have a limited dynamic range and can get easily overwhelmed by signals due to cleaners and solvents. False alarms, especially in HS applications, are costly and reduce the overall effectiveness of the monitoring system. Conversely, classical chemical analytical tools (such as Gas Chromatography) that have the ability to discriminate CWAs in the presence of these interfering molecules tend to be very high priced and incapable of continuous sampling. Thus they cannot provide real time monitoring for the CWAs and TICs, and therefore cannot serve the intended purpose of protecting a populated building or industrial facility due to the built-in time delay disadvantage. Homeland Security applications for CWA and TIC monitors require reliable, robust and easily deployed detectors that can monitor the ambient atmosphere of common commercial environments with minimal cost and installation requirements. HS monitors must be capable of continuously sampling the ambient air in
commercial and institutional environments. They must be networkable for rapid response and capable of 24/7 operation with minimal downtime. They must detect CWAs and TICs with specificity in the presence of interferents such as common solvents and cleansers that have the potential to cause false alarms. HS monitors must exhibit very few false alarms. Finally, detectors in HS applications must not be impacted by changes in ambient humidity or temperature. This report describes the results of tests on an effective system for HS CWA and TIC monitoring applications, the AIRGARD air analyzer. This system uses Fourier transform infrared (FTIR) spectroscopy, a detection technique that exhibits very high specificity while maintaining high sensitivity. ADVANTAGES OF FTIR DETECTORS IN HS APPLICATIONS FTIR spectrometers have very high sensitivities. Almost all chemical compounds have molecular bonds that absorb infrared radiation. This inherent detectability of a molecule, when combined with an appropriate path length for optical absorption and the high signal to noise ratios possible with Fourier transform detection methods, produces very low detection limits in FTIR analysis. With appropriate optical cell and detectors, a typical FTIR detection limit can be at ppb levels for most common infrared absorbers. Infrared spectroscopy is inherently a multidimensional technique in which many different frequencies are measured simultaneously. Since most molecules contain multiple chemical bonds, each of which absorbs at a different frequency, simultaneous measurement of absorbance over a broad frequency range can determine the unique absorption vs. frequency distribution (spectrum) that is the fingerprint of a given molecule. This fingerprinting ability of FTIR detection results in very high selectivity for the detection of designated molecules. These features of FTIR spectroscopy also enable the use of advanced analytical algorithms for data analysis that produce very low noise level data and reliable mathematical reductions of all of the components which comprise a given spectrum. Using these advanced chemometric algorithms, each component can be characterized and quantified such that CWAs and TICs can be detected and speciated in the presence of multiple interferents. These attributes of FTIR combine to produce a highly reliable detection methodology that produces very few false alarms. THE AIRGARD SYSTEM Configuration The AIRGARD air analyzer is a self-contained FTIR monitor designed to rapidly detect a variety of CWAs and TICs at part-per-billion (ppb) concentrations. The system was designed and constructed in collaboration with Aerospace Corporation, Los Angeles, CA under the DARPA Sensors for Immune Building Program. The interferometer, gas cell, light source, detector, sample pump and computer are all internal to the AIRGARD case with the only external connections being the power cord and interface links (1 Ethernet and 1 USB). This design allows the AIRGARD to be networked for remote monitoring with continuous 24/7 operation; no user interaction is needed. Figure 1 shows a schematic of the AIRGARD air analyzer along with an image of the deployable monitor.
The FTIR within the AIRGARD system uses a compact, rugged interferometer to record mid-infrared spectra in the wavelength range 5000 600 cm -1 (2.0 16.7 μm) at a nominal resolution of 4 cm -1. The system employs a multi-pass gas sampling cell that has gold plated mirrors for enhanced signal to noise. The geometry of the cell is optimized for minimal spectral response time, for fine granularity in the continuous monitoring data. The internal miniature diaphragm sampling pump produces a flow rate of 6 to 12 L/min. The spectrometer uses a MCT (mercury-cadmium-telluride) detector, which is Stirling cryocooled to permit 24/7 operation of the unit. Data analysis is performed using proven spectral analysis software. The AIRGARD typically needs no operator intervention; it runs autonomously 24/7/365 and reports back to a central server via the secured XML protocol. Alternatively, an operator can log onto an AIRGARD via a client-based software package and monitor the AIRGARD in this manner. System operational alarms point to the need for maintenance intervention; these alarms are also transmitted via the Ethernet link. The system requires one major maintenance procedure; that is the replacement of the cooled detector assembly. If the AIRGARD is run 24/7/365, this maintenance is required every 13 to 14 months. Performance Characteristics The AIRGARD air analyzer achieves high sensitivity and selectivity through an optimized system for the optics/detector coupled with the AIRSAGE data analysis algorithm. The gas sampling cell in the analyzer has the highest pathlength-to-volume ratio of any gas cell in a commercial FTIR system. The cell design produces an optimal gas throughput in a cell that has a 10.18 m pathlength with only 400mL of gas volume. This cell has extremely fast gas turnover with a T80 of as fast as 4 seconds. (T80 is the time it takes to fully replace 80% of the gas sample in the cell with a new sample.) Figure 2 shows a graph of the AIRGARD response to the introduction of ethylene in a test for response time. It can be seen that the system detects this contaminant rapidly; it exhibits a T80 of 6.1 sec in this test (recent modifications to the AIRGARD have reduced this number to ~4 sec). The mercury-cadmium-telluride solid state IR detector is mated to a Stirling cryocooler, for very low noise performance and high sensitivity in the detector. The combination of optical and detector designs results in typical detection limits (dependent on the absorptivity of the molecule) in the 1 to 10 part per billion range. Figure 3 provides a demonstration of this sensitivity. The graph shows the AIRGARD response to a release of NF 3 gas displaced a distance of ¼ mile from the sensor location. Within 38 minutes of the release (the time delay related to airflow around the location of the release), the AIRGARD monitor detected the presence of NF 3 in the ambient atmosphere at the test facility. The AIRGARD air analyzer uses the AIRSAGE algorithm to determine which components are present in a sample; this algorithm easily detects the target species in ambient air or other background environments. The AIRGARD has the ability to speciate threat agents by their infrared signature; for example, it can differentiate between all G-agents and provide the exact identification of a threat rather than generically reporting a G agent. The fast gas turnover in the sample cell, coupled with the speed and reliability of the data analysis, work to produce the fine granularity needed for continuous monitoring. An examination of the graph in Figure 2 clearly shows this granularity, with concentration data computed and recorded every 0.5
sec. The on-board spectral library includes a standard list of CWA and TIC gases as well as spectral data for over 350 compounds. As well, the library is continuously being augmented. All that needs to be supplied is a calibrated infrared spectrum of the new agent, which is then easily inserted into the threat agent list for validation. Table 1 shows the calculated sensitivity limits of the AIRGARD for selected CWAs in both mg/m 3 and ppb. TESTING OF THE AIRGARD AIR ANALYZER The AIRGARD was subjected to tests sponsored by the Department of Defense (DoD) as well as the Department of Homeland Security (DHS). In these tests, the AIRGARD was challenged in a variety of government, public and institutional environments including the Pentagon, a State Department mailroom, the Anaheim Convention Center, an active transportation facility, the Battelle Memorial Institute Laboratories, and the Edgewood Chemical and Biological Laboratories. On these systems, the AIRGARD air analyzer has been tested in these real-world, live environments well over 4 instrument-years with no validated, and detectable false alarm events having been recorded. Further the instrument has undergone thousands of hours of lab testing to assure it meets the design requirements. Two representative test scenarios are described below: CWA Monitoring at a Major Transportation Facility The AIRGARD was integrated into an internal network that is operated and maintained by the Security arm of a major transportation authority, and monitored over a 9 month period. The AIRGARD, contained in a ventilated housing, sampled air from a train platform environment. An off-line simulant challenge was performed prior to long term operations to both confirm that the instrument was performing properly, and to determine whether it would detect the nerve agent simulant at roughly 1-2 ppm concentrations alone and in the presence of interferent. Table 2 shows the results of the challenge testing. With one exception, no unexpected false alarms occurred. In that one case, it was suspected that a too-rapid introduction of 100% nitrogen immediately following a Windex challenge had caused the false alarm. That challenge was repeated, allowing additional time between the delivery of the two samples to the AIRGARD sample cell. In this subsequent test, no false alarm occurred. In one other test where nitrogen gas rapidly followed a challenge gas, it appeared that the sample line going to the AIRGARD did not clear fast enough, and the AIRGARD alarmed for a short period after delivery of the simulant was complete. The air temperature over the duration of the testing period varied between 40 F to 95 F; as well, wide variations in humidity and air composition occurred over the testing period due to train operation and human intervention. A subset of these challenge tests were repeated at the end of the nine months, again with no false alarms.
Table 3 shows the results from a series of tests performed by the transportation authority in order to evaluate the response time of the AIRGARD to the selected CWA simulants. An initial test evaluated the response to a 0.5% headspace glass cleaner. Sampling for this series of tests was carried out using Tedlar bags and sample introduction was done by simply squeezing the contents of the bag into the input of the AIRGARD. When tested using the glass cleaner, the system responded within 20 seconds, reporting detection of 2 BUTX, H2O and NH3 (the components of the cleaner) and generating no alarms after a full minute, at which point the test was halted. The system was flushed with nitrogen and then exposed to 2 ppm of CWA Simulant #2. Within 45 seconds, the system began to successfully alarm on Simulant #2, and it continued to alarm for the remaining minute of the test. Following this test, the system was flushed with nitrogen and then exposed to a sample containing 2 ppm of CWA Simulant #1. Within 10 seconds, Simulant #1 alarms came up and continued for the duration of the test. After each challenge, the AIRGARD quickly returned to a no alarm state when flushed with nitrogen. On one day during the testing period, a janitor working for the transportation authority used NMP (Nmethyl pyrrolidone) to remove graffiti from the walls of the train platform. Ancillary sensors that were being used by the Authority (located in the same area as the AIRGARD) immediately began to alarm for G- agent release. However, the AIRGARD did not alarm on this innocuous chemical release. In conclusion, these tests showed that, over a 9 month period, in a very dirty environment in which temperatures, humidity and air composition varied greatly, the AIRGARD worked to its specification. During the entire period of static operational testing, the AIRGARD had zero false alarms. TIC Detection During the DHS-sponsored tests, known as ARFCAM, an AIRGARD air analyzer was exposed to IDLH (Immediate Danger to Life and Health) levels of specific TICs under 10 varied conditions of temperature, humidity and background interferents. A controlled test hood was used to deliver a number of chemicals to the analyzer. Up to 15 tests were conducted per TIC, varying the following conditions: Temperature (10 C, 23 C, 38 C) Relative Humidity (30%, 75%) Interferents present or not [at 1% headspace, a relatively high concentration] Concentration (Immediately Dangerous to Life and Health (IDLH), Permissible Exposure Limit (PEL)) The TICs tested were hydrogen cyanide (AC), cyanogen chloride (CK), ammonia (NH 3 ), ethylene oxide (EtO), arsine (SA), formaldehyde (FORM), acrolein (ACROL), acrylonitrile (ACRY), phosgene (CG), sulfur dioxide (SO 2 ), and hydrogen chloride (HCl). Chlorine (Cl 2 ), which is not infrared active and not observable by the technology used in the AIRGARD, was not tested. The results of the TIC testing are summarized in Table 4. The AIRGARD alarmed properly to each of the 11 TICs at their IDLH level, and
did not alarm in the presence of 1% headspace for any of the interferents. Detections at the lower concentration PEL levels were largely successful, with detection in each test of 5 repeated cycles for seven of the TICs, and for 2 of the 5 cycles for another TIC. With 1% headspace of interferent gases presented along with the TICs at their IDLH, the AIRGARD was not affected by all interferents tested other than Windex, correctly alarming when the TICs were introduced. In the presence of 1% Windex, those TICs that were not detected at the PEL levels were not detected. In 97.3% of these tests the AIRGARD successfully detected and alarmed on each TIC within 15 seconds of exposure, with no false alarms recorded throughout testing. CONCLUSION The AIRGARD air analyzer was evaluated as a continuous monitor for the presence of CWAs and TICs in public environments. The system exhibits both the sensitivity and selectivity towards CWAs and TICs that are necessary for Homeland Security and DoD applications. Target concentration goals for these applications were met, with the AIRGARD successfully detecting the targeted CWA or TIC within a sampling matrix of varying temperature, humidity and chemical interferents.
Table 1: Selected sensitivity limits for the AIRGARD system.
Test Type Composition Alarmed Did Not Alarm Confidence 1 Check Simulant #2 #2 2 Blank Nitrogen Negative 3 Challenge Ammonia @ 25 ppm Positive Ammonia 4 Blank Nitrogen Negative 5 Challenge Simulant #1 @ 1ppm #1 6 Blank Nitrogen NA 7 Challenge Simulant #2 @ 2ppm #2 8 Blank Nitrogen Negative 9 Challenge Simulant #1 @ 1ppm #1 10 Blank Nitrogen Negative 11 Challenge 1% Windex in Nitrogen Negative 12 Blank Nitrogen Positive N Mustard 13 Blank Nitrogen Negative 14 Challenge 1% Windex in Simulant #1 @ 1ppm #1 15 Blank Nitrogen Negative 16 Challenge 1% Pynol in Nitrogen Negative 17 Blank Nitrogen Negative 18 Challenge 1% Pynol in Simulant #1 @ 1 ppm #1 19 Blank Nitrogen Negative 20 Challenge 1% Pynol in Simulant #2 @ 2 ppm #2 21 Blank Nitrogen #2 22 Blank Nitrogen Negative 23 Challenge 1% Wax Stripper in Nitrogen Negative 24 Blank Nitrogen NA 1% Wax Stripper in Simulant #2 @ 2 25 Challenge ppm #2 26 Challenge 1% Windex in Nitrogen Negative 27 Blank Nitrogen Negative 28 Blank Nitrogen Negative 29 Challenge 1% Windex in Nitrogen Negative 30 Blank Nitrogen Negative Table 2. Results for AIRGARD air analyzer CWA testing in an active transportation facility
Test Agent Start Time Alarm Time/End Time if no alarm Result 0.5% Headspace glass cleaner 21:09 21:10 No alarm Nitrogen 21:11 21:12 No alarm Simulant #2 21:16:20 21:18 Alarm Nitrogen 21:20 21:21 No Alarm Simulant #1 21:22 21:22:10 Alarm Nitrogen 21:23 21:22 No alarm Table 3. Response time evaluations for the AIRGARD Air Analyzer.
Table 4. ARFCAM TIC testing results.
COMPUTING SUBSYSTEM POWER SUBSYSTEM Single Board Computer Power Supplies AC Power Inlet Line Ethernet and USB Hard Drive FTIR SUBSYSTEM Control Electronics IR Source Brik Interferometer Temp. Press. Sensor Sensor Multi Pass Gas Cell Detector Detector Cooler Temp. Control ΔP Sensor SAMPLING SUBSYSTEM Inlet Filter Heated Line Pump (a) (b) Figure 1. (a) A schematic showing the internal components of the AIRGARD Air Analyzer; (b) the fielddeployable AIRGARD unit.
100 Normalized Ethylene Concentration (%) 80 60 40 20 0 01:09.1 01:17.8 01:26.4 01:35.0 01:43.7 01:52.3 02:01.0 02:09.6 Time (minute:second) Figure 2. AIRGARD response to ethylene gas contamination.
Figure 3. The response of the AIRGARD analyzer to the release of NF3 gas ¼ mile away from the detector.