Implementation of a new technology for point detection John Petinarides, M. Todd Griffin General Dynamics ATP 4205 Westinghouse Commons Drive Charlotte, NC 28277 Raanan A. Miller, Erkinjon G. Nazarov, Anthony D. Bashall Sionex Corporation 300 Second Avenue Waltham, MA 02451 ABSTRACT General Dynamics ATP (GDATP) and Sionex Corporation (Sionex) are carrying out a cooperative development for a handheld chemical agent detector, being called JUNO™, which will have lower false positives, higher sensitivity, and improved interference rejection compared with presently available detectors. This enhanced performance is made possible by the use of a new principle of ion separation called Differential Mobility Spectrometry (DMS). The enhanced selectivity is provided by the field tunable nature of the Sionex differential mobility technology (microDMxTM) which forms the analytical heart of the JUNO system and enables fingerprinting of molecules by characterization of the ionized molecular behavior under multiple electric field conditions. This enhanced selectivity is valuable in addressing not only the traditional list of chemical warfare agents (CWA) but also the substantial list of Toxic Industrial Compounds (TICs) and Toxic Industrial Materials (TIMs) which may be released in warfare or terrorist situations. Experimental results showing the ability of the microDMx to reject interferences, detect and resolve live agents are presented. An additional breakthrough in the technology was realized by operating the device at a reduced pressure of around 0.5 atmospheres. This reduced pressure operation resulted in roughly doubling the spectrometers resolution over what has previously been reported [1]. Advances have also been made in power consumption and packaging leading to a device suitable for portable, handheld, applications. Experimental results illustrating the performance of the microDMx technology employed in JUNO are highlighted. Keywords: Differential Mobility Spectroscopy, Chemical Warfare Agent Detection, Analytical Instrumentation *
[email protected]; Phone: (980) 235-2247; Fax: (980) 235-2247; 4205 Westinghouse Commons Drive, Charlotte, NC 28277
1. INTRODUCTION With the continuing threat from chemical and biological weapons, the need for more effective and reliable detectors continues to be an issue for both the military and homeland security. Most, if not all, of today’s deployed detection devices were developed to address the relatively narrow range of classic warfare agents of the cold war era. However, with the escalation of world terrorism there is a need to deal with a broader range of threats that include a substantial list of TIC’s and TIMs. This places an even greater burden on detector technologies which must offer even higher selectivity without compromising sensitivity. The requirement is for fast response times with significantly lower false positives. Most of the currently deployed detectors are based on Ion Mobility Spectrometry (IMS) technology, developed to maturity over the last several decades. We believe that microDMx technology will provide a much higher level of effectiveness and reliability in meeting current needs for point detection, and demonstrate the potential for evolutionary development to even greater performance, miniaturization and cost reduction. Sionex has miniaturized these devices using microfabrication techniques to produce significantly smaller, mass producible, and reproducible sensors. The key attributes of the sensor technology are its high sensitivity and ability to
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separate overlaying spectral peaks (selectivity) by simply varying the Radio Frequency (RF) field and compensation voltages. In cooperative development with Sionex, GDATP is able to use its extensive expertise in Chem/Bio detection and systems integration to maximize the potential of the microDMx sensor and develop a new miniature point detector, JUNO.
2. METHODOLOGY The detection and identification of CWAs, TICs and TIMs is recognized as a major concern as demonstrated by Operation Desert Storm and the Sarin incident on the Tokyo subway. Operation Desert Storm provided some valuable feedback on the existing technology and highlighted some deficiencies with the best available technology at that time, conventional IMS. There are a variety of different interferences present in realworld conditions such as; Aqueous Fire Fighting Foam (AFFF), diesel fuel, gasoline, pesticides, paints and floor waxes that lead to a high rate of false positives in the currently deployed conventional IMS detectors. These frequent false alarms produced in the dusty, smoke-ridden, environment of the desert with oil fires burning lead to lack of use in the field. Conventional IMS devices which are currently deployed lack the required specificity since the compound identification is based solely on a single characteristic of the ion, its low field mobility. In conventional IMS the false positives are caused by the fact that many ion species can have the same, or very similar, low field mobility coefficients. 3.0 Application of DMS technology JUNO, which is being designed to detect traditional CWAs and the more readily available TICs and TIMs, is based on microDMx technology. The microDMx detection principle differs fundamentally from conventional IMS in that it takes advantage of the fact that an ion’s mobility is field dependant as the field strengths are increased above those employed in conventional IMS, as illustrated in Figure 1. The way in which the mobility changes, differential mobility, in response to the electric field provides substantially more information relating to a molecule’s identity, consequently leading to a significant reduction in false positives.
Figure 1: Shows the electric field dependent mobility of typical ions. The horizontal arrow outlines the high-field region where the microDMx device operates. Conventional IMS operates at much lower field strengths. Figure 1 shows how the mobility of typical ions changes with electric field. The arrow indicates the electric field operational region for the microDMx. Mobilities of trace chemical species in this region are generally larger (both positive and negative values) and better separated compared to those in the low field region where conventional IMS operates. The Sionex microDMx sensor consists of a microfabricated chip as shown in Figure 2. The microDMx sensor
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chips are built like integrated circuit components. Since control of the device is based solely on embedded firmware and software, the device can be easily updated or upgraded to detect new compounds or iterations of existing compounds by simply downloading new parameters either locally or remotely via a wired or wireless infrastructure. The small volume of the device also makes contamination less of a concern since the device can be purged very efficiently and rapidly.
Figure 2: Sionex microDMx Sensor Chip With breakthrough advances in electronics developed at Sionex the power consumption has been dramatically reduced along with the electronics size in order to realize a practical, portable, small personal-sized detector based on this technology. 2.1 Operation Principle The initial stage of detecting a trace compound is its ionization producing ion species; ionization is achieved currently by the use of a 63Ni radioactive source, and a proprietary plasma ionization source is also being developed. As ions are transported through the microDMx sensor chip by a transport gas flow, an asymmetric oscillating RF field is applied perpendicular to the flow between the ion filter plates. The ions experience the asymmetric field and move with a “zigzag” motion as the field is applied. Only ions whose displacement during high field cycle equals their displacement during the low field cycle are detected. Any ions whose net vector allows them to touch either of the two plates are neutralized and not detected. Once neutralized, the molecules will be transported by the transport gas out of the filter. To make the microDMx sensor chip tunable, which selects the ions of interest, a perpendicular, DC tuning field, known as the compensation voltage, is also applied. This DC field is superimposed on the oscillating asymmetric field. The compensation voltage can be adjusted to allow only specific ions to pass through the sensor to the detector. This compensating voltage can be scanned allowing ions with a range of differential mobilities to be detected. The basic operation of the microDMx sensor chip is illustrated in Figure 3.
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3. DATA Experiments were performed with calibrated standards of CWA simulants; diethyl methylphosphonate (DMMP), diisopropylmethylphosphonate (DIMP), and methyl salicylate (MS). Three independent vapor generator systems (Vici Metronics Inc, Model 190) were used to generate controlled air mixtures of the simulants at different concentrations. Permeation sources were purchased from KIN-TEK with calibrated emission rates of 160 ng/min DMMP at T=80C, 301 ng/min DIMP at 100C, and 5240 ng/min MS at 100C. Gas flow rates in all three systems were the same 100 cc/min. The maximum sample concentrations that could be provided was 1.6 mg/m3 for DMMP, 3.01 mg/m3 for DIMP, and 52.4 mg/m3 for MS. The concentration of simulants in mixtures could be varied by appropriate dilutions. All experiments with actual CWAs were performed at the U.S. Army, Chemical Biological Center at Edgewood, MD in accordance with standard procedures. 3.1 Sensitivity The microDMx device contains no ion-gate, and is therefore more sensitive than gated approaches where large portions of the sample signal are discarded. The microDMx can be operated in a mode where it is sampling effectively 100% of an incoming trace gas sample. In this mode, ion filter, the compensation voltage is fixed such that a particular ion species, identified by its differential mobility, is permitted to reach the detector. In the ion filter mode a 100% duty cycle can be realized. This is in contrast to conventional IMS which typically uses a gate which is pulsed “open” for approximately 1% of a measurement cycle resulting in only about 1% of the ions being sampled. Another benefit from the absence of a gate in the microDMx, to improve sensitivity, the signal can be integrated over a relatively long period of time leading to further improvements in sensitivity. When used in the spectroscopic mode a range of compensation voltages are scanned. This reduces the “effective” duty cycle, but since the range of compensation voltages that are scanned can be selected by the operator the “effective” duty cycle for any type of ion species is significantly higher than in conventional IMS. The sensitivity of microDMx is higher than conventional IMS with the ability to detect compounds in the ppt range. Figure 4 illustrates a spectral scan for methyl salycilate, a blister agent simulant, on the left and a plot of the response function for methyl salycilate on the right. The detection of 45ppt was achieved on an un-optimized development system and with very simple noise averaging of a few scans. Clearly this 45ppt detection does not represent the limit of detection but a typical response.
Figure 4: Concentration dependence and typical detection for Methyl Salicylate, a blister agent stimulant. The lowest measured concentration was 45 parts-per-trillion. The breadboard JUNO system incorporating the microDMx technology was exposed to actual CWAs; in particular VX was used for illustrative purposes. Figure 5 shows the response of one of these systems to the nerve agent VX. The concentration 0.04 mg/m3 was easily detected without any preconcentration. The microDMx responded to the VX exposure of 0.04 mg/m3 in 1 second. A further advantage of the microDMx technology is its ability to simultaneously detect both positive and negative ion species as illustrated for VX.
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Figure 5(a): Positive Ions
Figure 5(b): Negative Ions
Figure 5: 0.04 mg/m3 VX. Positive ions, VRF = 1482V, 350cc/min, 10% background moisture.
3.2 Selectivity Increased sensitivity is invaluable in applications requiring miosis level detection. However, a detector which has high sensitivity without selectivity leads to an even higher rate of unwanted false positives. As previously mentioned, enhanced selectivity in the microDMx is provided by changing the electric field strength applied to the ionized molecules. In practice, this translates to changing the field strength of the asymmetric oscillating RF field (Vrf). Changing the RF field, results in a corresponding shift in the spectral peak position, as measured by the compensation voltage. Changing the RF field leads to tunable resolution accessed by changing the RF filtering amplitude and thus changing the operating point on the mobility vs. electric field curve. In the microDMx the various RF field values are all generated under the automatic control of the microprocessor. The tunable resolution makes it possible to separate monomers from dimers and other clusters and to use these cluster peaks to aid in the identification of compounds, see Figure 6 (top curves). Tunable resolution also enables the RIP to be displaced away from the peaks of interest. The RIP is a background peak that frequently interferes with the detection of targeted compounds in conventional IMS. This property enables the detection of trace compounds in backgrounds that produce interfering signals, or in some situations where the RIP or other compounds interfere with successful detection. Tracking how the spectral peak position shifts with RF field provides a great deal of information unique to that ion species.
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Figure 6: Resolution of nerve and interferent simulants at different RF field strengths.
The microDMx technology simultaneously detects both positive and negative ion peaks, which further helps to improve selectivity. In this case, the absence or presence of peaks in the positive ion channel versus the negative channel provides more information on the specific compound identity. The ratio’s between intensities of positive ions and negative ions for a given sample also provides additional information which and enhance the confidence of the detection. Figures 7 and 8 illustrate this for the nerve agent GA. These spectral plots were measured at a Vrf of 1,482v which corresponds to a field strength of 29,640v/cm. The spectrum for the positive ions is very different from the negative ion spectrum.
Figure 7(a): GA Positive Ions
Figure 7(b): GA Negative Ions.
The combination of the positive and negative ion channel information, together with the information provided by monitoring the spectral peak shifts as a function of the applied RF field, results in a powerful tool for chemical identification. Topographical plots provide a mechanism for viewing the 3 dimensional data where the change in RF field strength versus compensation voltage is shown on the x and y axes, while the peak intensity is in the z axis represented by the
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color density. A topographic plot for both the positive and negative ion response generated from the agent GA are shown in Figure 9. As the strength of the RF voltage is increased the peak resolution is enhanced. At Vrf = 550v the peaks overlap while increasing the Vrf = 1,000v results in the ability to distinguish multiple peaks. A topographic plot for VX is shown in Figure 10. When comparing the topographic plots for GA and VX it is apparent that the behaviors are very different, especially at high RF field strengths.
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One of the major concerns expressed regarding DMS systems in the past has been the broad peaks and relatively low peak resolution compared with conventional IMS. Sionex has addressed this apparent shortcoming by operating the microDMx at slightly reduced pressures relative to atmosphere. Under these reduced pressure conditions, down to 0.5 atmospheres, the resolution of the microDMx is significantly increased. The effect of reduced pressure is illustrated in Figure 11 for three CWA simulants, DMMP, DIMP, and MS. The top spectra show the results obtained at atmospheric pressure while below are the results at reduced pressure, 0.65 and 0.5 atmospheres respectively.
Figure 11: Increased resolution by reduced pressure (simulants)
A further advantage of reducing pressure in the system is that the amplitude of RF voltages required to filter the ions can be reduced, this results in a lower power requirements. One of the critical aspects of a CWA detector is how well it can reject interferants to prevent false alarms. One particular interferant that has proved extremely challenging for conventional IMS is to resolve CWAs, or simulants, from Aqueous Fire Fighting Foam (AFFF). The AFFF peak tends to overlap with that of the agent peak. Figure 12 presents experimental results for a series of warfare agent simulants selectively mixed with 1% headspace of AFFF. The different plots will be described starting with the topmost plot and progressing downward. The top plot shows RIP for a microDMx system with background air but no sample present with the sensor at atmospheric pressure. Next, the AFFF interferant is added. This results only in a slight shift to the left (more negative compensation voltage) of the RIP peak. The CWA simulant DMMP is then introduced into alone into the spectrometer and the typical monomer and dimmer peaks appear together with a corresponding reduction in the RIP peak intensity. When 1% AFFF is added, the DMMP peaks are not affected and only a slight leftward shift of the RIP is observed. The same experiment was repeated with DIMP and the effect of AFFF was negligible. Introducing MS and monitoring the negative ion peaks gave similar data illustrating the lack of interference with AFFF. The conclusion is that 1% AFFF has virtually no effect on the microDMx CWA simulant spectra. Similar results were obtained with live agents as well.
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4. DISCUSSION OF RESULTS The JUNO system will have at its heart a next generation microDMx sensor chip, optimized for CWA, TICs and TIMs detection. While sensor performance ultimately determines the effectiveness of a point detector, a good understanding of the operational requirements and user interface issues is essential when designing and optimizing the various components that make up the complete detector system. In addition to the system architecture and algorithm detection software, the key elements that support the microDMx are the heated sample inlet that ensures fast response and cleardown; a membrane that acts as a first stage filter; together with the gas management system to provide a clean air reference environment for the sensor. The choice of materials and the design of mechanical interfaces within the sampling and gas management system play a significant part in maximizing overall detector performance. Also of importance to the user are such things as chemical hardening, ease of maintenance, consumables and life cycle costs. These issues, as well as the previously mentioned functional issues, are key elements of the design philosophy applied throughout the development of the JUNO.
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5. CONCLUSIONS The microDMx technology has clearly demonstrated the capability to take CWA Point Detection to a higher level of performance than achieved with current detector technologies, giving users high sensitivity along with greater selectivity and lower false positive rates. The microDMx sensor technology is being integrated into a miniature point detector, Juno. Juno is demonstrating a clear capability of not only meeting current user needs for point detection but also the capability/potential for evolutionary development that will address the expanding needs for the detection of new threat agents. The JUNO detection system has the built in capability to expand its detection library through simple controls and the development of algorithm software. Ongoing development of the micrDMx technology/sensor, is demonstrating the potential for further miniaturization that will lead to more advanced detection capabilities and detector miniaturization.
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Eiceman, G.A.; Karpas, Z.; Ion Mobility Spectrometry, CRC Press, Boca Raton, Fl. 1993 Miller, R.A.; Nazarov, E.G.; Sensors and Acutuators A, 2001, 91, 3072318
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