Abstract:
A DMS-IMS chemical sensing system employs two ion-separation technologies in tandem to extract signals of specific chemicals from the glut of signals present. The sensing system generally includes an atmospheric pressure ion generation system, a Differential Mobility (DMS) system, a time-of-flight IMS (TOF-IMS) system, and an ion detector system. The DMS extracts a narrow range of trace chemicals from an environmental sample for subsequent analysis, and a TOF-IMS then analyzes the resulting narrow range of isolated chemicals, allowing compound-specific detection thresholds at sub-ppb concentrations.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a chemical sensing system, and more particularly to a sensing system that utilizes two ion-separation technologies in tandem to detect specific chemicals in the presence of common environmental chemical backgrounds. 
     Sensing to identify and quantitate specific chemicals is of interest for a variety of purposes, including assuring human safety in environments that may contain threats from Toxic Industrial Chemicals (TICs) or Chemical Warfare Agents (CWAs), and in industrial process control. Chemical sensing for environmental monitoring must be compatible with safety requirements, as specifically promulgated by OSHA and other government organizations through time-weighted average Permissible Exposure Limits (PELs). These standards are also broadly applied to potential chemical threats. Many TICs, including pesticides, acid vapors, and carcinogens, and the CWAs have PELs that range from part per million (ppm) and part per billion (ppb) levels to part-per-trillion (ppt) levels in air. At those low concentrations the air we breathe can have hundreds of other chemicals (scents, simple hydrocarbons, and other non-toxic vapors) in addition to near percent loads of water and carbon dioxide. Similarly, industrial process streams may have the chemicals to be monitored embedded in complex mixtures. Accurately and sensitively detecting the compounds of interest in the presence of complex and variable chemical backgrounds has historically required advanced laboratory analytical separations and complex instrumentation, such as gas chromatograph-mass spectrometry (GC-MS). GC-MS is a tandem or “hyphenated” analytical technique that represents a laboratory level standard for chemical mixture analysis, where the resolving power of two “orthogonal” measurement techniques that respond to different physical/chemical properties is used to extract signals from a complex sample matrix. 
     There is a looming possibility that civilians and first responders may be exposed to environments involving hazardous chemicals including CWAs. Low-cost portable and fixed site sensors providing reliable detection of TICs and CWAs at ppb and sub-ppb levels are required to provide rapid, on-site characterization and warning of the threat in the event of an attack or incident. No portable sensor technology exists that can detect specified lists of TICs and CWAs at the sub-ppb levels in real time and with acceptable false-alarm rates in the presence of common environmental chemical backgrounds. 
     Variations of Ion-Mobility Spectrometry (IMS) have been utilized for chemical vapor detection. The variants include Time-of-Flight IMS (TOF-IMS), Radio Frequency IMS (RFIMS), and a technique known either as Field Asymmetric IMS (FAIMS) or Differential Mobility Spectrometry (DMS). As with TOF-IMS, the FAIMS and DMS variants are highly sensitive with moderate specificity. However, both TOF-IMS and DMS are increasingly subject to false detection as sensor gain is increased and sub-ppm limits of detection are pursued, this due to the more complex nature of samples at ppb and ppt levels. While DMS technology has the sensitivity to approach PEL level detection of CWAs without preconcentration, resolution becomes a limiting factor in the presence of trace environmental chemical backgrounds. 
     Accordingly, it is desirable to provide a chemical sensing system that detects specified lists of TICs and CWAs at the sub-ppb levels in real time and in the presence of common environmental chemical backgrounds. 
     SUMMARY OF THE INVENTION 
     The sensor system according to the present invention includes a tandem DMS-IMS chemical sensing system that employs two ion-separation technologies in tandem to extract signals characteristic of specific chemicals in the presence of common environmental chemical backgrounds. The sensing system generally includes an ion generation system, a DMS system, a TOF-IMS system, and an ion detector system. 
     The DMS system extracts a narrow range of trace chemicals from an environmental sample for subsequent analysis, and one or more TOF-IMS detectors then analyze the resulting narrow range of isolated chemicals. The resulting ‘hyphenated technique’ of DMS-IMS allows compound-specific detection thresholds of sub-ppb concentrations with a sensor that is not significantly more complex than an IMS alone. 
     The combination of the differential mobility of the DMS and ion mobility of the IMS provides significant resolving power. The physical-chemical principles upon which the resolving properties of these devices are based differ sufficiently that their ion separations are highly orthogonal. Under this condition, the effective resolving power of the detection system approximates the product of the resolving power of each system alone. This allows the system to detect chemicals in air at ultra-trace levels while minimizing false positive detections. Moreover, the tandem sensor technologies upon which this higher performance is based are still particularly well suited to miniaturization, low power implementation and relatively low manufacturing cost. 
     The present invention therefore provides a chemical sensing system that detects specified lists of TICs and CWAs at sub-ppb levels in real time and in the presence of common environmental chemical backgrounds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
         FIG. 1  is a block diagram of a chemical sensing system according to the present invention; 
         FIG. 2  is a block diagram of a controller for a chemical sensing system; 
         FIG. 3  is a general schematic view of a chemical sensing system according to the present invention; 
         FIG. 4  is a schematic view of a Differential Mobility Spectrometer drift cell and representational ion traces passing therethrough; 
         FIG. 5  is a table illustrating various CWAs and TICs that a tandem DMS-IMS analyzer may be programmed to detect. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates a general schematic view of a chemical sensing system  10 . The sensing system  10  generally includes an atmospheric pressure ionization (API) system  12 , a Differential Mobility Spectrometry (DMS) system  14 , and a Time-of-Flight Ion Mobility Spectrometry (TOF-IMS) system  16 . The TOF-IMS system  16  includes an ion detector system  18 , which communicates with a controller  20  (also schematically illustrated in  FIG. 2 ). Instrument control and analysis functions to detect specified lists of TICs and CWAs are programmed into the controller  20  and are reconfigurable via a communication system  21 . The communication system  21  could perhaps be via serial or parallel interface, could be established via a Wi-Fi network, or could be based on a self-configuring RF network, with software for remote commanding and data analysis layered onto the selected communications link. 
     Generally, the API system  12  rejects simple hydrocarbon interferences while efficiently ionizing target molecules. From the API system  12  ions are communicated to the DMS system  14  under a gas flow. The DMS system  14  is adjusted or “tuned” to allow a narrow range of target ions such as CWAs and TICs to pass through, neutralizing most of the background ions created in the API source. From the DMS system  14 , the target ions are communicated into the TOF-IMS system  16  and the detector system  18 , which measures the ion mobility spectra of all species present for comparison to the TOF-IMS spectra of known CWAs and TICs. 
     Referring to  FIG. 3 , the system  10  employs both the DMS system  14  and the TOF-IMS system  16  in tandem to extract signals of specific chemicals including specified lists of TICs and CWAs in the presence of common environmental chemical backgrounds. Since differential mobility is largely a function of chemical type with highest resolution for low-mass ions, and time-of-flight ion mobility is fundamentally dependent on molecular weight and ion cross-section, the result is a highly sensitive, highly specific sensing system. 
     The API system  12  receives filtered air through a filtered air conduit  22  and sample air through a sample air conduit  24 . Preferably, the API system  12  includes an atmospheric pressure ionization source including a photoionization source, corona discharge source, beta decay source, or alpha decay source, in which air and/or chemical vapors ionize as they enter from the sample air conduit  24 . Airflow through the chemical sensing system  10  is effected by a sample pump  15  that draws the sample air into a recirculating, scrubbed flow in which humidity and organic vapor concentrations are preferably maintained at relatively low level. 
     The DMS system  14  includes a DMS drift cell  26 , a planar structure with two electrodes that defines an axis A. Materials that obtain a charge in the ionization source  12  at the front of the DMS drift cell are pushed between the pair of electrodes  28  ( FIG. 4 ). Voltages are applied to the electrodes  28  to effect an asymmetrically-oscillated electric field perpendicular to the ion path A. The controller  20  applies a brief high electric field followed by a lower field of opposite polarity for a longer time, such that the time-weighted applied field is nearly zero. In the time required for an ion to transit the DMS drift cell  26  to the TOF-IMS system  16 , about 1 millisecond, the RF cycle is repeated approximately 1000 times. 
     As the bulk flow of ions transit the DMS drift cell  26 , ions are forced by the oscillating field into a zig-zag or saw-tooth path, here greatly exaggerated ( FIG. 4 ). Ions with the same mobility or effective molecular cross-section under high and low-field conditions will travel a zig-zag path averaging down the middle of the DMS drift cell. Most molecular ions, and notably many CWAs and TICs, do not have equal high- and low-field mobilities. This happens when the molecule deforms in high fields, changing its cross-section. It also results when the molecular ion has sufficient affinity for clustering with water molecules in low fields, but collisionally declusters in high fields. For such molecules, the different voltages interact with these properties to cause the ions to drift toward either the upper electrode plate  28   t  or the bottom electrode plate  28   b , a little with each cycle. Over the many cycles the ions undergo, any unbalanced up or down net movement eventually forces the ions into contact with the electrode plates  28   t ,  28   b  where they are neutralized. 
     At any one time, most ions entering the DMS drift cell  26  impact upon the plates  28 , are neutralized, and then are carried out of the system  10  as neutral molecules. Only those ions that successfully transit the length of the DMS drift cell without contacting the electrodes  28 , i.e. those ions in tune with the applied electric field, are analyzed and detected by the TOF-IMS system  16  detectors. In practice, offset voltages, also known as compensation voltages, are added to the RF voltages, biasing the electric fields such that different ions are selected to pass through the DMS drift cell, thereby scanning the range of differential mobilities. The compensation voltage at which an ion is transmitted through the DMS is a measure of that ion&#39;s differential mobility. 
     The DMS system  14  provides continuously scanned and/or step-scanned functions to allow passage of specific ions using a combination of RF field amplitude and DC compensation voltage to modify the DMS drift cell  26  transmission. Chemical vapors or dopants may be added to the sample air in the drift cell to complex with target ions in the DMS drift cell, increasing target ion differential mobility, or to quench the formation of some interfering ions, depending on the chemicals to be detected. 
     For example only, a nerve agent like sarin has a phosphorous-oxygen core (P═O) that, when protonated, has an affinity for water. In humid air, the ion sarin forms in a DMS is Sarin.H+.nH 2 O, with the number (n) of associated water molecules dependent on temperature and humidity. Under very high fields, the ion is pulled so rapidly through the air that its collisions with air molecules decluster most of the water, leaving Sarin.H+0.1H 2 O, with a smaller collisional cross-section. When the field reverses and drops in amplitude, the ion is allowed to recluster with water molecules from the air around it and gets bigger again. The net effect is that the ion is smaller under the high-field portion of the RF cycle, larger under the low-field portion, and this size difference leads to a tendency to drift upward as the ion travels its sawtooth path down the DMS cell. 
     Thus, when the DMS system  14  is tuned to measure sarin, this upward drift must be countered. An offset voltage CV is applied to the electrode plates  28   t ,  28   b  so that as the ions are pulled up by the sawtooth, they are electrostatically driven back toward the center of the DMS. This reduces the efficiency of the ‘up’ sawtooth, and increases the efficiency of the down sawtooth, allowing any sarin present, and any other ions of similar differential mobility to transit down the DMS system  14 . Under this condition, ions of 0 differential mobility that were transiting the system under a CV of 0 volts are now dragged into the bottom electrode plate  28   b.    
     Only those ions that transit the DMS system  14  without contacting the upper or lower electrodes  28   t  and  28   b  are available to the TOF-IMS system  16  for analysis. Target species are caused to transit the DMS system  14  by setting the DMS system electrodes to specific compensation voltages, and the TOF-IMS system detects any target species present by measuring the signals associated with ions transiting the DMS system  14 . 
     The TOF-IMS system  16  includes one or more IMS drift cells arranged to detect ions passed by the DMS system  14 .  FIG. 2  illustrates one possible configuration, showing both a positively-biased TOF-IMS drift cell  30  and a negatively-biased IMS drift cell  32  coupled to the DMS drift cell  26  to simultaneously analyze both the positive and negative ions that are in tune with the DMS applied electric field. When two TOF-IMS drift cells  30  and  32  are used, they are preferably arranged to define an ion flow axis B transverse to axis A. Other geometries can be conceived for interfacing TOF-IMS drift cells to a DMS system, and are part of this invention. 
     The positive and negative ions in the DMS system exhaust are pulled toward a pair of ion gate electrodes  34  and  36  under an applied electric field. Once at these electrodes, the ions are gated into one of two TOF-IMS drift cells,  30  for the positive ions and  32  for negative ions. The pulses of ions gated into each TOF-IMS cell are drawn down the length of the cells under an applied electric field, while the controller  20  measures the number of microseconds the ions take to get from the gate electrode at one end to the ion detector at the other end. The controller  20  drives the ion gate electrode  34 ,  36  modulation patterns ( FIG. 2 ). 
     Each TOF-IMS drift cell  30 ,  32 , is terminated with an ion detector  38 ,  40 , which communicates with the controller  20  ( FIG. 2 ). Each ion detector  38 ,  40  is preferably a Faraday collector. Other detectors such as a charge-coupled Faraday collector, delta-doped silicon charge coupled device, or ion multiplier that is sensitive to the ions transmitted down the TOF-IMS drift cells  30 ,  32  may alternatively or additionally be utilized. 
     Determination of drift time by the TOF-IMS system  16  is effected by time-of-flight detection, measuring signals as a function of delay time following the admittance of ions into the drift cells  30 ,  32 . Ion drift times may be determined using single gate/analyze time-of-flight measurements, or using a gate correlation technique. The measured IMS drift times are then converted to mobility by normalization to drift field, temperature, and cell pressure. A TOF-IMS system  16  sorts the ions by molecular weight and collisional cross-section, obtaining a resolution of approximately one part in 100. The TOF-IMS system  16  also provides simultaneous positive and negative ion detection, providing real-time confirmation of target detection for certain analytes, including nerve and blister agents. Extensive detection libraries of the DMS differential mobilities and time-of-flight IMS mobilities of particular CWAs and TICs are preferably stored within the controller  20  for comparison and identification ( FIG. 1 ). 
     The combination of the differential mobility of the DMS and ion mobility of the TOF-IMS provides significantly higher resolution than either DMS or TOF-IMS, providing a resolving power approximately equal to the product of each technology alone. This permits the system  10  to detect chemicals in air at ultra trace levels while minimizing false positive detections, using a technology compatible with portable sensors. In short, the DMS-IMS chemical sensing system  10  represents a significant advantage over either technology alone in sensitivity and resolution, and possesses the required characteristics for application to trace chemical detection. 
     Referring to  FIG. 5 , detection—indicated by a Y in columns  3  and  4 —for CWAs and TICs as provided by the DMS-IMS chemical sensing system  10  is readily achieved. Confirmation is also available in every scan using dual positive and negative ion detection capability for those CWA materials that exhibit dual mode sensitivity (Y+/−). 
     The foregoing description is exemplary rather than defined by the limitations within. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.