Abstract:
A micromechanical field asymmetric ion mobility filter for a detection system includes a pair of spaced substrates defining between them a flow path between a sample inlet and an outlet; an ion filter disposed in the path and including a pair of spaced filter electrodes, one electrode associated with each substrate; and an electrical controller for applying a bias voltage and an asymmetric periodic voltage across the ion filter electrodes for controlling the paths of ions through the filter.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/331,333, filed Jan. 11, 2006, which is a continuation of U.S. application Ser. No. 10/866,645, filed Jun. 10, 2004, which is a continuation of U.S. application Ser. No.  10 / 321 , 822 , filed Dec. 16, 2002, now U.S. Pat. No. 6,806,463, which is a continuation-in-part of U.S. application Ser. No. 09/358,312 filed Jul. 21, 1999, now U.S. Pat. No. 6,495,823. The entire teachings of the above applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a Field Asymmetric Ion Mobility (FAIM) filter, and more particularly, to a micromachined FAIM filter and spectrometer. 
         [0003]    The ability to detect and identify explosives, drugs, chemical and biological agents as well as air quality has become increasingly more critical given increasing terrorist and military activities and environmental concerns. Previous detection of such agents was accomplished with conventional mass spectrometers, time of flight ion mobility spectrometers and conventionally machined FAIM spectrometers. 
         [0004]    Mass spectrometers are very sensitive, highly selective and provide a fast response time. Mass spectrometers, however, are large and require significant amounts of power to operate. They also require a powerful vacuum pump to maintain a high vacuum in order to isolate the ions from neutral molecules and permit detection of the selected ions, and are also very expensive. 
         [0005]    Another spectrometric technique which is less complex is time of flight ion mobility spectrometry which is the method currently implemented in most portable chemical weapons and explosives detectors. The detection is based not solely on mass, but on charge and cross-section of the molecule as well. However, because of these different characteristics, molecular species identification is not as conclusive and accurate as the mass spectrometer. Time of flight ion mobility spectrometers typically have unacceptable resolution and sensitivity limitations when attempting to reduce their size, that is a drift tube length less than 2 inches. In time of flight ion mobility, the resolution is proportional to the length of the drift tube. The longer the tube the better the resolution, provided the drift tube is also wide enough to prevent all ions from being lost to the side walls due to diffusion. Thus, fundamentally, miniaturization of time of flight ion mobility systems leads to a degradation in system performance. While these devices are relatively inexpensive and reliable, they suffer from several limitations. First, the sample volume through the detector is small, so to increase spectrometer sensitivity either the detector electronics must have extremely high sensitivity, requiring expensive electronics, or a concentrator is required, adding to system complexity. In addition, a gate and gating electronics are usually needed to control the injection of ions into the drift tube. 
         [0006]    FAIM spectrometry was developed in the former Soviet Union in the 1980&#39;s. FAIM spectrometry allows a selected ion to pass through a filter while blocking the passage of undesirable ions. Conventional FAIM spectrometers are large and expensive, e.g., the entire device is nearly a cubic foot in size and costs over $25,000. These systems are not suitable for use in applications requiring small detectors. They are also relatively slow, taking as much as one minute to produce a complete spectrum of the sample gas, are difficult to manufacture and are not mass producible. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore an object of this invention to provide a FAIM filter and detection system which can more quickly and accurately control the flow of selected ions to produce a sample spectrum than conventional FAIM devices. 
         [0008]    It is a further object of this invention to provide such a filter and detection system which can detect multiple pre-selected ions without having to sweep the bias voltage. 
         [0009]    It is a further object of this invention to provide such a filter and detection system which can even detect selected ions without a bias voltage. 
         [0010]    It is a further object of this invention to provide such a filter and detection system which can detect ions spatially based on the ions&#39; trajectories. 
         [0011]    It is a further object of this invention to provide such a filter and detection system which has a very high resolution. 
         [0012]    It is a further object of this invention to provide such a filter and detection system which can detect selected ions faster than conventional detection devices. 
         [0013]    It is a further object of this invention to provide such a filter and detection system which has a sensitivity of parts per billion to parts per trillion. 
         [0014]    It is a further object of this invention to provide such a filter and detections system which may be packaged in a single chip. 
         [0015]    It is further object of this invention to provide such filter and detection system which is cost effective to implement and produce. 
         [0016]    The invention results from the realization that an extremely small, accurate and fast FAIM filter and detection system can be achieved by defining a flow path between a sample inlet and an outlet using a pair of spaced substrates and disposing an ion filter within the flow path, the filter including a pair of spaced electrodes, one electrode associated with each substrate and a controller for selectively applying a bias voltage and an asymmetric periodic voltage across the electrodes to control the path of ions through the filter. 
         [0017]    The invention results from the further realization that by providing an array of filters, each filter associated with a different bias voltage, the filter may be used to detect multiple selected ions without sweeping the bias voltage. 
         [0018]    The invention results from the realization that by varying the duty cycle of the periodic voltage, no bias voltage is required. 
         [0019]    The invention results from the further realization that by segmenting the detector, ion detection may be achieved with greater accuracy and resolution by detecting ions spatially according to the ions&#39; trajectories as the ions exit the filter. 
         [0020]    This invention features a micromechanical field asymmetric ion mobility filter for a detection system. There is a pair of spaced substrates defining between them a flow path between a sample inlet and an outlet, an ion filter disposed in the path and including a pair of spaced filter electrodes, one electrode associated with each substrate and an electrical controller for applying a bias voltage and an asymmetric periodic voltage across the ion filter electrodes for controlling the paths of ions through the filter. 
         [0021]    In a preferred embodiment there may be a detector, downstream from the ion filter, for detecting ions that exit the filter. The detector may include a plurality of segments, the segments separated along the flow path to spatially separate the ions according to their trajectories. There may be confining electrodes, responsive to the electrical controller, for concentrating selected ions as they pass through the filter. The confining electrodes may be silicon. The silicon electrodes may act as spaces for spacing the substrates. There may be heater for heating the flow path. The heater may include the ion filter electrodes. The electrical controller may include means for selectively applying a current through the filter electrodes to heat the filter electrodes. The substrate may be glass. The glass may be Pyrex®. There may be an ionization source, upstream from the filter, for ionizing a fluid flow from the sample inlet. The ionization source may include a radioactive source. The ionization source may include an ultraviolet lamp. The ionization source may include a corona discharge device. There may be a clean air outlet for introducing purified air into the flow path. There may be a pump in communication with the flow path, for regulating a fluid flow through the flow path. 
         [0022]    The invention also features a field asymmetric ion mobility filter and detection system. There is a housing having a flow path between a sample inlet and an outlet, an ion filter disposed in the flow path and including a pair of spaced filter electrodes, an electrical controller for applying a bias voltage and an asymmetric periodic voltage across the ion filter electrodes for controlling the path of ions through the filter, and a segmented detector, downstream from the ion filter, its segments separated along the flow path to spatially separate the ions according to their trajectories. 
         [0023]    In a preferred embodiment there may be confining electrodes, responsive to the electrical controller, for concentrating the ions as they pass through the filter. The confining electrode may be silicon. The silicon electrodes may act as a spacer for spacing the filter electrodes. There may be a heater for heating the flow path. The heater may include the ion filter electrodes. The electrical controller may include means for selectively applying current through the filter electrodes to heat the filter electrodes. There may be an ionization source upstream from the filter for ionizing fluid flow from the sample inlet. The ionization source may include a radioactive source. The ionization source may include an ultraviolet lamp. The ionization source may include a corona discharge device. There may be a clean air inlet for introducing purified air into the flow path. There may be a pump in communication with the flow path for regulating a fluid flow through the flow path. 
         [0024]    The invention also features a field asymmetric ion mobility filter array. There is a housing defining at least one flow path between a sample inlet and an outlet, a plurality of ion filters disposed within the housing, each ion filter including a pair spaced filter electrodes, and an electrical controller for applying a bias voltage and an asymmetric periodic voltage across each pair of ion filter electrodes for controller the path of ions through each filter. 
         [0025]    In a preferred embodiment each ion filter may be associated with one of the flow paths. There may be a detector downstream from each ion filter for detecting ions that exit each said filter. Each detector may include a plurality of segments, the segments separated along the flow path to spatially separate the ions according to their trajectories. There may be a plurality of confining electrodes, responsive to the electrical controller, for concentrating the ions as they pass through each filter. Each confining electrode may be silicon. The silicon electrode may act as a spacer for spacing the filter electrodes. There may be a heater fro heating the at least one flow path. The heater may include each pair of ion filter electrodes. The electrical controller may include means for selectively applying a current through each pair of filter electrodes to heat the filter electrodes. There may be an ionization source upstream from each filter for ionizing a fluid flow from the sample inlet. The ionization source may be a radioactive source. The ionization source may be an ultraviolet lamp. The ionization source may be a corona discharge device. There may be a clean air inlet for introducing purified air into at least one flow path. There may be a pump in communication with each flow path for regulating a fluid flow through each flow path. 
         [0026]    The invention also features an uncompensated field asymmetric ion mobility filter for a detection system. There is a housing having a flow path between a sample inlet and an outlet, an ion filter disposed in the path and including a pair of spaced filter electrodes, an electrical controller for applying an uncompensated asymmetric periodic voltage across the ion filter for controlling the path of ions through the ion filter, and a selection circuit for selectively adjusting the duty cycle of the periodic voltage to target a selected specie or species of ion to be detected. 
         [0027]    In a preferred embodiment there may be a detector downstream from the ion filter for detecting ions that exit the filter. The detector may include a plurality of segments, the segments separated along the flow path to spatially separate the ions according to their trajectories. There may be a confining electrode, responsive to the electrical controller, for concentrating the ions as they pass through the filter. The confining electrode may be silicon. The silicon electrode may act as a spacer for spacing the filter electrodes. There may be a heater for heating the flow path. The heater may include the ion filter electrodes. The electrical controller may include means for selectively applying a current through the filter electrodes to heat the filter electrodes. There may be an ionization source, upstream from the filter, for ionizing a fluid flow from sample inlet. The ionization source may include a radioactive source. The ionization source may include an ultraviolet lamp. The ionization source may include a corona discharge device. There may be a clean air inlet for introducing purified air into the flow path. There may be a pump in communication with the flow path for regulating a fluid flow through the flow path. 
         [0028]    The invention also features a field asymmetric ion mobility filter. There is a housing having a flow path between a sample inlet and an outlet, an ion filter disposed in the flow path and including a pair of spaced filter electrodes, a pair of confining electrodes transverse to the flow path, and an electrical controller for applying a first bias voltage and an asymmetric periodic voltage across the ion filter electrodes and for applying a second bias voltage across the confining electrodes for controlling the path of ions through the filter. 
         [0029]    In a preferred embodiment there may be a detector downstream from the ion filter for detecting ions that exit the filter. The detector may include a plurality of segments, the segments separated along the flow path to spatially separate the ions according to their trajectories. The confining electrodes may be silicon. The silicon electrodes may act as a spacer for spacing the filter electrodes. There may be a heater for heating the flow path. The heater may include the ion filter electrodes. The heater may include the confining electrodes. The electrical controller may include means for selectively applying a current through the filter electrodes to heat the filter electrodes. The electrical controller may include means for selectively applying a current through the confining electrodes to heat the confining electrodes. There may be an ionization source upstream from the filter for ionizing fluid flow from the sample inlet. The ionization source may include a radiation source. The ionization source may include an ultraviolet lamp. The ionization source may be a corona discharge device. There may be a clean air inlet for introducing purified air into the flow path. There may be a pump in communication with the flow path for regulating a fluid flow through the flow path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0030]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
           [0031]    Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
           [0032]      FIG. 1  is a schematic block diagram of the micromachined filter and detection system according to the present invention; 
           [0033]      FIG. 2  is a schematic representation of the ions as they pass through the filter electrodes of  FIG. 1  toward the detector; 
           [0034]      FIG. 3A  is a graphical representation of the bias voltage required to detect acetone and the sensitivity obtainable; 
           [0035]      FIG. 3B  is a representation, similar to  FIG. 3A , of the bias voltage required to detect Diethyl methyl amine; 
           [0036]      FIG. 4  is a cross sectional of the view of the spaced, micromachined filter according to the present invention; 
           [0037]      FIG. 5  is a three dimensional view of the packaged micromachined filter and detection system, including fluid flow pumps, demonstrating the miniaturized size which maybe realized; 
           [0038]      FIG. 6  is an exploded view of one embodiment according to the present invention in which an array of filters and detectors are disposed in a single flow path; 
           [0039]      FIG. 7  is an exploded view, similar to  FIG. 6 , in which the array of filters is stacked and one filter and detector is associated with a single flow path. 
           [0040]      FIG. 8  is a cross sectional representation of a single flow path of the arrayed filter and detector system of  FIG. 7 ; 
           [0041]      FIG. 9  is a graphical representation demonstrating simultaneous multiple detections of benzene and acetone; 
           [0042]      FIG. 10  is a schematic block diagram, similar  FIG. 1 , in which the filter is not compensated by a bias voltage and the duty cycle of the periodic voltage is instead varied to control the flow of ions through the filter; 
           [0043]      FIG. 11  is a graphical representation of an asymmetric periodic voltage having a varying duty cycle which is applied to the filter of  FIG. 9  to filter selected ions without a bias voltage; and 
           [0044]      FIG. 12  is a schematic diagram of a filter and detector system in which the detector is segmented to spatially detect ions as they exit the filter. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]    A description of preferred embodiments of the invention follows. 
         [0046]    FAIM spectrometer  10 ,  FIG. 1 , operates by drawing a gas, indicated by arrow  12 , via pump  14 , through inlet  16  into ionization region  18 . The ionized gas is passed between parallel electrode plates  20  and  22 , which comprise ion filter  24 , following flow path  26 . As the gas ions pass between plates  20  and  22 , they are exposed to an asymmetric oscillating electric field between electrode plates  20  and  22  induced by a voltage applied to the plates by voltage generator  28  in response to electronic controller 
         [0047]    As ions pass through filter  24 , some are neutralized by plates  20  and  22  while others pass through and are sensed by detector  32 . Detector  32  includes a top electrode  33  at a predetermined voltage and a bottom electrode  35 , typically at ground. Top electrode  33  deflects ions downward to electrode  35 . However either electrode may detect ions depending on the ion and the voltage applied to the electrodes. Moreover, Multiple ions may be detected by using top electrode  33  as one detector and bottom electrode  35  as a second detector. Electronic controller  30  may include for example, amplifier  34  and microprocessor  36 . Amplifier  34  amplifies the output of detector  32 , which is a function of the charge collected by detector  34 , and provides the output to microprocessor  36  for analysis. Similarly, amplifier  34 ′, shown in phantom, may be provided where electrode  33  is also utilized as a detector. 
         [0048]    As ions  38 ,  FIG. 2 , pass through alternating asymmetric electric field  40 , which is transverse to gas flow  12 , electric field  40 , causes the ions to “wiggle” along paths  42   a ,  42   b  and  42   c . Field  40  is typically in the range of ±(1000-2000) volts dc and has a maximum field strength of 40,000 V/cm. The path taken by a particular ion is a function of its mass, size, cross-section and charge. Once an ion reaches electrode  20  or  22 , it is neutralized. A second, bias or compensation field  44 , typically in the range of ±2000 V/cm or ±100 volts dc, is concurrently induced between electrodes  20  and  22  by as bias voltage applied to plates  20  and  22 , also by voltage generator  28 ,  FIG. 1 , in response to microprocessor  36  to enable a preselected ion species to pass through filter  24  to detector  32 . Compensation field  44  is a constant bias which offsets alternating asymmetric field  40  to allow the preselected ions, such as ion  38   c  to pass to detector  32 . Thus, with the proper bias voltage, a particular species of ion will follow path  42   c  while undesirable ions will follow paths  42   a  and  42   b  to be neutralized as they encounter electrode plates  20  and  22 . 
         [0049]    The output of FAIM spectrometer  10  is a measure of the amount of charge on detector  32  for a given bias voltage  44 . The longer filter  24  is set at a given compensation bias voltage, the more charge will accumulate on detector  32 . However, by sweeping compensation voltage  44  over a predetermined voltage range, a complete spectrum for sample gas  23  can be achieved. The FAIM spectrometer according to the present invention requires typically less than thirty seconds and as little as one second to produce a complete spectrum for a given gas sample. 
         [0050]    By varying compensation bias voltage  44  the species to be detected can be varied to provide a complete spectrum of the gas sample. For example, with a bias voltage of −3.5 volts acetone was detected as demonstrated by concentration peaks  46 ,  FIG. 3A  in concentrations as low as  83  parts per billion. In contrast, at a bias voltage of −6.5 volts, diethyl methyl amine, peaks  48 ,  FIG. 3B , was detected in concentrations as low as 280 parts per billion. 
         [0051]    Filter  24 ,  FIG. 4 , is on the order of one inch is size. Spectrometer  10  includes spaced substrates  52  and  54 , for example glass such as Pyrex® available from Corning Glass, Corning, N.Y., and electrodes  20  and  22 , which may be example gold, titanium, or platinum, mounted or formed on substrates  52  and  54 , respectively. Substrates  52  and  54  are separated by spacers  56   a  and  56   b  which may be formed by etching or dicing silicon wafer. The thickness of spacers  56   a  and  56   b  defines the distance between electrodes  20  and  22 . Moreover, applying the same voltage to silicon spacers  56   a - b , typically ±(10-1000 volts dc) transforms spacers  56   a - b  into electrodes which produce a confining electric field  58 , which guides or confines the ions&#39; paths to the center of flow path  26 . This increases the sensitivity of the system by preserving more ions so that more ions strike detector  34 . However, this is not a necessary limitation of the invention. 
         [0052]    To maintain accurate and reliable operation of spectrometer  10 , neutralized ions which accumulate on electrode plates  20  and  22  must be purged. This may be accomplished by heating flow path  26 . For example, controller  30 ,  FIG. 1 , may include current source  29 , shown in phantom, which provides, in response to microprocessor  36 , a current I to electrode plates  20  and  22  to heat the plates, removing accumulated molecules. Similarly, current I may instead be applied to spacer electrodes  56   a  and  56   b ,  FIG. 4 , to heat flow path  26  and clean plates  20  and  22 . 
         [0053]    Packaged FAIM spectrometer  10 ,  FIG. 5 , may be reduced in size to one inch by one inch by one inch. Pump  14  is mounted on substrate  52  for drawing a gas sample  12  into inlet  16 . Clean dry air may be introduced into flow path  26 ,  FIG. 1 , by recirculation pump  14   a  prior to or after ionization of the gas sample. Electronic controller  30  may be etched into silicon control layer  60  which combines with substrates  52  and  54  to form a housing for spectrometer  10 . Substrates  52  and  54  and control layer  60  may be bonded together, for example, using anodic bonding, to provide an extremely small FAIM spectrometer. Micro pumps  14  and  14   a  provide a high volume thoughput which further expedites the analysis of gas sample  12 . Pumps  14  and  14   a  may be, for example, conventional miniature disk drive motors fitted with small centrifugal air compressor rotors or micromachined pumps, which produce flow rates of 1 to 4 liters per minute. One example of pump  14  is available from Sensidyne, Inc., Clearwater, Fla. 
         [0054]    While the FAIM spectrometer according to the present invention quickly produces a spectrum for a particular gas sample, the time for doing so may be further reduced with an array of filters  32 . FAIM spectrometer  10 ,  FIG. 6 , may include filter array  62 , a single inlet  16  and single flow path  26 . Sample gas  23  is guided by confining electrodes  56   a - h  to filter array  62  after passing by ionization source  18 , which may include an ultraviolet light source, a radioactive device or corona discharge device. Filter array  62  includes, for example, paired filter electrodes  20   a - d  and  22   a - e  and may simultaneously detect different ion species by applying a different compensation bias voltage  44 ,  FIG. 2 , to each electrode pair and sweeping each electrode pair over a different voltage range greatly reducing the sweep time. However, array  62  may include any number of filters depending on the size of the spectrometer. Detector array  64 , which includes detectors  32   a - e , detects multiple selected ion species simultaneously, thereby reduce the time necessary to obtain a spectrum of the gas sample  12 . The electrode pairs share the same asymmetric periodic ac voltage  40 . 
         [0055]    Clean dry air may be introduced into flow path  26  through clean air inlet  66  via recirculator pump  14   a ,  FIG. 5 . Drawing in clean dry air assists in reducing the FAIM spectrometer&#39;s sensitivity to humidity. Moreover, if the spectrometer is operated without clean dry air and a known gas sample is introduced in the device, the device can be used as a humidity sensor since the resulting spectrum will change with moisture concentration from the standardized spectrum for the given sample. 
         [0056]    However, rather than each filter  32   a - e  of filter array  62  sharing the same flow path  26 , individual flow paths  26   a - e ,  FIG. 7 , may be provided so that each flow path has associated with it, for example, inlet  16   a , ionization region  18   a , confining electrodes  56   a ′,  56   b ′, ion filter electrode pair  20   a ,  22   a , detector electrode pair  33   a ,  35   a , and exit port  68   a.    
         [0057]    In operation, sample gas  12  enters sample inlet  16   a ,  FIG. 8 , and is ionized by, for example, a corona discharge device  18   a . The ionized sample is guided towards ion filter  24   a  by confining electrodes  56   a . As ions pass between ion filter electrodes  20   a  and  22   a , undesirable ions will be neutralized while selected ions will pass through filter  24   a  to be detected by detector  32   a.    
         [0058]    As shown in  FIG. 9 , multiple, simultaneous detections were made of Benzene, peaks  50  and acetone peaks  51 , demonstrating the advantage of the arrayed filters and detectors according to the present invention. 
         [0059]    It has also been found that a compensation bias voltage is not necessary to detect a selected specie or species of ion. By varying the duty cycle of the asymmetric periodic voltage applied to electrodes  20  and  22  of filter  24 ,  FIG. 10 , there is no need to apply a constant bias voltage to plate electrodes  20  and  22 . Voltage generator  28 , in response to control electronics  30  varies the duty cycle of asymmetric alternating voltage  40 . By varying the duty cycle of periodic voltage  40 ,  FIG. 11 , the path of selected ion  32   c  may be controlled. As an example, rather than a limitation, the duty cycle of field  40  may be one quarter: 25% high, peak  70 , and 75% low, valley  72 , and ion  38   c  approaches plate  20  to be neutralized. However, by varying the duty cycle of voltage  40   a  to 40%, peak  70   a , ion  38   c  passes through plates  20  and  22  without being neutralized. Typically the duty cycle is variable from 10-50% high and 90-50% low. Accordingly, by varying the duty cycle of field  40 , an ion&#39;s path may be controlled without the need of a bias voltage. 
         [0060]    To improve FAIM spectrometry resolution even further, detector  32 ,  FIG. 12 , may be segmented. Thus as ions pass through filter  24  between filter electrodes  20  and  22 , the individual ions  38   c ′- 38   c ″″ may be detected spatially, the ions having their trajectories  42 ′- 42 ′″ determined according to their size, charge and cross section. Thus detector segment  32 ′ will have one a concentration of one species of ion while detector segment  32 ″ will have a different ion species concentration, increasing the spectrum resolution as each segment may detect a particular ion species. 
         [0061]    Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. 
         [0062]    Other embodiments will occur to those skilled in the art and are within the following claims: 
         [0063]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.