Abstract:
System and method for operating an ionizer using a combination of amplitude modulation and pulse width modulation to control the plasma temperature and the type of ions needed for analytic equipment. Ion density can be controlled by the repetition rate. The ionizer may utilize a non-radioactive ionization source, and be coupled to a differential mobility spectroscopy (DMS) analyzer.

Description:
This application claims priority to U.S. Provisional Patent Applications Ser. Nos. 61/587,352 and 61/618.947, which are hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to utilization of gas ionization for detection of chemical compounds, such as in air. 
     BACKGROUND INFORMATION 
     Ion Mobility Spectrometry (“IMS”) for the detection of chemical warfare agents (“CWAs”), toxic industrial chemicals (“TICs”), drugs, and explosives is primarily based on gas ionization by radioactive sources (e.g.,  63 Ni,  241 Am, and  3 H), since these sources meet requirements of a portable device for field use: small sized and lightweight, good mechanical stability, and do not require any additional power. Furthermore, they are very reliable while displaying a good sensitivity with regard to the detection of quite a large number of compounds of interest. However, for well-known reasons (e.g., radiation safety, regulation, record keeping, disposal problems) there is a growing interest in replacing radioactive sources by alternative gas ionization techniques. 
     One type of ionization source is a dielectric barrier discharge as ionizer, which has two metal electrodes separated by an insulator. However, in this configuration, one of the electrodes is exposed to the gas being ionized, which can lead to electrode erosion and gas contamination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a non-radioactive ionization source configured in accordance with embodiments of the present invention. 
         FIG. 1B  shows a digital image of a differential mobility spectroscopy (“DMS”) analyzer configured in accordance with embodiments of the present invention, 
         FIG. 2  illustrates a block diagram of an ionizer and driver configured in accordance with embodiments of the present invention. 
         FIG. 3  shows exemplary oscilloscope images showing AC voltage frequency (left image) and pulse repetition rate (right image). 
         FIG. 4  illustrates a schematic of pulse operation showily, exemplary voltage pulse repetition rate. 
         FIG. 5  illustrates a flow diagram of a stabilization process in accordance with embodiments of the present invention. 
         FIG. 6  illustrates a principle of operation of a DMS as an ion filter using the principles of ion mobility, 
         FIG. 7A  illustrates a dielectric side of a substrate of an ionizer in accordance with embodiments of the present invention. 
         FIG. 7B  illustrates an electrode side of a substrate of an ionizer in ac accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide a nonradioactive ionization source that exhibits stability, low power consumption, and long lifetime to replace radioactive sources. The nonradioactive ionization source may be used in place of radioactive sources for instruments such as a differential mobility spectroscopy (“DMS”) analyzer and other ion mobility spectrometers (“IMS”), such as a time of flight ion mobility spectrometer and field asymmetric ion mobility spectrometers. As illustrated in  FIG. 6 , a principal of operation of such a DMS has volatile organic compound (“VOC”) analyte molecules ionized as they enter the DMS. The DMS is essentially an ion filter operating in a gas environment. In embodiments of the present invention, the gas environment is filtered and dried (de-humidified) air at near atmospheric pressure. As previously noted, the most common technique used to create gas ions is to place a radioactive source material (either beta emitter or alpha emitter) next to the gas flow. The inventors succeeded in developing an ion generator that does not utilize radioactive sources. 
     Referring to  FIG. 1A , the radioactive gas ionization source is replaced with a non-radioactive gas ionization (“NRI”) source  100 . The NRI  100  may include plates comprising two parallel electrodes  101 ,  102  coated with a dielectric material  103 ,  104 . The electrodes  101 ,  102  may be held apart by spacers  105  in a configuration in which the dielectric surfaces face each, other and allow gas to flow through the channel between them. In such a configuration, the metal electrodes  101 , are not exposed to the gas or ions and thus are protected from plasma erosion. As shown in  FIG. 1B , a DMS analyzer  110  may be coupled to the ionizer  100 . The carrier gas flows into the assembly of  FIG. 1B  and through the ionizer  100 , The carrier gas is ionized in the ionizer  100  and continues to flow through the assembly. Analyte is introduced into the ionized carrier gas where charge exchange occurs creating analyte ions. The mixture of carrier gas, carrier gas ions, analyte, and analyte ions then flows into the DMS  110  for analysis and thereafter the gas is exhausted. 
     To create ions, a plasma is generated between the plates by application of an alternating voltage (“AC”) across the electrodes  101 ,  102 , As a nonlimiting example, the peak voltage may be in a range of approximately 3 kV to 8 kV, With a larger gap between the plates, a higher voltage is needed: using a smaller gap allows lowering the peak voltage. Typically, this AC voltage is driven in a non-regulated manner, such as with a continuous waveform (“CW”), and the voltage is merely adjusted to a level where the plasma ignites. Using this method of driving the NRI, the plasma is difficult to keep stable and power consumption is poorly regulated. Furthermore, the non-regulated driving method also creates negative ions (e.g., nitrous oxide and other compounds) that are highly electronegative and thus do not easily share their charge with other compounds. Thus, a non-regulated NRI creates an ample number of negative ions, but these ions are effectively useless in that they do not result m ionization of the analyte compounds of interest. Moreover, if using a high power driver in air, a lot of ozone may he created. 
     To address such problems, a driving method and circuitry in accordance with embodiments of the present invention is disclosed herein to reduce power consumption and maintain stability of the plasma. A more regulated driver may be used to generate as soft plasma more useful for ion mobility, since it may be better to create ions that have low electronegativity (e.g., negative oxygen and water molecules such as O 2   −  and H 2 O − ) that will share their charge with molecules and volatile organic compounds of interest in the sampled gas being characterized by the ion mobility tool. 
     By adjusting the gas flow and electrode dimensions, the ionization of the vas can be better controlled, To maintain a soft plasma, the gas should not be excited once it has already been ionized. By increasing the gas flow rate, ions pass out of the ionization region before a new voltage cycle is applied to the ionizer. A similar result may also achieved by shortening the electrode so that ions pass out of the ionization region before the new voltage cycle applies. 
     Referring to  FIG. 2 , an ionization source driver in accordance with embodiments of the present invention comprises: 
     1. A power controller  201 : Controls sinusoidal voltage (e.g., 0 to 8 kV (peak to peak) of frequencies from 300 to 500 kHz) applied, to the discharge electrodes  101 ,  102 ; 
     2. A timer  202 : Switches (or, pulses) the power controller  201  ON and OFF in accordance to a predetermined pulse width modulated frequency (e.g., with a repetition rate from 0.5 to 3.0 KHz, and pulse width from 50 μS to 500 μS); 
     3. A plasma detector  203 : Monitors current through the discharge electrodes  10   102  and generates a signal sent to the timer  202  when the plasma ignites in the ionizer  100 . 
     In general, as previously noted, the plasma ignition process is very unstable. To ignite a plasma in the NRI  100 , the AC voltage applied to the discharge electrodes  101 ,  102  may depend on humidity, gas pressure, gas flow rate, gas composition, insulator properties, NRI gap, and other factors. In order to produce the desired ions, the plasma should be as “cold” as possible (i.e., of a lower power). The higher the power of the plasma (i.e., a “hot” plasma.), the more NO x  ions are created. NO x  ions inhibit, ionization of the analyte ions being studied in an IMS system. Therefore, the power (AC voltage) applied to the discharge electrodes  101 ,  102  should be as low as possible, but yet sufficient to ignite the plasma. The high power makes the NRI ignition process even more unstable. 
     To stabilize the process and produce a stable stream of desired ions, the following method was developed and the device (ionization source driver) designed. The method turns the AC voltage ON according to a repetition rate set by a user (i.e., a predetermined pulse width modulated frequency), and then turns the AC voltage OFF when a plasma discharge is detected. This allows the gas to cool and prevents continuous, hotter ionization. The process may then be stabilized using pulse width modulation with feedback from a plasma discharge detector  203 . 
     Refer next to the flow diagram illustrated in  FIG. 5 ,  FIG. 4  is a schematic of an exemplary AC voltage pulse operation, which operates as described below with respect to  FIG. 5 . When the device illustrated in  FIG. 2  begins operation in step  501 , the AC voltage is set in step  502  by the power controller  201  to zero (i.e., (VIAL In steps  503  and  504 , at time t 1  the timer  202  turns the power controller  201  ON (AC voltage ON), and waits for a predetermined period of time (essentially steps  503 - 507  in a loop until time t 2 ). If during this time (step  505 ), plasma discharge was not detected by plasma detector  203 , the AC voltage setting in step  506  will be increased (e.g., either during the time period t 1 -t 2 , or for the next cycle beginning with t 3 ). The timer  202  may then turn the power controller  201  OFF in steps  507 - 508  at time t 2  For a next cycle beginning with time t 3  (return of process to step  503 ), when the power controller  201  is ON, the AC voltage will now be higher than in the previous cycle. This process repeats until the plasma discharge is ignited and detected by detector  203  in step  505  (e.g., corresponding to time  141 , and any other subsequent instances of time when the AC voltage is ON). At this time, the AC voltage is high enough to ignite the plasma discharge. 
     When a plasma discharge is detected b detector  203 , it sends a signal to the timer  202  (e.g., at time t 4 ). In response, the tinier  202  turns the power controller  201  OFF (e.g., at time t 4 ) in step  508 , and the AC voltage setting remains the same for the next cycle. During a next cycle, when the power controller  201  is ON, the AC voltage will not change. As such, the process has become stable. 
     The timer  202  may turn the AC voltage ON, according to any repetition rate, including but not limited to a predetermined pulse width modulated frequency, and turn the AC voltage OFF, when plasma discharge is detected. 
       FIG. 3  shows an oscilloscope trace with an exemplary waveform for an AC driving voltage. The amount of ions produced by the ionizer may be adjusted by changing the Frequency of the AC voltage or the pulse repetition rate. 
     Referring to  FIGS. 7A-7B , in alternative embodiments, the ionizer plates may be made from alumina (e.g., approximately 1 mm thick) with electrodes  101 ,  102  printed with a conductor (e.g., Au/Pt). Spacers  105  may be from PIPE (e.g., 380 μm to 500 μm thick) to hold the two apart in the configuration of  FIG. 1A . The spacers  105  form a channel to direct the gas flow through the ionizer  100  where the plasma is created before the ions are carried out of the ionization region, The gas is ionized when it is in a static, no flow state, but in a no flow case, the ionization creates a lot of ozone and NO x  ions in air. Flowing the gas at approximately  300  seem (standard cubic centimeters per minute) to 500 sccm may be performed for an IMS application, but the ionizer works at more than approximately 1000 seem as well. The metallic electrode may be a printed metal pad of approximately 5 mm×3 mm with a lead to connect to the electronic drivers. 
     The plates may be made from alumina or another dielectric, such as glass or printed circuit board material. The metal electrode may be a printed or painted metallic ink, or a thicker metal structure, such as a metal tape, wire, or thin metal. The structure may also be a metal support for the electrode coated with as dielectric material. The shape of the electrodes does not need to be rectangular, but may be circular or another shape to conform to a specific application.  FIG. 7A  illustrates a dielectric side of the plates, while  FIG. 7B  illustrates a metal electrode side of the plates. 
     By protecting the electrodes with a ceramic or dielectric, an ionizer will have a longer lifetime and will generate a cleaner plasma.