Abstract:
A sensor for detecting volatile organic compounds in ambient air by positioning a detection cell adjacent a gas discharge device to cause molecules of organic compounds to become ionized, and applying an electric field across the collection cell to attract ions and free electrons formed in the cell to develop a current, and amplifying the current magnitude so created.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for detecting organic compounds in ambient air. More particularly, the apparatus is based on a photoionization sensor of sufficiently small size so as to be useful in portable multi-sensor instruments. 
     The apparatus uses a very small, high-energy, vacuum ultraviolet radiation device, which is attached to a chamber exposed to the ambient air of interest. The chamber is subjected to an electric field. The radiation device is a gas discharge lamp connected to a suitable voltage source. A certain percentage of the organic compounds in this ambient air chamber will become ionized, ie., converted into positively charged ions and negatively charged electrons. The major constituents of the ambient air, such as nitrogen (N 2 ) and oxygen (O 2 ), are unaffected by the radiation device because the energy of the radiation (8.5-11.7 eV) is too low to cause ionization of these constituents. The positive and negative ionization charges are collected by suitable electrodes, thereby generating a current which may be measured to provide an indication of the concentration of organic compounds found in the ambient air. 
     Therefore, the apparatus is very useful for detection of a wide range of volatile organic compounds in ambient air, in concentrations as low as in the parts-per-billion (ppb) range, without interference from air components. 
     SUMMARY OF THE INVENTION 
     An ultraviolet radiation source is constructed of a glass housing having a window at one end and being filled with an inert gas such as krypton or argon. A dielectric plate having a pattern of holes drilled proximate its center is placed adjacent the window after a thin metal layer is placed on either side of the plate, and each of the layers is covered with a thin layer of dielectric material. The gas in the lamp housing is excited by a capacitively-coupled radio frequency voltage, causing ultraviolet illumination in the pattern of holes. The apparatus is placed near a source of ambient air containing volatile organic compounds, and the ultraviolet illumination causes ionization of some of the organic compound molecules which have migrated into the pattern of holes. A DC voltage is placed across the metal layers on either side of the dielectric plate, and the charges from the ionized molecules are collected in the metal layers to cause a current to flow; the current is measured to provide a measure of the concentration of volatile organic compounds in the ambient air. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a simplified diagram illustrating the overall functioning of the apparatus; 
     FIG. 2A shows an exploded view of the gas discharge lamp and dielectric plate; 
     FIG. 2B shows an expanded view of a portion of the dielectric plate; 
     FIG. 3 shows an alternative construction of the dielectric plate; 
     FIG. 4 shows a miniature hybrid electronic circuit built on the dielectric plate; 
     FIG. 5 shows one form of electronic circuit for energizing and driving the components of the sensor; 
     FIG. 6 shows an alternative form of electronic circuit driver; 
     FIG. 7 shows excitation electrodes formed on the outer glass surface of the gas discharge cell; 
     FIG. 8 shows the exterior view of the sensor after assembly of all components; and 
     FIG. 9 shows the assembly of FIG. 8 with the outer cover removed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a simplified view of the apparatus is shown. A gas discharge lamp  30  emits ultraviolet radiation (UV)  20  as a result of being capacitively excited by an external radio frequency voltage (not shown). The radiation passes through a window  100  into an adjacent cell volume  102  which contains molecules  10  of a volatile organic compound. As a result, some percentage of the molecules are ionized by the UV radiation, converting the molecule into positively charged ions and free electrons, according to the equation: 
     
       
           M (molecule)+photon= M   + (ion)+ e   − (electron).  
       
     
     A pair of electrodes  40 ,  50  is positioned near the cell volume; one electrode  40  is connected to a high voltage DC source  70 , and the other electrode  50  is connected to the input of an amplifier  60 . The electric field created by these electrodes forces both the electrons and the ions to migrate toward respective electrodes, where they are collected to produce a very small current flow. The current flow is amplified by the amplifier  60 , and the amplifier output signal is displayed or recorded by a connected display/recorder  62 . 
     FIG. 2A shows an isometric exploded view of the gas discharge lamp  30 , the detector cell  11 , and FIG. 2B shows a further expanded view  12  of the layered construction of the detector cell  11 . The gas discharge lamp  30  is preferably made with an outer glass housing  90  and a window  100  made from magnesium fluoride, with krypton gas sealed inside the glass housing  90 . Ultraviolet radiation produced by excitation of the Krypton gas readily passes through the window  100 . 
     Adjacent the window  100  is placed a detector cell  11  constructed as a wafer from alumina ceramic material, which has excellent dielectric properties. Detector cell  11  has a plurality of holes  22  forming a hole pattern through the wafer. Each side of the wafer is plated with a metal layer  32 ,  42 , and each metal layer is coated with a thin layer of dielectric material  52 ,  53 . The layers of dielectric material serve to reduce photoemission from the detector cell  11 . 
     Conductors  73 ,  74  are attached to the metal layers  32 ,  42 . The hole pattern serves as a plurality of open volumes where the ionization of gas and collection of produced charges takes place, as will be explained more fully hereinafter. A high DC voltage is applied to conductor  73 , and conductor  74  is connected to the input of amplifier  60 . The electrostatic field developed between the metal layers  32  and  42  causes a current flow to the amplifier  60  input, proportional to the ionization of the organic molecules which have collected in the plurality of holes  22 . 
     FIGS. 3A and 3B show an alternative construction of the detector  90 , where a third conductive layer  104  is embedded in the detector wafer  11  between the metal layers  32 ,  42  of FIG.  2 B. The purpose of the third conductive layer  104  is to prevent the flow of unwanted current between the electrode conductors  73 ,  74  over the surface of the cell&#39;s dielectric material under conditions when the sample gas has a high moisture content. Conductor  104  is connected to the electrical ground of the circuitry, Conductor  104  is shaped to occupy area outside the cell&#39;s holes  22 , and it does not affect the electric field inside the holes. 
     FIG. 4 illustrates an alternative construction wherein the electronic circuitry, including the amplifier  60 , an A/D converter (not shown), and other related circuitry can be formed on the same dielectric substrate  22  as the detector cell  11 . 
     FIG. 5 shows an electric circuit which can be used as part of the present invention. A miniature transformer  80  has a secondary winding  82  which is connected to the gas discharge tube&#39;s excitation electrodes  83  which, for example, may be plated against the outer surface of the glass lamp  30 . A secondary winding tap  84  is connected via a diode  85  to provide a rectified DC voltage Vpol which can be used as the voltage applied between electrodes  40 ,  50  (FIG.  1 ). The electric circuit requires a +5V input power supply and conventional filter and feedback circuits, and may be miniaturized for construction. 
     FIG. 6 shows an alternative circuit design, utilizing a piezoelectric transformer. An AC voltage is applied to the input  93  of this transformer, causing vibration which causes generation of an output voltage to terminals  94 . The output voltage is applied to the electrodes  83  which are placed about the surface of glass discharge lamp  30 . 
     FIG. 7 shows the gas discharge lamp  30  wherein the excitation electrodes  83  are preferably applied directly to the surface of the lamp by a metal sputtering process. Conductors  86  can be attached to the electrodes  83  with conductive epoxy. 
     FIG. 8 shows an exterior view of the invention after all components have been assembled, and are inserted inside a standard plastic housing  96 . The hole pattern  22  is exposed at one end of the housing  96  and plug-in connector pins  98  project from the other end of the housing. A typical size for the assembly, including the housing  96  and connector pins  98 , is about 20 millimeters (mm), with an outside diameter of about 20 mm. The dimensions of the sensor and the pin layout of its connectors are the same as available on industry-standard electrochemical sensors. Therefore, the sensor is mechanically compatible with a majority of commercial portable gas analyzers (based on electrochemical types of sensors) and can be implemented in those gas analyzers without redesigning them. 
     FIG. 9 shows the assembly without the outer housing  96 . The assembly comprises a base  108  through which the connector pins  98  project. The connector pins  98  are suitably connected to a printed circuit board  110  which is attached to the base  108 . The electrical transformer  80  (see FIG. 5) is attached to the printed circuit board  110 , as are the conductors  86  which lead to the excitation electrodes  83 . The detector hole pattern  22  is placed adjacent the window in the gas discharge lamp  30 , and a further electronic circuit board carries the electronics associated with the detector. 
     In operation, the detector cell is placed in an ambient gas location, where the hole pattern  22  is exposed to receive samples of the gas under test. As samples of this gas migrate into the holes of the pattern  22 , a certain percentage of the molecules will become ionized, and the ions will be collected by the electrodes as a current. The current is fed into an amplifier and associated circuitry, to produce a signal representative of the measured gas concentration, and the signal may be displayed or recorded as needed. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof; and it is, therefore, desired that the present embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.