Abstract:
A photoionization detector comprising multiple ionization cells for detecting the presence of chemical compounds in multiple fluid samples. The photoionization detector uses a single source of ultraviolet (UV) radiation to emit high energy photons into at least two ionization cells. Each ionization cell comprises at least one fluid inlet and at least one fluid outlet. Independent samples of fluid can be introduced into each cell and exposed to the UV radiation such that any molecules with an ionization energy lower that the energy of the photons are ionized. Each ionization cell also comprises a set of at least two electrodes arranged to generate an electric field such that ionized molecules are attracted to the electrode at lower voltage potential resulting in an electrical current.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority from U.S. provisional patent application Ser. No. 60/322,778, for Photoionization Detector With Multiple Ionization Cells, filed Sep. 18, 2001, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to photoionization detectors for detecting the presence of chemical compounds in a fluid and more particularly to a photoionization detector that comprises multiple ionization cells. 
   2. Description of the Background Art 
   Photoionization detectors (PIDs) are conventionally used to detect the presence of chemical compounds in air. When a molecule is exposed to high-energy photons of the appropriate photon energy, the molecule will become ionized. A pair of electrodes are arranged to be exposed to the ionized molecule such that the electrodes are electrically insulated from each other and are maintained at a voltage differential from each other. The ion is repelled from the higher voltage electrode and attracted to the lower voltage electrode. When a fluid stream is exposed to the high-energy photons, and a number of ions are produced, a measurable current is generated. 
   Conventionally, a PID that is used in an instrument for the purpose of detecting chemical compounds in air includes; an ultraviolet (UV) radiation source as a source for high energy photons and associated electronic circuitry for driving the UV radiation source; an ionization cell into which the output of the UV radiation source is directed, a means for a sample of gas to enter and exit the ionization cell; and two or more electrodes electrically insulated from each other and held at a voltage differential and associated electronic circuitry to maintain the voltage differential, the electrodes having a size, shape, and orientation to effectively produce a current when exposed to ions. 
   In a classical PID design, a glass discharge UV lamp is used to produce high-energy photons. Typically, the lamp is constructed of a sealed glass volume filled with a gas such as helium, argon, krypton, or xenon with a window made from a material that is highly transmissive of UV radiation such as magnesium fluoride, lithium fluoride, barium fluoride, strontium fluoride, calcium fluoride or sapphire and the glass volume also contains two electrodes. By maintaining the electrodes at a voltage differential, the gas in the glass volume is momentarily excited. The excited gas then returns to the ground state and in doing so emits photons. U.S. Pat. No. 4,398,152 to Leveson describes a gas discharge UV lamp that eliminates the need for electrodes in the glass volume and excites the gas in the glass volume by inductively coupled radio frequency and produced UV radiation that is uniform across the cross-section of the UV lamp. The glass volume is placed in a holder made of polytetrafluoroethylene around which a coupling inductor is wound and connected at one end to an oscillator circuit to generate radio frequency. 
   The electrodes can have a variety of designs, including, a concentric format with one electrode in the middle of a cylindrically shaped electrode, two disc shaped electrodes in parallel spaced apart with an electric insulator, two thin rods oriented in parallel. The electrodes must be spaced appropriately. Improperly spaced electrodes, either too closely spaced or too widely spaced, will have a negative impact on the overall sensitivity of the detector. Specifically, in the case of too closely spaced electrodes, the electric field is smaller and fewer ions are exposed to the electric field; in the case of too widely spaced electrodes, ions have a greater chance of colliding with a free electron and recombining and the electric field is weaker and would not as strongly attract the ions. 
   Typically PIDs are designed with small internal volumes and volumes that are continually swept with gas to achieve good analytical performance. Unduly large internal volumes allow the sample to diffuse and generate results that are less accurate. Volumes that are poorly swept, such as holes or tubes that have no exit, or unnecessary changes in cross-sectional areas of the flowpath also allow the sample to diffuse and can allow the sample to reside in the poorly swept volume even after an analysis is complete, and such residual sample can then contaminate a subsequent analysis. 
   Typically PIDs are made from materials that have high chemical inertness so as to minimize the interaction of the PID with the sample and potentially contaminate the sample and to minimize adsorption of gas into the material to be potentially later released into subsequent analyses. 
   The sample can be introduced into the ionization cell in a number of ways, including for example: by placing a pump downstream of the PID, the vacuum generated by the pump causing gas to flow through the PID in which case the PID is somewhat evacuated; by placing a pump upstream of the PID, the pressure generated by the output of the pump causing gas to flow through the PID in which case the PID is somewhat pressurized; an injection of sample into the PID by syringe or other means; by injection of the sample into a carrier gas flow stream and passing the sample and carrier gas through a chromatographic column and then through the PID in which case the ionization cell is somewhat pressurized. 
   In any case, care must be taken to adequately seal the PID from its ambient environment so as not to unduly allow sample or carrier gas to leak out of the PID prior to ionization in the case of a pressurized system, and so as not to allow gases from the ambient environment to be introduced into the ionization cell and potentially contaminate the sample in the ionization cell. 
   A PID alone typically cannot provide a user with information to distinguish the specific molecules and concentrations of a number of types of molecules that are in a sample. The UV source will emit photons of a specific energy and any molecules that have an ionization energy lower than the energy of the photons will become ionized. If there is a single type of molecule present in the sample that has an ionization energy lower than the energy of the photon, a current will be generated and if compared against a known reference of the same type of molecule, a concentration of the molecule in the sample can be determined. However, if there is more than one type of molecule that is thus ionized, the user will be unable to discern the molecules that are present, the concentrations that are present, nor the number of different types of molecules present. 
   This drawback of PIDs is typically addressed by passing the fluid to be measured through a chromatographic column prior to introduction into the ionization cell. The various chemical compounds that are in the fluid will be separated from each other in the chromatographic column and will be introduced into the PID individually. The concentrations of each individual chemical compound can then be measured, and in many cases the chemical compounds can be identified based on the timing at which the chemical compounds are eluted from the chromatographic column. 
   Though it is advantageous to use a chromatographic column to introduce a fluid to the PID, there is a significant delay between the time a fluid sample is collected and the time the various chemical compounds are eluted from the chromatographic column to be measured by the PID. Conversely, if no chromatographic column is used, there is relatively no delay between the time a fluid sample is collected and the time it is introduced into the PID. There exists many applications that require a specific analysis that identifies and quantifies the chemicals in a fluid sample and there exists many applications that require minimal time delay. Also, analyses that require identification and quantification of the chemical compounds tend to also require the measurement of very low concentrations of the chemical compounds and thus require greater sensitivity of the detector. Analyses that require minimal time delay and do not require identification and quantification of chemical compounds often require the measurement of relatively high concentrations of the chemical compounds and thus require a detector capable of measuring a broad range of concentrations of chemical compounds. Though it is advantageous for a chemical compound detection instrument to allow for both types of operation, existing PIDs cannot reconcile the conflicting requirements of the variety of applications. 
   SUMMARY OF THE INVENTION 
   The present invention improves on existing photoionization detectors (PID) by arranging multiple ionization cells to be exposed to a single source of high energy photons. The ionization cells are arranged such that each ionization cell can receive an independent fluid stream, allow for molecules in the fluid stream of each ionization cell to be ionized, and generate a current in each cell by the presence of a set of electrodes maintained at a voltage differential. 
   An ultraviolet radiation source is used as the source of high energy photons and the UV radiation is arranged to pass into a series of ionization cells, in which any ionizable compounds are ionized. The sequence of ionization cells can be continued so long as the intensity of the UV radiation remains sufficient to produce a measurable number of ions. The ionization cells each have at least one inlet and at least one outlet through which fluids may enter and exit the cells. The fluids in the respective ionization cells are substantially prevented from contacting each other and potentially contaminating each other and the cells are substantially sealed from the ambient environment. 
   Each ionization cell has a set of at least two electrodes that are maintained at a high voltage potential to attract the ions to the electrode at the lower voltage potential and produce a measurable current. The electrodes are spaced appropriately to be exposed to a large portion of the ionized molecules and are close together to be exposed to the ionized molecules before they recombine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of one embodiment of a photoionization detector with two ionization cells in accordance with this invention. 
       FIG. 2  is a cross-sectional view of the construction of one embodiment of a photoionization detector with two ionization cells in accordance with this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an exploded view and  FIG. 2  shows a sectional view of a photoionization detector in accordance with an embodiment of this invention. Detector housing  110  holds a first ionization cell  140 , a window assembly  130 , a second ionization cell  120 , and an ultraviolet (UV) radiation source  150 . 
   In one embodiment, UV radiation is generated by a UV lamp  152 . The UV lamp body is made of glass and is filled with krypton, argon, xenon, or helium and a UV lamp window  152   a  on the end of the lamp, the UV lamp window  152   a  made of a material that is transmissive of UV radiation such as magnesium fluoride, lithium fluoride, barium fluoride, strontium fluoride, calcium fluoride or sapphire. The lamp is placed in a radio frequency (RF) inductive coupler  151  that is comprised of a RF holder  151   b  made of polytetrafluoroethylene (PTFE) and an inductive coupler coil  151   a  wound around the RF holder  151   b , the inductive coupler coil  151   a  being connected at one end to an RF oscillator circuit. Excitation of the gas in the UV lamp  152  by the RF field generates UV radiation which propagates into a first ionization cell  140 . 
   A fluid can be introduced into a first fluid port  145   a  of a first ionization cell housing  145 . The fluid propagates along a passage  145   b  into the first ionization cell  140 . Molecules in the fluid that have an ionization energy lower than the photon energy of the UV radiation source  150 , will be ionized. The ionized molecules are exposed to an electric field generated by a first electrode  142   a  and a second electrode  142   b  which are maintained at a voltage differential and positioned an appropriate distance from each other, in one embodiment by an electrical insulator  141 , such that an adequate portion of the ions reach the electrodes before they collide with a free electron and recombine. The ions are attracted to the electrode at lower voltage and result in a measurable current through the electrodes. The electrodes are electrically insulated from the detector housing  110 , in one embodiment by the ionization cell housing  145  which is made from an electrically insulating material such as PTFE and in one embodiment is made from polyetheretherketone (PEEK). The gas exits the first ionization cell  140  through a second fluid port (not shown) in the first ionization housing  145 . 
   The UV radiation propagates through the first ionization cell  140  and through a window assembly  130 . In one embodiment, the window assembly  130  is comprised of a window retainer  132  that holds a window  131  and is held in place by a window clamping ring  131   a . The window  131  is made of a material that is transmissive of UV radiation such as magnesium fluoride, lithium fluoride, barium fluoride, strontium fluoride, calcium fluoride or sapphire. 
   The UV radiation propagates into a second ionization cell  120 . A fluid can be introduced into the second ionization cell  120  through a first fluid port  110   a  in the detector housing  110 . The fluid propagates through a passage  110   b  into the second ionization cell  120 . Molecules in the fluid that have an ionization energy lower than the photon energy of the UV radiation in the second ionization cell  120  will be ionized and exposed to an electric field generated by a first electrode  122   a  and a second electrode  122   b  which are maintained at a voltage differential and positioned an appropriate distance from each other, in one embodiment by an electrical insulator  121 . The electrodes are electrically insulated from the detector housing  110  by an electrically insulating material such as PTFE or PEEK and in one embodiment, there is a second electrical insulator  121  made from PTFE between the first electrode  122   a  and the detector housing  110 , and the ionization cell housing  125  is made from PEEK. The fluid exits the second ionization cell  120  through a second fluid port (not shown) in the detector housing  110 . In some applications and in one embodiment it is beneficial for there to be a third electrode to reduce the effects of electrical leakage induced by contamination on the inside surfaces of the detector. 
   In one embodiment, there are a first and a second ionization cell. In other embodiments, there may be a second or more window assemblies through which the UV radiation may further propagate and a third or more ionization cells each with two electrodes and fluid inlet and outlet ports. In one embodiment, the number of ionization cells that can be arranged is limited by the intensity of the UV radiation in the more distant ionization cells, the intensity of UV radiation decreasing with distance from the UV radiation source. Also, each window and the fluid in each ionization cell absorb a portion of the UV radiation. If the ionization cells are arranged particularly close to the UV radiation source, a greater number of ionization cells may be viable. The spacing of the ionization cells is also determined by consideration of the spacing of the electrodes in each cell, closer spaced electrodes possibly allowing closer spaced ionization cells, and thus a greater number of ionization cells. Changing the spacing of the electrodes requires careful consideration of the resulting impact on the detector&#39;s sensitivity, because electrodes that are spaced closer than the optimal spacing results in fewer ions being exposed to the electric field and can be compensated for by a variety of means such as: increasing the voltage differential of the electrodes, increasing the intensity of the UV radiation. 
   Depending on the application, the components that are exposed to the fluids in question should be chemically inert such that the fluid is not contaminated by the components, and so that the components do not unduly adsorb molecules from the fluid and potentially contaminate fluids that enter the detector subsequently. In applications that require a very high degree of chemical inertness, materials such as special grades of stainless steel, glass, sapphire, nickel, or platinum should be used. Also, the degree of chemical inertness that is required may differ between the first ionization cell  140  and the second ionization cell  120 , depending on the particular application for which each ionization cell is used. In one embodiment, the first ionization cell  140  receives a fluid that elutes from a chromatographic column and the second ionization cell  120  receives a fluid directly from the ambient environment. In this embodiment, a high degree of sensitivity to detect the presence of ionizable molecules of a few parts per billion is required of the first ionization cell  140  and wide range of sensitivity is required of the second ionization cell  120  but sensitivity is less important. In this embodiment, the components of the first ionization cell  140  are made from highly chemically inert materials, for example, the first ionization cell housing  145  is made from polyetheretherketone (PEEK), the electrical insulator  141  is made from PTFE, the window retaining ring is made from PEEK, and the electrodes are made from stainless steel. In this embodiment, the components of second ionization cell  120  are made from less highly chemically inert materials, for example, the detector housing which receives the fluid that is introduced into the second ionization cell is made from electroless nickel plated aluminum and fluid is transmitted through tubing made from fluorocarbon polymer (VITON), the electrodes made from stainless steel, the second ionization cell housing made from PEEK, and the electrical insulators made from PTFE. 
   Because the UV radiation intensity lessens with distance from the UV radiation source, fluids that are introduced into ionization cells that are more distant from the UV radiation source will ionize less readily than fluids introduced into ionization cells that are close to the UV radiation source. The respective cells into which fluids are introduced should be chosen with the UV radiation intensity in consideration. In one embodiment, the fluid that is introduced into the first ionization cell is more likely to have a low concentration of ionizable molecules, possible a few parts per billion and the fluid that is introduced into the second ionization cell is likely to have a wide range of concentrations of ionizable molecules and it is less important for a very low concentration of ionizable molecules to be readily detected. 
   It is desirable that the different fluids that enter the first ionization cell  140  and second ionization cell  120  do not unduly come into contact which each other and potentially contaminate each other. It is also desirable that the fluids in the respective ionization cells do not unduly leak to the ambient environment and potentially affect the sensitivity of the detector or that fluids from the ambient environment do not unduly leak into the respective ionization cells and potentially contaminate the fluids in the ionization cells. The extent to which the ionization cells must be thus sealed from the ambient environment and from each other depends on the specific application, but factors that must be considered include: the means by the fluid is propagated through the ionization cells and the resulting magnitude of the pressure or vacuum that results in the ionization cells, and the properties of the fluids that are in question and the concentration and types of molecules that are present in the fluids. In one embodiment, the fluids that are introduced into the first ionization cell  140  are propagated in a carrier gas and are passed through a chromatographic column and the first ionization cell  140  is thus somewhat pressurized. In this embodiment, the fluids that are introduced into the second ionization cell  120  are propagated by a pump that is positioned at the exit of the second ionization cell  120  and thus the second ionization cell is somewhat evacuated. 
   In any case, an adequate seal can be achieved by a variety of means including, for example, by producing highly smooth and flat surfaces on the various spacers, electrodes, and housings of the detector and applying a force on the arrangement to press the various components against each other to sufficiently block the flow of fluid between any two components; or by using seals such as gaskets and o-rings between components to effect a seal. In using seals, careful consideration of the materials from which the seals are made must be observed. Seals made from elastomers can adsorb the fluid in the ionization cells and potentially release the adsorbed fluids into subsequent analyses or potentially emit foreign contaminants into the fluid stream. Seals offer benefits of allowing less special attention being paid to the smoothness of the surfaces of the various components and potentially result in cost savings of the overall assembly. In one embodiment, the electrical insulator  141  has a groove (not shown) formed in either planar side to accept an elastomeric o-ring (not shown) and achieve a seal between the first electrode  142   a  and the electrical insulator  141  and the second electrode  142   b  and the electrical insulator  141 . In one embodiment, the window retainer  132  has a groove (not shown) formed in either planar side to accept an elastomeric o-ring (not shown) and achieve a seal between the window retainer  132  and the second ionization cell housing  125  and the window retainer  132  and the electrode  142   a . In one embodiment, a force is applied to the arrangement by pressing on the end of the lamp with a lamp retainer (not shown). 
   Depending on the application, the surfaces of the window  131  and the lamp window  152   a  can become contaminated and may require periodic cleaning or replacing. In one embodiment, the window retainer  132  is secured in a holder (not shown) that protrudes through a slot (not shown) in the detector housing  110 , the holder being removable by the user, and the UV lamp is secured in a lamp retainer (not shown) that is removable by the user. In another embodiment, contamination of the window  131  and lamp window  152   a  occurs more slowly and are not made to be easily removable by the user. 
   The embodiments described above are illustrative and not limiting of the invention. Many additional and alternative embodiments will be apparent to those skilled in the art without departing from the essential characteristics of the invention as claimed below.