Abstract:
A monitor of the concentration of an oxidative gas or vapor comprises a chemical coupled to a temperature probe, such as by a carrier. Addtionally, the monitor is used in a method of monitoring the concentration of an oxidative gas or vapor and in a sterilization system operated by a user. By utilizing an output signal from the temperature probe to measure the heat produced in an exothermic reaction between the monitored oxidative gas or vapor and the chemical of the monitor, the concentration of the monitored oxidative gas or vapor can be determined. The present invention represents an improvement over the monitors described in the prior art since it is more simplified, and can provide information on the local concentration of an oxidative gas or vapor at various positions within a chamber, and can be operated in size-restricted volumes. Additionally, a method of using the monitor is described, as well as a sterilization system which utilizes the monitor.

Description:
FIELD OF THE INVENTION 
     The invention relates to devices and techniques for monitoring the concentrations of an oxidative gas or vapor. 
     BACKGROUND OF THE INVENTION 
     Medical and surgical instruments have traditionally been sterilized using heat (e.g., exposure to steam), or chemical vapors (e.g., formaldehyde or ethylene oxide). However, both heat and chemical sterilizations have drawbacks. For example, many medical devices, such as fiberoptic devices, endoscopes, power tools, etc. are sensitive to heat, moisture, or both. Additionally, formaldehyde and ethylene oxide are both toxic gases which pose potential health risks to health workers. After sterilization with ethylene oxide, the sterilized articles require long aeration times to remove any remaining toxic material. This aeration step makes the sterilization cycle times undesirably long. 
     Sterilization using hydrogen peroxide vapor has been shown to have some advantages over other chemical sterilization processes (e.g., see U.S. Pat. Nos. 4,169,123 and 4,169,124). The combination of hydrogen peroxide vapor and a plasma provides additional advantages, as disclosed in U.S. Pat. No. 4,643,876. U.S. Pat. No. 4,756,882 discloses the use of hydrogen peroxide vapor, generated from an aqueous solution of hydrogen peroxide, as a precursor of the reactive species generated by a plasma. The combination of plasma and hydrogen peroxide vapor in close proximity with the sterilized articles acts to sterilize the articles. 
     Furthermore, use of low concentrations of hydrogen peroxide vapor has other advantages when used for chemical sterilization. Hydrogen peroxide is easy to handle, can be stored for long periods of time, is efficacious, and mixes readily with water. In addition, the products of decomposition of hydrogen peroxide are water and oxygen, which are both non-toxic. 
     However, there are problems with using hydrogen peroxide for sterilization. First, in order to be effective, devices must be exposed to a specified concentration of hydrogen peroxide. If the concentration of hydrogen peroxide is not sufficient, the article may require longer time and/or higher temperature to achieve sterilization. Second, if too much hydrogen peroxide is present, there is a risk of damaging the sterilized articles, particularly if they contain nylon, neoprene, or acrylic. For hydrogen peroxide absorbent materials, too much peroxide may leave an unacceptable residue on the sterilized article that may be incompatible with the user or patient. In addition, the use of too much hydrogen peroxide increases the cost of sterilization. Third, hydrogen peroxide concentration levels can decrease during the course of the sterilization process due to various factors, such as reactions with some surfaces which are undergoing sterilization, or permeation into and through some plastic materials. Fourth, hydrogen peroxide vapor can condense onto the walls of the sterilization chamber or onto equipment in the chamber, potentially degrading or harming the equipment. It is therefore important to be able to determine the concentration of hydrogen peroxide vapor in the sterilization chamber so that enough hydrogen peroxide is present to be effective, yet not so much that the sterilized articles or other equipment are damaged. 
     Furthermore, the concentration of hydrogen peroxide vapor can vary from one section of the sterilized articles to another. Even under equilibrium conditions, there may be regions of the sterilization chamber which are exposed to higher or lower concentrations of hydrogen peroxide due to restrictions of diffusion caused by other equipment in the chamber, or by the sterilized articles themselves. In particular, an enclosed volume with only a narrow opening will have a lower concentration of hydrogen peroxide than one with a wider opening. Under dynamic conditions (e.g., hydrogen peroxide is introduced into the chamber via an inlet port while at the same time, it is pumped out of an outlet port), the hydrogen peroxide concentration at a particular position in the chamber is a function of various factors, including the inlet flow, outlet pumping speed, and geometrical configuration of the system&#39;s inlet and outlet ports, sterilization chamber, and other equipment in the chamber, including the sterilized articles. 
     Various methods for determining hydrogen peroxide concentration levels in sterilization chambers have previously been disclosed. Ando et al. (U.S. Pat. No. 5,608,156) disclose using a semiconductor gas sensor as a means for measuring vapor phase hydrogen peroxide concentrations. The reaction time of the sensor is several tens of seconds, and the relation between the sensor output and the concentration of the hydrogen peroxide vapor varies with changes in pressure. Most hydrogen peroxide vapor sterilization procedures involve several treatment steps, usually including at least one step in vacuum. The response of the sensor to hydrogen peroxide through the treatment steps will therefore change, depending on the pressure used in each treatment step. 
     Cummings (U.S. Pat. No. 4,843,867) discloses a system for determining the concentration of hydrogen peroxide vapor in situ by simultaneous measurements of two separate properties, such as dew point and relative humidity. A microprocessor is then used to fit the two measurements into a model to calculate the hydrogen peroxide concentration. The method uses an indirect approximation based on a number of empirical assumptions, and the accuracy will vary depending on how closely the conditions in the sterilization chamber resemble those used to develop the model. This method also does not yield information concerning the differing concentrations of hydrogen peroxide at various positions within the sterilization chamber. 
     Van Den Berg et al. (U.S. Pat. No. 5,600,142) disclose a method of using near-infrared (NIR) spectroscopy to detect hydrogen peroxide vapor. Hydrogen peroxide has an absorption peak at about 1420 nm (nanometers) which can be used to determine its concentration. However, water is always present when hydrogen peroxide is present, since water is a decomposition product of hydrogen peroxide. Because water also absorbs near-infrared radiation at 1420 nm, it interferes with the determination of the hydrogen peroxide concentration. In order to correct for this interference, the water vapor concentration is determined separately by an absorption measurement at wavelengths which hydrogen peroxide does not absorb. This measured water vapor concentration is then used to correct the absorbance at 1420 nm for the contribution due to water. However, this correction measurement also suffers from contributions due to contaminants, such as various organic molecules, which absorb in the spectral region of the correction measurement. Since one does not normally know what organic molecules are present, the correction factor is therefore somewhat unreliable. 
     Furthermore, the NIR method requires absorption measurements at two different wavelengths and making corrections for the presence of water vapor, organic contaminants, or both. The electronic equipment for doing these corrections is complex and expensive, and the correction for the presence of organic compounds is subject to error. Additionally, the calculated hydrogen peroxide concentration is an average concentration over the volume which absorbs the near-infrared radiation, not a localized measurement of concentration at particular positions within the sterilization chamber. 
     U.S. Pat. No. 4,783,317 discloses an apparatus for monitoring the concentration of hydrogen peroxide in liquid media, e.g. aqueous solutions for scrubbing the flue gases emanating from waste-incineration plants or large capacity firing systems. By exploiting the exothermic reaction of hydrogen peroxide with reducing agents (e.g. gaseous sulfur dioxide), the apparatus is able to measure the concentration of hydrogen peroxide in the liquid medium. The U-shaped apparatus comprises a thermally insulated measuring cell, a supply line which supplies a partial stream of the liquid from the source to the measuring cell, and a discharge line which returns the liquid to the source. In the measuring cell, the liquid is combined with a small stream of a reducing agent from a separate supply line, and the temperature of the mixture is monitored by a sensor. By comparing this temperature to the temperature of the liquid prior to entering the measuring cell, the apparatus measures temperature rise due to the ongoing exothermic reaction which is a function of the concentration of hydrogen peroxide in the liquid. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides an apparatus for monitoring the concentration of an oxidative gas or vapor comprising: a chemical which reacts with the oxidative gas or vapor to produce a heat change, and a temperature probe which measures the heat change. The chemical is coupled to the temperature probe, such as through a carrier. Preferred carriers include vacuum grease, tape, epoxy, or silicone. The carrier can also comprises a gas-permeable pouch or gas-impermeable enclosure with at least one hole. 
     In another aspect, the apparatus described above can form part of a sterilization system with a control system to produce a desired level of oxidative gas or vapor. The sterilization system ordinarily comprises: a chamber, a door, and a source of oxidative gas or vapor. 
     In still another aspect, the present invention provides a method of monitoring the concentration of an oxidative gas or vapor comprising: providing a chemical which undergoes a reaction with the oxidative gas or vapor to be monitored so as to produce a heat change, providing a temperature probe which detects the heat produced by the reaction between the chemical and the oxidative gas or vapor to be monitored and which produces an output signal which is a function of the concentration of the oxidative gas or vapor. The chemical coupled to the temperature probe is exposed to the oxidative gas or vapor, and the output signal from the temperature probe is measured so as to determine the concentration of the oxidative gas or vapor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B,  1 C,  1 D, and  1 E schematically illustrate various preferred embodiments of the present invention comprising a carrier, a chemical, and a temperature probe. 
     FIG. 2 schematically illustrates a sterilization system utilizing one preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1A,  1 B,  1 C,  1 D, and  1 E illustrate embodiments of the present invention. In a preferred embodiment of the present invention, a concentration monitor  10  comprises a carrier  12 , a chemical  14 , and a temperature probe  16 . All of the elements of the concentration monitor  10  must be compatible with its operating conditions. For use in a sterilization system utilizing hydrogen peroxide vapor with or without plasma, the carrier  12 , chemical  14 , and temperature probe  16  must all be compatible with operations under sterilization conditions and with exposure to hydrogen peroxide vapor and plasma. Persons skilled in the art recognize that there is a wide variety of materials and structures which can be selected as the carrier  12  in these preferred embodiments. The carrier  12  couples the chemical  14  in close proximity to the temperature probe  16  so as to minimize the thermal losses between them. Examples of adequate carriers include, but are not limited to, vacuum grease, tape, epoxy, or silicone. Additionally, the carrier  12  can either be configured to expose the chemical  14  directly to the environment, or to enclose the chemical  14  in a gas permeable pouch, such as Tyvek tubing, or a gas impermeable enclosure with a hole or holes. In certain embodiments, the chemical can be coupled directly to the temperature probe without use of a carrier. For example, the chemical  14  can be formed as an integral part of the temperature probe  16  or, if the chemical  14  is sufficiently adhesive, it can be directly coupled to the probe  16 . 
     The chemical  14  is a chemical compound which undergoes an exothermic reaction with the oxidative gas or vapor to be monitored, producing a detectable amount of thermal energy (i.e., heat) upon exposure to the oxidative gas or vapor to be monitored. Persons skilled in the art are able to choose an appropriate chemical  14  which yields a sufficient amount of heat upon exposure to the relevant range of concentrations of the oxidative gas or vapor to be measured. Examples for use in a hydrogen peroxide sterilization system include, but are not limited to, potassium iodide (KI), catalase, magnesium chloride (MgCl 2 ), iron (II) acetate, and platinum on alumina. In addition, a combination of these chemical compounds may be chosen as the chemical  14 . Furthermore, persons skilled in the art are able to select the appropriate amount of chemical  14  to yield a sufficient amount of heat upon exposure to the relevant range of hydrogen peroxide concentrations. 
     Various configurations are compatible with use in the preferred embodiments illustrated in FIGS. 1A,  1 B,  1 C,  1 D, and  1 E. FIG. 1A shows a temperature probe  16  coated with a thin layer of carrier  12 , such as vacuum grease, double-sided tape, epoxy, or silicone, on the tip of the probe  16  and the chemical  14  is coated on the outside of the carrier  12 . FIG. 1B shows the chemical  14  is mixed with the carrier  12  and applied onto the tip of the temperature probe  16 . For example, the chemical is dispersed in the silicone, such as polydimethyl siloxane or other similar two-part silicone, prior to curing. The probe  16  is then dip-coated with the chemical-containing silicone. The silicone containing the chemical  14  that has been dip-coated onto the probe  16  is then cured at conditions consistent for two-part silicones. The chemical  14  is accessible for reaction as the hydrogen peroxide diffuses into the silicone matrix. FIG. 1C show the chemical  14  is enclosed onto the tip of the temperature probe  16  with a carrier  12 . The carrier  12  is a gas-permeable Tyvek pouch with a heat-sealed area  17 . The carrier  12  can also be a gas-impermeable film, or CENTRAL SUPPLY ROOM (“CSR”) wrap pouch, or any enclosure with one or more holes to allow the diffusion of gas or vapor to react with the chemical  14  retained in the enclosure. FIG. 1D shows a chemical  14  coupled to a heat-conducting material  18  with a carrier  12 , and the heat-conducting material  18  is coupled to the temperature probe  16  with a substrate  19 . The substrate  19  can be tape, adhesive, or any other coupling means. The heat-conducting material  18  can be metallic wire or any other materials which can properly conduct heat to the temperature probe  16 . FIG. 1E show a chemical  14  coupled to a temperature probe  16  with a carrier  12 , and two parts of the temperature probe  16  can be connected and disconnected with a male connector  20  and a female connector  21 . 
     The temperature probe  16  is a device which measures the temperature at a particular location. One preferred embodiment of the present invention utilizes a fiber-optic temperature probe, such as a Luxtron 3100 fluoroptic thermometer, as the temperature probe  16 . This fiber-optic temperature probe is coated with Teflon and therefore is very compatible to any oxidative gas or vapor. Another preferred embodiment utilizes a temperature probe  16  which is a thermocouple probe which utilizes a junction of two metals or alloys. The thermocouple junction produces a voltage which is a known function of the junction&#39;s temperature. Measurements of this voltage across the thermocouple junction can therefore be converted into measurements of the junction&#39;s temperature. Thermocouple junctions can be made quite small (e.g., by spot welding together two wires of 0.025-millimeter diameter composed of differing alloys), so they can be positioned into size-restricted volumes. In yet another preferred embodiment, a thermistor can be used as a temperature probe  16 . 
     Table 1 illustrates the increases of temperature measured by a concentration monitor  10  with potassium iodide (KI) as the chemical  14 . The tip of the fiber-optic temperature probe was first coated with a thin layer of Dow Coming high vacuum grease (part number 2021846-0888). About 0.15 grams of KI powder was then applied onto the vacuum grease. This configuration is the same as illustrated in FIG.  1 A. The measurements were conducted by suspending the concentration monitor  10  in a vacuum chamber heated to 45° C., evacuating the chamber, recording the initial probe temperature, injecting hydrogen peroxide into the chamber, recording the temperature after all hydrogen peroxide was vaporized, evacuating the chamber to remove the hydrogen peroxide, and venting the chamber. The measurements were repeated with different concentrations of hydrogen peroxide injected into the chamber. The same temperature probe  16  was reused for all the measurements, and the results are shown in Table 1. As can be seen from Table 1, KI produces a measurable increase of temperature with increasing concentration of hydrogen peroxide. Additionally, this concentration monitor  10  can be reused many times. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Concentration of H 2 O 2  (mg/L) 
                 Temperature increase (° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0.2 
                 3.0 
               
               
                   
                 0.4 
                 8.3 
               
               
                   
                 0.8 
                 19.2 
               
               
                   
                 1.3 
                 24.2 
               
               
                   
                 2.1 
                 33.7 
               
               
                   
                   
               
             
          
         
       
     
     Table 2 provides data on the measured temperature increases with varying concentrations of hydrogen peroxide for a concentration monitor  10  utilizing different chemicals  14 . Same test conditions and probe configurations were used in these temperature measurements. As can be seen from Table 2, each of the chemicals produced a measurable temperature rise which increased with increasing hydrogen peroxide concentration. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Temperature increase (° C.) 
               
             
          
           
               
                   
                 Chemical 
                 0.4 mg/L 
                 1.0 mg/L 
                 2.1 mg/L 
               
               
                   
                   
               
             
          
           
               
                   
                 Platinum on Alumina 
                 13.5 
                 17.2 
                 — 
               
               
                   
                 Catalase 
                 1.1 
                 — 
                 6.9 
               
               
                   
                 Iron (II) acetate 
                 62.5 
                 83.1 
                 — 
               
               
                   
                 Magnesium Chloride 
                 0.8 
                 — 
                 4.4 
               
               
                   
                   
               
             
          
         
       
     
     The utility of using a thermocouple junction as the temperature probe  16  is illustrated in Table 3. For these measurements, the concentration monitor  10  was configured as illustrated in FIG.  1 A. The test conditions of Table 1 were also used for these measurements. Table 3 illustrates that significant temperature increases were also observed using a thermocouple temperature probe. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Concentration of H 2 O 2  (mg/L) 
                 Temperature increase (° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0.2 
                 2.7 
               
               
                   
                 0.4 
                 11.9 
               
               
                   
                 0.8 
                 19.3 
               
               
                   
                 2.1 
                 24.2 
               
               
                   
                   
               
             
          
         
       
     
     The utility of using double-sided tape as the carrier  12  is illustrated by Table 4, which presents the temperature increases measured by a fiber-optic temperature probe  16 . A thin layer of 3M Scotch double-sided tape was first applied to the tip of the fiber-optic probe  16 . About 0.15 grams of KI powder was then coated onto the tape. Table 1 test conditions were repeated for these measurements. It is apparent from Table 4 that measurable increases of temperature were detected for increasing H 2 O 2  concentration when using double-sided tape as the carrier  12 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Concentration of H 2 O 2  (mg/L) 
                 Temperature increase (° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0.4 
                 9.3 
               
               
                   
                 1 
                 16.8 
               
               
                   
                 2.1 
                 31.2 
               
               
                   
                   
               
             
          
         
       
     
     The utility of using epoxy as the carrier  12  is illustrated by Table 5, which presents the temperature increases measured by a fiber-optic temperature probe  16 . The concentration monitor  10  was constructed by applying a thin layer of Cole-Parmer 8778 epoxy on an aluminum wire. About 0.15 grams of KI powder was then applied and dried onto the epoxy. Finally, the aluminum wire was attached to the temperature probe  16 . Table 1 test conditions were repeated for these measurements. It is apparent that measurable increases of temperature were detected for increasing H 2 O 2  concentration when using epoxy as the carrier  12 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Concentration of H 2 O 2  (mg/L) 
                 Temperature increase (° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0.4 
                 7.8 
               
               
                   
                 1 
                 12.9 
               
               
                   
                 2.1 
                 20.1 
               
               
                   
                   
               
             
          
         
       
     
     The utility of using an enclosure as the carrier  12  to enclose the chemical  14  is illustrated by Tables 6 and 7, which illustrate the increase of temperature detected by a fiber-optic temperature probe  16  with KI contained in an enclosure. For Table 6, the enclosure was PVC shrink tubing with holes. The holes were small enough to trap the KI powder but large enough to allow the diffusion of gas or vapor into the PVC tubing. For Table 7, the enclosure was gas-permeable Tyvek tubing fabricated from heat-sealed 1073B Tyvek. The inner diameter of the enclosure was about 0.5 centimeters, and its length was approximately 1.5 centimeters. For Table 6, about 0.2 grams of KI powder was enclosed in the PVC tubing and the concentration monitor  10  was re-used for all measurements. For Table 7, about 0.2 grams of KI powder was enclosed in the Tyvek pouch and the concentration monitor  10  was also re-used for all measurements. Table 1 test conditions were used for these measurements. It is apparent that measurable increases of temperature were detected for increasing H 2 O 2  concentration when using both embodiments of a gas-permeable pouch as the carrier  12 . The results also demonstrate that the concentration monitor  10  can be re-used and the measurements are reproducible. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
             
             
               
                   
                   
               
               
                   
                 Concentration of 
                 Temperature increase (° C.) 
               
             
          
           
               
                   
                 H 2 O 2  (mg/L) 
                 Trial #1 
                 Trial #2 
                 Average 
               
               
                   
                   
               
             
          
           
               
                   
                 0.2 
                 1.1 
                 1.1 
                 1.1 
               
               
                   
                 0.4 
                 9.5 
                 8.8 
                 9.2 
               
               
                   
                 1.0 
                 13.6 
                 13.6 
                 13.6 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
             
             
               
                   
                   
               
               
                   
                 Concentration of 
                 Temperature increase (° C.) 
               
             
          
           
               
                   
                 H 2 O 2  (mg/L) 
                 Trial #1 
                 Trial #2 
                 Average 
               
               
                   
                   
               
             
          
           
               
                   
                 0.4 
                 9.7 
                 8.4 
                 9.1 
               
               
                   
                 1.0 
                 17.3 
                 16.8 
                 17.1 
               
               
                   
                 1.4 
                 23.6 
                 23.6 
                 23.6 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 2 schematically illustrates a sterilization system  25  utilizing one preferred embodiment of the present invention. The sterilization system  25  has a vacuum chamber  30  with a door  32  through which items to be sterilized are entered into and removed from the chamber  30 . The door is operated by utilizing a door controller  34 . The vacuum chamber also has a gas inlet system  40 , a gas outlet system  50 , and a radio-frequency (rf) system  60 . Comprising the gas inlet system  40  is a source of hydrogen peroxide (H 2 O 2 )  42 , a valve  44 , and a valve controller  46 . The gas outlet system  50  comprises a vacuum pumping system  52 , a valve  54 , a valve controller  56 , and a vacuum pumping system controller  58 . In order to apply radio-frequency energy to the H 2 O 2  in the vacuum chamber  30 , the rf system  60  comprises a ground electrode  62 , a powered electrode  64 , a power source  66 , and a power controller  68 . The sterilization system  25  is operated by utilizing a control system  70  which receives input from the operator, and sends signals to the door controller  34 , valve controllers  46  and  56 , vacuum pumping system controller  58 , and power controller  68 . Coupled to the control system  70  (e.g., a microprocessor) is the concentration monitor  10 , which sends signals to the control system  70  which are converted into information about the H 2 O 2  concentration in the vacuum chamber  30  at the location of the concentration monitor  10 . The sterilized article  80  is shown to be positioned in the chamber  30  with concentration monitor  10  located in the load region to monitor the concentration of hydrogen peroxide in the load region. Persons skilled in the art are able to select the appropriate devices to adequately practice the present invention. 
     The heat produced between the oxidative gas or vapor and the chemical  14  may not be the same for different configurations of the concentration monitor  10 , carrier  12 , and chemical  14 . Therefore, for a given type of concentration monitor  10 , a calibration curve needs to be established to determine the relationship between the concentration of oxidative gas or vapor and the heat produced. Once the calibration curve is established, the heat detected during the measurement can be converted to the concentration of the oxidative gas or vapor around the monitor  10 . 
     By coupling the operation of the sterilization system  25  with the H 2 O 2  concentration measured by the concentration monitor  10 , the sterilization system  25  is assured of operating with an appropriate amount of H 2 O 2  in the region of the articles to be sterilized. First, if the H 2 O 2  concentration is determined to be too low for adequate sterilization, the control system  70  can signal the inlet valve controller  46  to open the inlet valve  44 , thereby permitting more H 2 O 2  into the chamber  30 . Alternatively, if the H 2 O 2  concentration is determined to be too high, the control system  70  can signal the outlet valve controller  56  to open the outlet valve  54 , thereby permitting the vacuum pumping system to remove some H 2 O 2  from the chamber  30 . Furthermore, if the sterilization system is being operated in a dynamic pumping mode (i.e., H 2 O 2  is introduced into the chamber  30  via the inlet valve  44  while at the same time, it is pumped out via the outlet valve  54 ), then either the inlet valve  44  or the outlet valve  54 , or both can be adjusted in response to the measured H 2 O 2  concentration to ensure an appropriate level of H 2 O 2 . 
     Because the concentration monitor  10  provides localized information regarding the H 2 O 2  concentration, it is important to correctly position the concentration monitor  10  within the sterilization chamber  30 . In some preferred embodiments, the concentration monitor  10  is fixed to a particular position within the sterilization chamber  30  in proximity to the position of the sterilized articles  80 . In other preferred embodiments, the concentration monitor  10  is not fixed to any particular position within the sterilization chamber  30 , but is placed on or near the sterilized article  80  itself. In this way, the concentration monitor  10  can be used to measure the H 2 O 2  concentration to which the sterilized article  80  is exposed. In particular, if the sterilized article  80  has a region which is exposed to a reduced concentration of H 2 O 2  due to shadowing or a reduced opening, then the concentration monitor  10  can be placed within this region to ensure that a sufficient H 2 O 2  concentration is maintained to sterilize this region. The small size of the concentration monitor of the present invention permits the concentration monitor to be placed in very restricted volumes, such as the inner volume of a lumen, or in a container or wrapped tray. In still other embodiments of the present invention, a plurality of concentration monitors  10  can be utilized to measure the H 2 O 2  concentration at various positions of interest. 
     This invention may be embodied in other specific forms without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner. The scope of the invention is indicated by the following claims rather than by the foregoing description. Any and all changes which come within the meaning and range of equivalency of the claims are to be considered within their scope.