ELECTROCHEMICAL SENSOR FOR THE SPECIFIC DETECTION OF PERACETIC ACID IN AQUEOUS SOLUTIONS USING PULSE AMPEROMETRIC METHODS

Abstract: An electrochemical sensor (A, A') is specific for the detection of peracetic acid in a solution which also contains hydrogen peroxide. A potential is applied between a reference electrode (120, 120') and a working electrode (118, 118'). A read voltage (fig. 7) is selectively pulsed to the working electrode and then through the solution to a counter electrode (122, 122'). The current flowing between the working electrode and the counter electrode is dependent on the peracetic acid concentration in the solution (fig. 6). By careful selection of the read voltage, the contribution of hydrogen peroxide to the current flow is virtually negligible. The sensor effectively measures peracetic acid concentrations in the range generally employed in sterilization and disinfection baths (100-3000 ppm.).

Description: ELECTROCHEMICAL SENSOR FOR THE SPECIFIC DETECTION OF PERACETIC ACID IN AQUEOUS SOLUTIONS USING PULSE AMPEROMETRIC METHODS Background of the Invention The present invention relates to the sterilization and disinfection arts. It finds particular application in conjunction with the detection of peracetic acid • concentrations in solutions used for sterilization or disinfection of medical, dental, and pharmaceutical equipment and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to detection of peracetic acid and other oxidizable chemicals, such as hydrogen peroxide, in solution. Peroxyacetic acid, or peracetic acid, is a useful sterilant and/or disinfectant for a variety of applications, including disinfection of waste and sterilization or disinfection of medical, dental, pharmaceutical, or mortuary equipment, packaging containers, food processing equipment, and the like. It has a broad spectrum of activity against microorganisms, and is effective even at low temperatures. It poses few, if any, disposal problems because it decomposes to compounds which are readily degraded in sewage treatment plants. Peracetic acid solutions also have the ability to be reused over a period of time, allowing instruments to be sterilized or disinfected throughout the day in the same bath of sterilant. In use, peracetic acid precursors are typically mixed with water and other chemicals in order to create a sterilant solution. Items to be sterilized or disinfected are then immersed in the sterilant. Decontaminated items are then rinsed to remove traces of the acid and other cleaning chemicals, before use. To ensure effective sterilization or disinfection within a preselected period of time, the concentration of peracetic acid is maintained above a selected minimum effective level. Disinfection is typically carried out at lower concentrations of peracetic acid than for sterilization. When the peracetic acid concentration is at or above the minimum effective level, complete sterilization or disinfection is expected. Because the peracetic acid tends to decompose over time, it is valuable to monitor the sterilant periodically to determine the level of peracetic acid. The level can be compared against preselected minimum levels, used to adjust contact time, used to control concentration, or the like. Currently, it is often assumed that the sterilant will remain at or above the minimum effective concentration. However, differences in the temperature of the sterilant, the quantity of items sterilized or disinfected, and the degree and nature of contamination of the items all result in considerable variations in the degradation of the sterilant. In addition, storage conditions and duration sometimes lead to degradation of the peracetic acid precursors before use. Methods currently used to detect peracetic acid are often unable to distinguish between peracetic acid and other compounds typically present in the solution, such as hydrogen peroxide and acetic acid. Dippable papers are easy to use, but lack accuracy, particularly at concentrations suitable for sterilization or disinfection. Chemical titration methods provide a more accurate measure of the peracetic acid in solution, but are time consuming to perform and are not readily automated. Frequently, more than one titration is performed to eliminate the contribution of hydrogen peroxide to the result. Recently, a number of electrochemical techniques have been developed for detection of oxidizable or reducible chemical species, such as peracetic acid. Consentino, et al., U.S. Patent No. 5,400,818, discloses a sensor for peracetic acid-hydrogen peroxide solutions. The sensor measures the resistivity of the solution, which is dependent on both the peracetic acid and the hydrogen peroxide concentrations, as well as other factors. Thus, the sensor is unable to differentiate between the two components. European Patent Application EP 0 333 246 A, to Unilever PLC, discloses an electrochemical sensor for detection of oxidizable or reducible chemical species using an amperometric method in which a fixed potential is maintained between a reference and a working electrode. The current at the working electrode is used to determine the concentration of peracetic acid. Other species present, however, influence the current flowing, and hence the accuracy of the results. Teske, U.S. Patent No. 5,503,720, discloses a process for the determination of reducible or oxidizable substances, such as peracetic acid in sewage waste. The process uses potentiostatic amperometry to detect peracetic acid concentrations. The technique, however, depends on the achievement of a steady state, which frequently takes several hours. Conventional electrochemical detection systems often employ a porous membrane, which separates the sample to be analyzed from the electrodes. Species to be measured pass through the membrane when traveling to the electrodes. This increases the time for measurements to be made and adds complexity and cost to the system. The present invention provides a new and improved sensor and method for the selective detection of peracetic acid, which overcomes the above-referenced problems and others. Summary of the Invention In accordance with one aspect of the present invention, a method of detecting a first oxidizing species in a solution which also contains a second oxidizing species is provided. The method comprises disposing a working electrode, a reference electrode, and optionally a counter electrode in the solution to be tested, selectively applying a read voltage to the working electrode, and detecting current flowing between either the working electrode and the reference electrode or the working electrode the counter electrode, where present. The method is characterized by the read voltage being selected such that the current flowing is dependent on a concentration of the first oxidizing species and substantially independent of a concentration of the second oxidizing species in the solution. In accordance with another aspect of the present invention, a sensor system is provided. The sensor system includes a chamber which receives a sample of the solution from the fluid flow path and electrode system comprising a working electrode, a reference electrode, the reference electrode also acting as a counter electrode, or additionally, a counter electrode. The electrodes are disposed within the chamber to contact the sample of solution. An amperometric controller is electrically connected with the working and reference electrodes which selectively pulses a preselected read voltage to the working electrode and detects an output current flowing in a circuit including the working electrode, the reference electrode or the counter electrode, where present, and the solution. The read voltage is selected such that the output current is substantially dependent on a concentration of the first oxidizing species and substantially independent of a concentration of at least a second oxidizing species in the solution. One advantage of one embodiment of the present invention is that it enables the peracetic acid concentration of a sterilizing or disinfecting solution to be determined rapidly, (i.e., in less than one minute) and without interference by other oxidizing species present in the solution. Another advantage of one embodiment of the present invention is that the sensor confirms that a minimum effective concentration of peracetic acid is maintained for effective sterilization or disinfection. Yet another advantage of one embodiment of the present invention is the provision of a disposable sensor probe that requires no calibration before use. Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. Brief Description of the Drawings The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. FIGURE 1 is a plumbing diagram of a peracetic acid sterilization and disinfection system according to the present invention; FIGURE 2 is a schematic diagram of an electrochemical system for detecting peracetic acid using pulse amperometry according to the present invention; FIGURE 3 is a top view of a disposable sensor for detecting peracetic acid according to one embodiment of the present invention; FIGURE 4 is a side view of a reusable sensor system, according to another embodiment of the present invention; FIGURE 5 is a side view through section V-V of the reusable sensor system of FIGURE 4; FIGURE 6 is a plot showing a pulse sequence applied between reference and working electrodes of the sensor system of FIGURE 1; FIGURE 7 is a plot showing cyclic voltammograms of current flow against electrode voltage for solutions containing (1) a buffering system, (2) a buffering system and 1500 pp peracetic acid, and (3) a buffering system and 500 ppm hydrogen peroxide and peroxide ions, for a pyrolytic graphite working electrode; FIGURE 8 is a plot showing cyclic voltammograms in peracetic acid solutions at different concentrations using a glassy carbon electrode; and FIGURE 9 is a plot showing cyclic voltammograms in peracetic acid solutions at different concentrations using a pyrolytic graphite carbon electrode. Detailed Description of the Preferred Embodiments With reference to FIGURE 1, a peracetic acid monitoring system or sensor A measures peracetic acid concentrations in the presence of hydrogen peroxide. The monitoring system will be described with reference to an automated liquid decontamination apparatus 1 which sequentially cleans items, such as endoscopes or other medical, dental, and pharmaceutical devices, and the like, and then sanitizes, sterilizes, or disinfects them with a decontaminant solution which contains peracetic acid. It should be appreciated, however that the monitoring system is also applicable to the measurement of peracetic acid concentrations in other treatment systems and peracetic acid-containing liquids. The term "decontamination" and other terms relating to decontaminating will be used herein to describe sanitizing, sterilization, disinfection, and other antimicrobial treatments which are designed to destroy microorganisms contaminating the items. The system 1 includes a decontamination cabinet 10 which defines an interior decontamination chamber 12. Items to be sterilized, disinfected, sanitized, or otherwise microbially decontaminated are loaded into the decontamination chamber through an opening in a front wall 13 of the cabinet, illustrated as closed by a door 14. Within the chamber, several spray jets or nozzles 16 spray a decontaminant solution over the items. Optionally, in the case of instruments with lumens, or other internal passages, some of the nozzles act as fluid ports 18 which are configured for interconnection with internal passages of the endoscopes and other objects with lumens, for supplying decontaminant solution and other liquids to the internal passages. A collection tank or sump 20 forms the base of the cabinet 10 and receives the sprayed decontaminant solution as it drips off the items. A high pressure pump 22 delivers the decontaminant solution under pressure to the nozzles 16 and fluid ports 18 through a fluid distribution system or manifold 24. A source 30 of a decontaminant solution preferably includes a well or mixing chamber 34. The well receives a dose of a concentrated decontaminant, such as an antimicrobial agent or reagents which react to form an antimicrobial agent on mixing with water. As shown in FIGURE 1, the well is preferably integral with the collection tank 20 of the chamber, although a separate well is also contemplated. A preferred antimicrobial agent is peracetic acid, either in concentrated liquid form, or as a reaction product of powdered reagents, such as acetyl salicylic acid and sodium perborate. A water inlet 42 supplies water, typically from a municipal water system to the well. The water mixes with detergents, corrosion inhibitors, the concentrated decontaminant, and other selected components in the well to form wash, decontaminant, or other solutions. Preferably, the concentrated decontaminant and the other components are supplied in a disposable package or cup 44 which is positioned in the well 34 prior to a decontamination cycle. The cup 44 holds a measured dose of the concentrated decontaminant. Optionally, a cleaner concentrate is also contained in the cup for forming a cleaning solution to clean the items prior to antimicrobial decontamination. The cup 44 may include a number of compartments which separately contain the cleaning concentrate and decontaminant concentrate for separate release into the system. In this way, the items are first cleaned and then microbially decontaminated. In a preferred embodiment, the cup holds a cleaning concentrate in a first compartment. A second compartment holds pretreatment components, such as buffers for adjusting the pH, surfactants, chelating agents, and corrosion inhibitors for protecting the components of the system and items to be decontaminated from corrosion by the decontaminant. A decontaminant, such as concentrated liquid peracetic acid solution (or reagents that react to form it) is held in a third compartment. A cup cutter 46, or other suitable opening member, is positioned at the base of the well 34 for opening selected compartments of the cup, in sequence. Alternatively, a solid or liquid concentrated decontaminant is supplied to the system from a separate bulk source (not shown) , or is supplied to the system as the decontaminant solution, in an already-diluted form. The water used for diluting the cleaner concentrate and decontaminant may be tap water or treated water, such as distilled water, filtered water, microbe free water, or the like. The quantity of water entering the system is regulated to provide a decontaminant solution of a desired concentration in the decontamination chamber 12. The water is preferably passed through a microporous filter 50 in the water inlet line 42 which filters out particles of dirt and microorganisms. A valve 52 in the water inlet 42 closes when the selected quantity of water has been admitted. A fluid supply pathway 60 connects the well 40, the pump 22, and the fluid distribution system 24. A heater 64, situated in the fluid supply pathway 60, heats the decontaminant solution and optionally the cleaning solution and rinse liquid to a preferred temperature (s) for effective cleaning, decontamination, and rinsing. A temperature of about 50-60°C is preferred for sterilization with peracetic acid. The pathway 60 returns the sprayed decontaminant solution from the sump 20 to the manifold 24, and thence to the nozzles 16 and the fluid ports 18 via a recirculation valve 68. At least a portion of the sprayed decontaminant solution is directed through the well 34 before being returned to the decontamination chamber. This ensures thorough mixing of the concentrated decontaminant and other components with the solution before returning the decontaminant solution to the nozzles 16, 18. The peracetic acid monitoring system A detects the concentration of peracetic acid passing through the fluid lines. FIGURE 1 shows the system connected with the line 60. It should be appreciated, however, that the sensor is also conveniently connected with or disposed in any of the fluid flow lines of the system or simply in a bath of decontaminant solution. A computer control system 80 controls the operation of the peracetic acid monitoring system A. Preferably, the control system also controls the operation of other elements of the system 1, including the introduction of the cleaner concentrate, the peroxy concentrate, and other reagents as well as the pump 22, the heater 64, the valves 52, 68 and the like. The control system 80 may control one or more additional systems 1, if desired. With reference now to FIGURE 2, the system A for selective detection of peracetic acid includes an electrode system 110, and an amperometric controller 112. The controller 112 both applies voltages and detects current flows in the system A. Although the controller is shown as a single unit in FIGURE 2, it should be understood that a combination of pieces of electrochemical equipment generally known in the art which serves these functions is also contemplated. The electrode system 110 is disposed in a reservoir 114 which receives a peracetic acid solution to be tested, or may be placed directly in the recirculation path of the automated processor 1. Preferably, as shown in FIGURE 1, the reservoir comprises a separate chamber, into which a sample of the circulating decontaminant solution is withdrawn at intervals. Because peracetic acid is generally in equilibrium with hydrogen peroxide when in solution, the solution to be tested invariably contains some hydrogen peroxide. The electrode system 110 can be a two or three electrode system. In a three electrode system it includes a working or sensing electrode 118, a reference electrode 120, and a counter electrode 122. In a two electrode system, the counter electrode is eliminated in favor of a reference electrode which also serves the functions of the counter electrode, although with some loss in stability of the system to be expected. The reference electrode produces a constant electrical potential (or base potential) and assures a stable reference potential when the current density flowing through the electrode is small. Suitable reference electrodes 120 include silver/silver chloride electrodes and mercury/mercury chloride (calomel) electrodes. The working electrode is formed from an electrically conductive material. It is preferably an electroactive for peracetic acid. Other features of good working electrode materials include minimal to no consumption of the electrode material during operation of the sensor, useful working potential range covering the detection potential of peracetic acid, provision of a high signal to background noise ratio for peracetic acid, and a low sensitivity to interfering substances, such as hydrogen peroxide. Suitable electrode materials for detection of peracetic acid in the presence of hydrogen peroxide include carbon, gold (either alone, or doped with an inert material) , or platinum. A particularly preferred working electrode 118 is formed from carbon. Carbon is an effective electroactive for peracetic acid and is highly selective for peracetic acid in the presence of hydrogen peroxide. Carbon electrodes are also relatively resistant to peracetic acid, giving them a longer useful life. Crystalline forms of carbon, such as graphite, provide effective working electrodes for measurements in the diffusion limited region. One preferred form of crystalline carbon is pyrolytic graphite (obtainable from Advanced Ceramics Corp) , a highly crystalline form, manufactured by decomposition of hydrocarbon gas at high temperatures in a vacuum furnace. Another form of crystalline carbon is sold under the tradename Isomolded graphite by Industrial Sales, Melrose Park, 111. Amorphous forms of carbon, such as glassy carbon, also provide effective working electrode materials for measurements in the diffusion limited region. The counter electrode 122, where used, is preferably formed from an inert conductive material, such as carbon. Alternatively, suitable counter electrodes are formed from silver, gold, platinum, or titanium. The external circuit is closed between the working electrode and the counter electrode. With reference to FIGURE 3, in one embodiment, a substrate 124, preferably formed from an inert polymeric or ceramic sheet, supports the electrode system 110 to form a disposable probe 125. Electric leads 126 electrically connect the electrodes and the controller 112 through connecting points 128. Optionally, the sensor probe also includes an insulation layer 130 which covers the substrate and the leads around the connection points. The insulation layer inhibits the leads from participating in the electrochemical reactions. Optionally, a thermistor 132 detects the temperature of the sample in the region around the probe. The sensor probe 125 is preferably constructed by photolithographic etalization or thick film printing technology, although other methods of probe formation are also contemplated. In one embodiment, components of the sensor, including electrodes, electrical connection points and electrical leads are all laid down on the substrate. Materials for the electrodes and connection points are separately dispersed in inks and printed onto the substrate. The inks are cured, for example, by heat, UV light, or the like. The probes produced are inexpensive and thus are suited to single use. Additionally, such probes can be used without prior calibration. The electrode materials are selected so that they will not become disbonded when immersed in a peracetic acid solution at temperatures between around 25°C and 75°C. The choice of ink also affects the conductivity to some degree. In another embodiment, shown in FIGURES 4 and 5, where like components are numbered with a prime ('), a sensor A 1 includes a durable, reusable electrode system 110 • is shown. The electrode system 110' comprises a working electrode 118 ', a reference electrode 120' and a counter electrode 122'. The electrodes are analogous to those described above for the disposable sensor, but in this embodiment, are constructed to be reusable. The electrodes are mounted in a housing 150 formed from stainless steel or other material with a large heat capacity. The housing 150 defines an interior chamber or reservoir 114 ' . Working faces of the three electrodes 118 • , 120', 122' project through walls 156 of the housing into the chamber. The electrodes are sheathed with and receive mechanical support from insulating material 158 so that only the working faces are exposed to the peracetic acid sample. Steel tubes 160 are threadably, or otherwise removably attached to the walls of the chamber and carry the electrodes therethrough for ease of insertion and removal of the electrodes from the chamber and for mechanical support exterior to the chamber. An inlet line 162 carries a sample of the circulating decontaminant solution into the chamber through an inlet 164 formed in one wall of the housing 150. A diaphragm valve 168 in the inlet line is normally closed, except when a sample is being taken. An overflow or drain line 170 carries fluid from the chamber via an outlet 172 defined through an opposite wall of the chamber. The overflow line leads to a drain via an inverted U-bend or trap 174 or returns the sample to the fluid flow line 60. It is preferred to direct the decontaminant solution to drain since this eliminates the need to assure sterility of reservoir surfaces of the sensor housing. The chamber 114' and housing 150 are configured such that the thermal mass of the housing is substantially greater than the volume of the decontaminant solution to be sampled. The internal volume of the chamber is preferably about 10-15 ml or less. One or more thermal elements 176, within the walls 156 of the housing, maintains the housing at a stable temperature, and thereby the sampled fluid. Preferably, the sample is heated to a measurement temperature only slightly above the maximum temperature expected in the circulating fluid. This allows the sample to reach the measurement temperature very quickly. For example, if the decontamination portion of the cycle operates at about 50-55°C, the walls are preferably maintained at about 60°C. Alternatively, the sample may be cooled by cooling elements, such as by Peltier elements, to achieve an optimum measuring temperature. A thermocouple 132", or other temperature detector, detects the temperature of the chamber walls or the sampled fluid in the chamber. A temperature detector 178 receives signals from the thermocouple and adjusts the thermal elements to maintain the walls at a constant temperature. Alternately, compensation for temperature fluctuations can be made in the calculation of concentration, the currents from the electrodes, or the like. Preferably, the large, heated mass of the housing quickly brings the sample to a reproducible as well as stable temperature. When a sample is to be taken, the valve 168 opens and allows the sampled fluid to flow into the chamber. The valve 168 remains open for sufficient time to allow the sampled fluid to flush the contents of the chamber through the overflow and replace the contents with freshly sampled fluid. Alternatively, to conserve fluid, the drain valve may be opened to allow the prior sample to flow from the chamber. The valve is then closed and the chamber filled with a rinsing sample, which may be the same as the measurement sample. The rinsing sample is then drained and the chamber filled with a sample for measurement. In the system 1, the pump 22 pressurizes the circulating decontaminant to about 70 psi. In this case, a flush and fill period of around three seconds is sufficient to fill the chamber with a fresh sample of decontaminant solution. The valve is then closed and the sample is held within the chamber for sufficient time to equilibrate the temperature and for the sampled fluid to become quiescent. Once this equilibration period is complete, a pulsed voltage sequence is applied to the electrodes, resulting in the generation of an electrical current which is correlated to the concentration of peracetic acid in the sample. The sampling and measurement steps are repeated, preferably every one to two minutes, to ensure that the peracetic acid concentration does not drop below a minimum acceptable level. With reference once more to FIGURE 2 , the amperometric controller 112 includes a voltage regulator 180 which applies a reference voltage (relative to the potential generated by the reference electrode) between the reference electrode 120 and the working electrode 118 of the embodiment of either FIGURE 3 or FIGURES 4 and 5. A voltage pulser 182 superimposes a read voltage between the reference and working electrodes in short pulses. The current flowing through the counter electrode is the same as that of the working electrode. Since reference electrodes do not tend to conduct electricity well, this may lead to resistance problems. Additionally, the stability at the reference electrode may be compromised when large currents are drawn. It is desirable for the counter electrode 122 to be held at a potential sufficient to prevent current from flowing through the reference electrode. This is readily achieved by using one or more operational amplifiers 184, connected between the reference electrode 120 and counter electrode 122. The amplifiers only allow current to flow through the sampled solution between the working and the counter electrode. This allows precise control of the applied potential while blocking the reference electrode against carrying electrical current. The reference potential of the reference electrode is thus used to maintain/control the desired voltage potential applied between the working and reference electrodes so that the signal generated is well controlled. The controller 112 also includes a current monitor 186 which detects the current flowing between the working and counter electrodes. At a given temperature and voltage, the current measured is dependent on both the peracetic acid concentration and the concentration of other oxidizing species, such as hydrogen peroxide, in the solution tested. The respective contributions of each of these components to the overall current measured is dependent on the selected read voltage. Over a limited read voltage range, the hydrogen peroxide (or other oxidizing species present) has a much smaller influence on the current than the peracetic acid. Thus, by carefully selecting a read voltage which minimizes the effect of other oxidizing species, the current measured is virtually independent of the concentration of hydrogen peroxide and shows a linear relationship with peracetic acid concentration. For solutions containing peracetic acid and hydrogen peroxide, the read voltage is preferably in the range of 0-2v, more preferably, -0.5 to -1.4 volts, more preferably about -1.0 volts, relative to an Ag/AgCl reference electrode, when the working electrode and counter electrode are both carbon. Within this range, a preferred range for glassy carbon and iso-carbon is -0.9 to -1.1 volts and for pyrolytic graphite, -1.0 to -1.4 volts, more preferably, from -l.l to -1.3 volts. When the working and counter electrodes are gold, a preferred read voltage is about -50mV. Obviously, if a different reference electrode and corresponding different base potential are employed, the read voltage relative to the reference electrode will change accordingly. The choice of range will also be partially dependent on pH, ionic strength, and temperature, the above ranges having been determined in near neutral solutions at temperatures of from about ambient to about 50°C. To determine an appropriate read voltage a voltammetric scan of the working electrode in both a peracetic acid-free solution (e.g. a buffer solution) is compared with a scan for the same solution but with peracetic acid present. Before use, the working electrode may be treated to prepare the electrode surface for sensing. The pretreatment step improves the reproducibility of the electrode surface and consequently that of the sensed current. In the pretreatment step, gross debris tends to be removed from the surface of the electrode. Pretreatment of the electrode may be achieved by subsequent oxidation and reduction of the electrode surface by means of positive and negative pulses. This pretreatment step may be carried out in the reservoir 114 or in a separate vessel. The electrode is immersed in an electrolyte, such as a buffer solution or a sample of the decontaminant solution to be used. For example, sequential application of positive (+1.5 to +2.8V vs an Ag/AgCl reference electrode) and negative (-1.8 to - 2.5 V vs the Ag/AgCl reference electrode) pulses (e.g., as a square or sine wave) of from about 1 to 8 seconds in duration are used. For example, about ten positive and negative pulses are generally effective. With reference now to FIGURE 6, a typical measurement sequence includes a preconditioning phase P and a read phase R. The preconditioning phase enhances the quality of the current signal received in the read phase, for example, by removing from the electrode components of the solution which have deposited on the electrode in prior measurement sequences. The precleaning step may also be used to ensure an optimal oxidation state of the electrode surface which catalyzes the detection process. A preferred preconditioning phase P includes applying a positive voltage pulse between the reference 122 and working electrode 118 (at about +1.2 to +2 volts vs Ag/AgCl, followed by a negative voltage pulse (of about -1.8 to -2.5 volts vs Ag/AgCl) , each of the pulses lasting from about 1 to 8 seconds (i.e. a frequency of about 0.01Hz to 10 Hz, more preferably, 0.1 to 1 Hz). A sensing pulse at the read voltage (about -0.5 to -1.4 volts) is then applied for about 10 seconds. The current flowing during the read pulse decays asymptotically to a fairly steady value. Preferably, the current is measured towards the end of the application of the read voltage when its value has substantially reached a steady state. For example, the current flowing during the last two to three seconds of the sensing pulse (the read phase) is measured and averaged to produce the signal that is used to measure the concentration of peracetic acid. This delays allows time for the discharging of capacitative currents and recharging of the double layer of the measurement electrode and establishment of a diffusion- limited current so that the current measured is derived from primarily faradaic reactions, rather than the primarily capacitative currents which occur during rapid voltage changes. The pulse sequence of FIGURE 6 is repeated a plurality of times during the antimicrobial stage of the cycle, each time with a new sample of the circulated solution. Any residue build-up at the end of cycle is electrochemically removed at the beginning of the next cycle. More specifically, after the last measurement, liquid is retained in the electrolytic cell either by retaining the last sample or by filling the cell with rinse water in the subsequent rinse stage. In the next cycle, the cell samples the solution after the buffers, wetting agents, and corrosion inhibitors have been circulated, but before the antimicrobial is added. With the sampled buffer solution, a series of voltage pulses are applied between the reference and working electrodes to drive off the residue, at a voltage about the voltage that causes hydrogen gas to form and below the voltage at which oxygen gas forms on the working electrode of the present configuration. The pulses are large enough to drive off the residue, but small enough that the carbon electrode is not electrochemically eroded. In the present system, alternating square wave pulses of +1.3 to +2.0 volts and - 1.8 to -2.5 volts are preferred. However, voltage pulses of -1.5 to -2.5 volt and +2.0 to +3.5 volts with durations of 1-10 seconds can also produce satisfactory results. FIGURE 7 illustrates the relationship between voltage and current flow for solutions containing fixed concentrations of hydrogen peroxide and peracetic acid. The solutions tested were (1) a buffering system- to provide a "background" reading, (2) buffering system and 1500 ppm peracetic acid, and (3) buffering system and hydrogen peroxide and peroxide ions, for a pyrolytic graphite working electrode. It should be appreciated that some hydrogen peroxide was inevitably present in the peracetic acid sample due to the equilibrium concentration of hydrogen peroxide in peracetic acid. At the read voltage (around -0.5 to -1.4V for the system described above, relative to silver/silver chloride) , the peracetic acid makes a much larger contribution to the current measured than the hydrogen peroxide present. Electrode systems 110 and applied voltages are readily designed which allow the contribution of peracetic acid to the current measured to be roughly ten times that of hydrogen peroxide, or greater. Unless the concentration of hydrogen peroxide in the solution to be tested is much greater than that of peracetic acid, the current output in the optimal read voltage range is, for all practical purposes, dependent on the peracetic acid concentration and almost independent of the hydrogen peroxide concentration. In some instances the optimum read voltage may not be achievable in the electrochemical system due to background noise. At low voltages (about -20mV) , the current output tends to be masked by background noise and therefore measurement of very low peracetic acid concentrations, in particular, may be difficult. Thus, the choice of a preferred read voltage is dependent on the likely concentrations of peracetic acid to be measured, the ratio of peracetic acid to hydrogen peroxide, and the degree of background noise in the system. Using a carbon electrode pushes the optimum read voltage away from the background noise region (-20mV) . Read voltage pulses at about -0.5 to -1.4 volts relative to silver/silver chloride are ideal for detecting peracetic acid concentrations in the range of 100 ppm to 3000 ppm, when the hydrogen peroxide concentration is less than, or not substantially greater than, the peracetic acid concentration. The choice of read voltage, within the preferred range, may also be selected to take advantage of a flat portion or plateau of the voltammogram, if one conveniently exists within the preferred range. Choosing a read voltage in a plateau region makes the current reading less sensitive to minor fluctuations in voltage. The choice of materials for the electrodes thus affects the selection of a preferred read voltage. Other factors, such as temperature and pH, also influence the selection. The current output increases with temperature. Preferably, the heated housing brings the temperature of the sample to a constant temperature for measurements to be made. Alternatively, where significant variations in temperature are anticipated, the detected current flows are preferably corrected for variations in the temperature. The thermistor 132, in this embodiment, is placed in contact with the sample to be tested and measures the temperature of the peracetic acid sample. Current measurements are then compensated for variations in temperature. The computer control system 80 optionally corrects the detected current flows for variations in temperature detected by the thermistor 132. The computer preferably accesses a look-up table 196 and determines the peracetic acid concentration corresponding to the current output measured. However, for automated processing systems where temperatures are controlled to within + 0.5-1.0°C, the effect of temperature on the current is relatively small and thus temperature compensation may be unnecessary. Preferably, the working electrode surface area is significantly smaller than that of the counter electrode. Thus, the current flow generated for a given peracetic acid concentration is limited by the working electrode surface area. The counter electrode has a larger surface area than the working electrode to avoid saturation of the electrode with electrons and a loss of the linear relationship between peracetic acid concentration and current flow at higher peracetic acid concentrations. The read voltage is pulsed between the reference and working electrodes (and hence to the counter electrode) at a fixed rate. A preferred duration of read voltage is about 10 seconds. Because of double layer discharging and rate limiting diffusion effects, the current output decreases asymptotically with time, eventually reaching a plateau region in which the current output is relatively constant with time. A sample time of around 5-15 seconds allows such steady state conditions to be established. The controller 112 then detects the current output, from which the peracetic acid concentration is determined. Between each sampling period, the working electrode is preconditioned again, as described. By repeating the sampling and the measurement of current output over a period of time, at intervals of about 30 seconds to two minutes, an accurate current measurement of the peracetic acid concentration in the sterilant solution or in the sample is obtained. When peracetic acid is present in the sample, the working electrode becomes enriched with electrons when the sensing pulse is applied. This excess of electrons will tend to cause the peracetic acid molecules in the vicinity of the electrode to become reduced (i.e., accept electrons from the surface of the electrode.) The movement of electrons from the electrode into the solution via this mechanism produces the electrical current that can be measured. The magnitude of the current produced is proportional to the concentration of the peracetic acid molecules close to the surface of the electrode. When the magnitude of the voltage is small (not negative enough) , the rate at which the peracetic acid molecules react is slow compared to the rate at which the peracetic acid at the surface is replenished by diffusion from the bulk solution. As the voltage becomes more negative, the rate at which peracetic acid is consumed increases and, as time progresses, the concentration of peracetic acid close to the electrode is depleted. This results in the current dropping exponentially and asymptotically reaching a limit determined by the rate at which peracetic acid can diffuse from the bulk solution to the surface of the electrode (i.e., a diffusion limited current) . The peracetic acid sensor A, as it is used herein, measures this diffusion limited current. The decontaminant solution contains a buffering system which acts as an electrolyte. When a voltage is applied, a small current will flow due to the electrical conductivity of the electrolyte. In addition, when chemical species are present that are susceptible to electrochemical conversion, an additional electrical current will be produced due to electrochemical conversions at the surfaces of the electrodes. In one embodiment, the computer control system 80 signals an alarm 202 when the peracetic acid concentration of the bath drops below a preselected minimum peracetic acid concentration. Or, the computer adjusts the length of the cycle to compensate for a reduced peracetic acid concentration. In another embodiment, the control system 80 adjusts the concentration of peracetic acid flowing through the system in response to the detected concentration. In this embodiment, the control system signals a valve 204 in fluid communication with the fluid line 60 to open and release an additional dose of the concentrated source of peracetic acid into the system from a supplementary dispenser, such as a reservoir 206, or other source of the concentrate. Other means of adjusting the peracetic acid concentration are also contemplated. Because the electrodes 118', 120', 122' in the reusable sensor A 1 tend to degrade over time, they should be replaced at intervals to maintain the accuracy of the sensor. Optionally, a calibration check is carried out prior to a sterilization cycle with a peracetic acid containing solution or solutions of known concentration, preferably concentrations in the range to be measured. The reference electrode 120' may be checked every decontaminant cycle by measuring the magnitude of the reference potential relative to the carbon electrodes 118', 120' and/or the stainless steel housing in the presence of an electrolyte. The electrolyte may be the pretreatment mixture of buffers, corrosion inhibitors, and the like, which is circulated through the system prior to addition of the peracetic acid decontaminant. It is also important to maintain the surface condition of the working electrode, since the active area of the working electrode affects the magnitude of the measured current. Conductivity measurement may be made periodically to provide information on the state of the electrode surface. For example, the conductivity is measured each cycle in the presence of the buffers and corrosion inhibitors. Provided that the ionic strength of the buffered solution does not vary significantly from cycle to cycle, the conductivity measurements can be used to provide an indication of the state of the working electrode surface. Theoretically, the electrical resistance between the housing and the working electrode is a function on both the surface area of the housing and the surface area of the working electrode. Since the surface area of the housing is significantly larger than that of the working electrode, the electrical resistance will be more sensitive to changes in surface area of the working electrode. When the electrodes 118', 120", 122' are to be reused, it is preferable to maintain the working surfaces in contact with an electrolyte or water between decontaminant cycles. Accordingly, a sample of the decontaminant solution is left in the chamber at the end of a cycle. Or, the chamber is filled with a fresh solution of electrolyte or rinse water. Particularly when the system 1 is not to be used for some time, the electrodes may be removed from the sensor A' and stored in an electrolyte solution or water until needed. In an alternative embodiment, one or more of the electrodes is disposable, while the remaining are reusable. For example, a card type sensor 125 of the type shown in FIGURE 3 may be used for the working and counter electrodes 118, 122 in combination with a reusable reference electrode 120" of the type shown in FIGURES 4 and 5. The card is disposed after a decontamination cycle, and the reference electrode 120' is reused. It will be appreciated that the peracetic acid monitoring system A, A' may also be used in a variety of other peracetic acid sterilization/ disinfection systems in which items to be microbially decontaminated are immersed in, or sprayed with a peracetic acid solution. The system may also be used to detect the concentration of peracetic acid in a bath containing peracetic acid or in fluid flow lines of a water treatment system, bleaching plant, or similar system. Where the solution to be tested does not act as an electrolyte, an electrolyte may be added to the sample to be analyzed prior to making measurements. While the system has been described with particular reference to detection of peracetic acid, the system is also applicable to detection of hydrogen peroxide and other oxidizing species. For example, other peracids, or mixtures of peracids are also useful antimicrobial agents. By selecting an appropriate read voltage to maximize the contribution of the species to the overall output current and by choosing counter and working electrodes which are particularly suited to the species to be detected, the system may be tailored to the specific detection of a variety of species. Without intending to limit the scope of the invention, the following examples show cyclic voltammograms for carbon electrodes at differing peracetic acid concentrations. EXAMPLES Cyclic voltammograms were performed on glassy carbon and pyrolytic graphite working electrodes in solutions at different peracetic acid levels. FIGURES 8 and 9 show exemplary voltammograms for glassy carbon and pyrolytic graphite, respectively. For both these electrode materials, there is a plateau region in which the current remains relatively stable. This plateau region overlaps the voltage range in which the electrode shows greater sensitivity to peracetic acid than to hydrogen peroxide.

Claims: Having thus described the preferred embodiments, the invention is now claimed to be: 1. A method of detecting a first oxidizing species in a solution which also contains a second oxidizing species, the method comprising disposing: (a) a working electrode (118) and a reference electrode (120) , or (b) a working electrode (118) a reference electrode (120), and a counter electrode (122), in the solution to be tested, selectively applying a read voltage to the working electrode, and detecting current flowing between either the working electrode and the reference electrode or the working electrode and the counter electrode (122) , where present, the method characterized by: the read voltage being selected such that the current flowing is dependent on a concentration of the first oxidizing species and substantially independent of a concentration of the second oxidizing species in the solution. 2. The method of claim 1, further characterized by: the read voltage being selected such that a contribution of the first oxidizing species to the current flowing is at least ten times that of an equivalent concentration of the second oxidizing species. 3. The method of either one of preceding claims 1 and 2 , further characterized by the read voltage being in the diffusion limiting range. 4. The method of any one of preceding claims 1-3 , further characterized by: the read voltage being selected to provide a high signal to noise ratio. 5. The method of any one of preceding claims 1-4, further characterized by: the read voltage is selected to be within a voltammetric plateau region where current is relatively insensitive to minor changes in read voltage. 6. The method of any one of preceding claims 1-5, further characterized by: converting the detected current flow into an indication of the concentration of the first oxidizing species in the solution. 7. The method of any one of preceding claims 1-6, further characterized by: the first oxidizing species including peracetic acid. 8. The method of claim 7, further characterized by: the working electrode being formed from carbon. 9. The method of claim 8, further characterized by: the read voltage being in the range of -0.5 to -1.4 volts, relative to a silver/silver chloride reference electrode. 10. The method of claim 9, further characterized by: the working electrode being formed from pyrolytic graphite and the read voltage being in the range of -1.1 to -1.3 volts, relative to a silver/silver chloride reference electrode. 11. The method of any one of preceding claims 7-10, further characterized by: the peracetic acid concentration being in the range of 100 to 3000 ppm. 12. The method of claim 11, further characterized by: the peracetic acid concentration being determined in under one minute. 13. The method of any one of preceding claims 1-12 , further characterized by: the second oxidizing species including hydrogen peroxide. 14. The method of any one of preceding claims 1-13, further characterized by: the selective application of the read voltage and the detection of the current flowing are repeated at intervals of from about ten to thirty seconds. 15. The method of any one of preceding claims 1-15, further characterized by: prior to the step of applying the read voltage: conditioning the working electrode by applying at least one positive pulse and applying at least one negative pulse to the electrode. 16. The method of any one of preceding claims 1-15, further characterized by: detecting a temperature of the solution adjacent the electrodes; and correcting the detected current flowing for a difference between the detected temperature and a preselected temperature. 17. The method of any one of preceding claims 1-16, further characterized by: increasing the first oxidizing species concentration in the solution when the concentration is below a preselected minimum level. 18. The method of any one of preceding claims 1-17, wherein the read voltage is applied as a pulse. 19. The method of any one of preceding claims 1-18 , further characterized by: prior to the step of disposing a working electrode and a reference electrode in the solution to be tested: circulating the solution though a treatment vessel (12) which contains items to be decontaminated; and the step of disposing the working electrode, reference electrode, and counter electrode, where present, in the solution to be tested including: withdrawing a sample of the circulated solution into a chamber (114) to contact the electrodes. 20. The method of claim 19, further characterized by: prior to the step of circulating the solution though the treatment vessel: circulation a preconditioning solution including buffers and wetting agents though the treatment vessel ; and withdrawing a sample of the preconditioning solution into the chamber; and pulsing voltages between the reference and working electrodes which electrochemically remove residues from the working electrode. 21. The method of either one of preceding claims 19 and 20, further characterized by: prior to the step of withdrawing a sample of the circulated solution into a chamber: filling the chamber with solution; and emptying solution from the chamber to flush residue from previous measurements from the chamber. 22. The method of any one of preceding claims 19-21, further characterized by: signaling an indication of the current measured to a control system (80) , which, in the event that the current measured is below a predetermined minimum level, conducts at least one of the following steps: aborting the decontamination process; extending the time of the decontamination process to compensate for the first oxidizing species concentration; controlling the addition of additional first oxidizing species to the circulating solution; and providing a signal which indicates that the first oxidizing species concentration is below the predetermined minimum level. 23. A sensor system comprising: a chamber (114) which receives a sample of the solution; an electrode system (110) comprising one of: (1) a working electrode (118) and a reference electrode (120), the reference electrode also acting as a counter electrode, and (2) a working electrode (118), a reference electrode (120), and a counter electrode (122), the electrodes disposed within the chamber to contact the sample of solution; and an amperometric controller (112) electrically connected with the working and reference electrodes, the system characterized by: the controller (112) selectively pulsing a preselected read voltage to the working electrode, and detecting an output current flowing in a circuit including the working electrode, the reference electrode or the counter electrode, where present, and the solution, the read voltage being selected such that the output current is dependent on a concentration of the first oxidizing species and substantially independent of a concentration of at least a second oxidizing species in the solution. 24. The sensor system of claim 23, further characterized by: the sensor system including: the working electrode, the reference electrode and the counter electrode disposed in the chamber, the amperometric controller selectively applying a potential between the working electrode and the reference electrode and measuring a current flowing between the working electrode and the counter electrode. 25. The sensor system of either one of claims 23 and 24, further characterized by: a heater for heating the sample to a preselected temperature prior to pulsing the read voltage. 26. The sensor system of any one of preceding claims 23-25, further characterized by: the working electrode being formed from carbon. 27. The sensor system of any one of preceding claims 23-26, further characterized by: the electrodes being laid down on a common substrate. 28. The sensor system of any one of preceding claims 23-27, further characterized by: a thermistor for detecting temperatures in a portion of the solution adjacent the electrodes for correcting the measured current for temperature fluctuations. 29. The sensor system of any one of preceding claims 23-28, further characterized by: a computer which receives current signals from the amperometric controller and determines a concentration of the first oxidizing species. 30. The sensor system of claim 29, wherein the computer includes a look-up table which determines the concentration of the first oxidizing species from the current output at a given temperature. 31. The sensor system of either one of preceding claims 29 and 30, further characterized by: a dispenser for dispensing additional amounts of the first oxidizing species into the solution, the computer directing the dispenser to dispense an additional amount of the first oxidizing species into the solution when a preselected minimum level of the first oxidizing species in the solution is detected. 32. A decontamination apparatus comprising a decontamination vessel (12) which receives items to be decontaminated, a fluid flow path (60) which circulates a solution comprising a first oxidizing species to the vessel, and a sensor system (110) fluidly connected with the fluid flow path for specifically detecting the first oxidizing species in the solution, the apparatus characterized by: the sensor system of any one of preceding claims 23-31.

Inventors: DESANTIS BRIAN J, LEWANDOWSKI JAN J, MCVEY IAIN F, SCHINDLY BRIAN E, THOMAS KAREN L, DESANTIS, BRIAN, J., LEWANDOWSKI, JAN, J., MCVEY, IAIN, F., SCHINDLY, BRIAN, E., THOMAS, KAREN, L.
Applicants: DESANTIS BRIAN J, LEWANDOWSKI JAN J, MCVEY IAIN F, SCHINDLY BRIAN E, STERIS INC, THOMAS KAREN L, DESANTIS, BRIAN, J., LEWANDOWSKI, JAN, J., MCVEY, IAIN, F., SCHINDLY, BRIAN, E., STERIS INC., THOMAS, KAREN, L.
Application year: 2001
Main classification: G01N27
IPC codes: G01N27