Abstract:
A wet chemical indicator for peracetic acid solutions includes an indicator for peracetic acid and an inhibitor. The inhibitor is selected to inhibit a selected peracetic acid concentration in a reproducible sample of a solution containing peracetic acid. The indicator exhibits an observable change when the selected concentration of peracetic acid is exceeded. In this manner, a rapid indication of whether the peracetic acid solution is of a required minimum strength is provided.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the sterilization and disinfection arts. It finds particular application in conjunction with the evaluation of peracetic acid sterilization or disinfection baths and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to other liquid sterilization and disinfection processes where the sterilant or disinfectant is effective above a minimum effective concentration, such as liquid hydrogen peroxide or sodium hypochlorite sterilization or disinfection systems. 
     Peracetic acid is a useful disinfectant and sterilant for a variety of applications including disinfection of waste and sterilization of medical equipment, packaging containers, and the like. Peracetic acid has the ability to be reused over a period of time, allowing instruments to be repeatedly sterilized or disinfected throughout the day in the same bath. 
     In use, peracetic precursors are mixed with water and other chemicals in a bath and the items to be sterilized or disinfected are immersed in the bath. Decontaminated items are then typically rinsed before use. To ensure effective sterilization or disinfection within a preselected period of time, the concentration of the peracetic acid is maintained above a minimum effective level, typically around 2300 parts per million for sterilization of medical instruments. For disinfection, peracetic acid concentrations of 5 ppm and above are used. For peracetic acid concentrations of at or above the minimum effective level, complete sterilization or disinfection is expected. Because the peracetic acid tends to decompose over time, it is important to evaluate the bath periodically to determine whether the minimum effective level of peracetic acid is present. 
     Currently, it is often assumed that the bath will be at or above the minimum effective concentration for a period of around eight hours where a selected initial concentration of peracetic acid is present in the bath. However, differences in ambient temperature, the quantity of items to be disinfected or sterilized and the level of contamination on the items can lead to considerable variation in the useful life of the bath. In addition, storage conditions sometimes lead to degradation of peracetic acid precursors before use. For medical instruments in particular, therefore, a more accurate method of evaluating the peracetic acid is required. Dippable chemically-treated papers are easy to use but lack accuracy, particularly at concentrations suitable for sterilizing and disinfecting. Although chemical titration methods provide an accurate measure of the concentration of peracetic acid in a solution, these methods are time-consuming and open to possible operator errors. Premeasured vials of titrating solution have been utilized to detect low and trace concentrations of peracetic acid, e.g., residual peracetic acid after sterilizing and rinsing. Manually measuring a unit volume of solution into the vial raises the specter of human error. 
     The present invention provides a new and improved wet chemical indicator for the evaluation of peracetic acid solutions which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a wet chemical system is provided for evaluating peracetic acid solutions. An indicator for peracetic acid and an inhibitor for inhibiting a selected concentration of peracetic acid in a sample quantity of a peracetic solution are disposed within a container. The inhibitor prevents the indicator from detecting peracetic acid at concentrations below the selected concentration. 
     In accordance with another aspect of the present invention, a method is provided for evaluating a peracetic acid sterilizing or disinfecting solution. An indicator for peracetic acid and an inhibitor for inhibiting a selected concentration of peracetic acid in a sample of a sterilizing or disinfecting solution are combined with the sample of the sterilizing or disinfecting solution to be tested. The combination of indicator, inhibitor, and peracetic acid sterilizing or disinfecting solution are observed for a change in a measurable property. The change indicates that the concentration of peracetic acid in the sterilizing or disinfecting solution exceeds the selected concentration. 
     One advantage of the present invention is that it provides a rapid and inexpensive method of determining the adequacy of a peracetic acid solution for sterilizing or disinfecting instruments. 
     Another advantage of the present invention is that it provides a clear yes-or-no indication of whether a minimum effective concentration of peracetic acid is present. 
     Another advantage of the present invention is that it permits efficient use of a peracetic acid sterilizing or disinfecting solution. 
     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. 
     FIG. 1 is a cross section of a wet chemical indicator for the evaluation of peracetic acid of the present invention; and, 
     FIG. 2 is a cross section of an alternative embodiment of a wet chemical indicator for the evaluation of peracetic acid of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a container  10  includes an approximately cylindrical transparent side  12  and a base  14 , sealed to the side. The side defines an opening  16 . A removable cap  18 , seals the opening  16 . The cap  18  preferably has a screw fitting  20  capable of engaging a similar fitting  22  on the side  12  of the container  10 . Alternatively, the cap  18  has a press-fit seal that engages the side  12  of the container  10 . 
     A calorimetrically oxidizable species, or indicator  30  and a inhibitor  32  are disposed within the container  10 . The inhibitor inhibits a selected concentration of peracetic acid, usually selected to be the minimum effective level for sterilization or disinfection. A measured quantity of a peracetic acid sterilizing or disinfecting solution to be tested is added to the container  10 , the cap  18  positioned to seal the opening  16  and the mixture shaken for a short period, typically less than ten seconds. A change in a property of the solution, such as a color change, indicates that the concentration of peracetic acid in the sterilizing or disinfecting solution is at or above the minimum effective level for sterilization or disinfection. 
     The indicator  30  is preferably one which exhibits a visible color change when mixed with peracetic acid, such as starch-iodide solution, n,n diethyl-p-phenylene diamine (DPD), ferroin indicator, ferrous thiocyanate or cerric sulphate. Starch-iodide is a particularly preferred indicator because of its long shelf life and because it exhibits a distinct color change (colorless to blue-black) in the presence of concentrations of peracetic acid. Typical contaminants found on medical instruments do not interfere with the detection of peracetic acid by the starch-iodide indicator at the contaminant concentration levels to be expected in sterilization or disinfection baths. A starch-iodide solution is readily prepared by mixing approximately equal parts of a starch solution and a potassium or (sodium) iodide solution. The ratio is not critical, however, the starch component serving only to emphasize the color change caused by the conversion of potassium iodide to iodine in the presence of peracetic acid. A suitable solution contains 0.05-1.0% by weight of starch and 0.05-1.0% by weight potassium or sodium iodide. 
     Another preferred indicator is DPD. This indicator is preferably used in combination with a halogen phosphate buffer (HPB). It is particularly useful in solutions containing chlorine because it is unaffected by the chlorine. However, the color change is not as distinct as with starch-iodine. In the presence of peracetic acid, the color of the solution changes from clear to a light pink, then to a dark pink as the concentration of peracetic acid increases. For accurate results, it is preferable to employ a spectrophotometer or a set of standard solutions of appropriate colors to evaluate the color change when the peracetic acid concentration is close to the selected minimum effective level. 
     The inhibitor  32  is a chemical which inhibits a known concentration of peracetic acid, preventing the indicator  30  from detecting peracetic acid below that concentration. For example, when the desired peracetic acid concentration (the trip point) is 2500 ppm, the inhibitor  32  dose is adjusted to inhibit 2500 ppm of peracetic acid. The indicator  30  then only detects the uninhibited peracetic acid in excess of that amount. The combination of the inhibitor  32  with the indicator  30  thus creates an easy to read yes-or-no indicator for peracetic acid sterilization and disinfection solutions. Absence of a color change indicates that the peracetic acid solution needs to be replenished, while a distinct color change indicates that the solution can be safely used for further sterilization or disinfection of instruments. 
     Preferred inorganic inhibitors  32  are sodium thiosulfate and salts of heavy metals including ferrous salts, copper salts and cobalt salts. Preferred organic inhibitors  32  include hydroquinone and derivatives, t-butyl catechol, ethanolamines, and phenols. A particularly preferred indicator  30  and inhibitor  32  combination is starch-iodide solution with sodium thiosulfate because the mixture is stable over fairly long periods of time, allowing the two chemicals to be stored together in the container  10 . Sodium thiosulfate is also an effective inhibitor for DPD. 
     By adjusting the concentration of the inhibitor  32 , different preselected concentrations of peracetic acid can be detected. Since the indicator  30  detects concentrations of as low as a few parts per million peracetic acid, peracetic acid solutions in the range of about 10 ppm to about 10,000 ppm are measurable. TABLE 1 gives examples of indicator and inhibitor concentration ranges suitable for testing peracetic acid solutions. 
     
       
         
               
             
               
               
               
             
           
               
                                                TABLE 1 
               
               
                   
               
               
                 Test Solutions for mixing with an equal quantity of a 
               
               
                 peracetic acid solution to generate a distinct color change at 
               
               
                 2500 ppm peracetic acid. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 FORMULA 1 
                   
               
               
                   
                 Indicator 
                 0.4% Sodium Iodide 
               
               
                   
                   
                 0.2% Starch 
               
               
                   
                 Inhibitor 
                 0.01N Sodium Thiosulfate 
               
               
                   
                 FORMULA 2 
               
               
                   
                 Indicator 
                 0.4% Sodium Iodide 
               
               
                   
                   
                 0.2% Starch 
               
               
                   
                 Inhibitor 
                 0.02N Sodium Thiosulfate 
               
               
                   
                 FORMULA 3 
               
               
                   
                 Indicator 
                 0.4% Sodium Iodide 
               
               
                   
                   
                 0.2% Starch 
               
               
                   
                 Inhibitor 
                 0.1N Cerric Ammonium Sulfate in 1.0N 
               
               
                   
                   
                 Sulphuric Acid 
               
               
                   
                 FORMULA 4 
               
               
                   
                 Indicator 
                 0.4% Sodium Iodide 
               
               
                   
                 Inhibitor 
                 0.02N Sodium Thiosulfate 
               
               
                   
                 FORMULA 5 
               
               
                   
                 Indicator 
                 0.5-2.5 g/l n,n Diethyl-p-Phenylene-Diamine 
               
               
                   
                   
                 (DPD) 
               
               
                   
                 Buffer 
                 Halogen Phosphate Buffer 
               
               
                   
                 Inhibitor 
                 0.01N Sodium Thiosulfate 
               
               
                   
                   
               
             
          
         
       
     
     Thus, for example, to make 100 ml of FORMULA 1, one would mix about 10.0 ml of 0.1 normal solution of sodium thiosulfate with 0.2 grams of starch and 0.4 grams of sodium iodide. Deionized water is added to 100 ml. 
     The exact quantity of the inhibitor used varies according to the desired peracetic acid trip point. TABLE 2 shows how the trip point varies with the quantity of inhibitor for a HPD/DPD/sodium thiosulfate indicator/inhibitor system. In each test, a peracetic acid solution of known concentration (established by conventional titration methods) was mixed with a 0.1N thiosulfate solution in HPB. 1000 ml of a solution of 2.5 g/l DPD was then added. Color changes were observed by eye after about five seconds. 
     TABLE 2 records the average color change of one to four tests as a number between 0 and 4, 0 indicating no color change, 4 indicating a change to dark pink. The results show that as the quantity of the inhibitor (sodium thiosulfate) is increased, the trip point shifts towards a higher peracetic acid concentration. For example, if a trip point color of 3 is selected, the peracetic acid concentration at the trip point shifts from 2400 ppm at 1400 μL of inhibitor to 2600 ppm at 2000-2100 μL of inhibitor and 2700 ppm at 2200 μL of inhibitor. Thus, the trip point is readily adjusted by varying the quantity of inhibitor. Sharper trip points would be obtained if the tests were evaluated with a spectrophotometer or standard color solutions. 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                                              TABLE 2 
               
             
             
               
                   
               
               
                 Inhibitor 
                 Peracetic Acid Concentration (ppm.) 
               
             
          
           
               
                 Volume μL 
                 &gt;2700 
                 &gt;2600 
                 &gt;2500 
                 &gt;2400 
                 &gt;2300 
                 &gt;2200 
               
               
                   
               
             
          
           
               
                 1400 
                 3 
                 3 
                 3 
                 3 
                 2 
                 0.5 
               
               
                 2000 
                   
                 3 
                 2 
                   
                 1.5 
               
               
                 2100 
                 3 
                 3 
                 1.5 
                 2 
                 2 
               
               
                 2200 
                 3 
                 2 
                 1.5 
                   
                 1.5 
                 2 
               
               
                   
               
             
          
         
       
     
     Where the indicator  30  and inhibitor  32  are unstable in combination, the indicator and inhibitor are preferably separately contained. For FORMULA 5, for example, the halogen phosphate buffer is conveniently combined with the inhibitor and is preferably kept separate from the indicator (DPD) until used for testing a sample of a peracetic acid solution. Optionally, a sealed frangible inner container  34 , such as a glass ampule, holding the indicator  30 , is disposed within the container  10 . After the peracetic acid has been added to the container  10 , the inner container  34  is broken, releasing the indicator  30  into the inhibitor-peracetic acid solution. Alternatively, the inner container  34  is broken immediately prior to addition of the peracetic acid solution. Preferably the container  10  is constructed from a flexible material such as polypropylene, allowing the inner container  34  to be broken by squeezing the side  12  of the container. Alternatively, a downward motion on the cap  18 , causes a dart  24  to fracture the inner container  34 . 
     Optionally, the indicator  30  and or the inhibitor  32  are solids which readily dissolve in and interact with the peracetic acid solution. 
     The measured quantity of peracetic acid solution is preferably obtained by drawing the solution into a calibrated eyedropper or syringe. Alternatively, a swab, capable of absorbing a known volume of peracetic acid solution, is dipped into the solution. The swab and absorbed peracetic acid solution are then introduced to the container  10 . A particularly preferred method of introducing a reproducible quantity of peracetic acid to the container  10  is by drawing the peracetic acid directly into the container under vacuum as shown in FIG.  2 . 
     With reference to FIG. 2, an alternative embodiment includes a container  110  with a base  114  and a side  112 , sealed to the base. The side tapers inwards, thereby defining an open area  116  adjacent to the base  114  and a narrow tube  118 , extending from the open area. The end of the tube  118  furthest from the base  114  defines an opening  120 . The container  110  is preferably constructed from a transparent rigid material, such as glass, capable of maintaining a vacuum within the open area  116 . The tube includes a frangible area  122 . 
     Measured quantities of an indicator  30  and an inhibitor  32  are disposed in the container  110  under vacuum. A frangible seal  124  is formed within the tube  118  to block the tube. This prevents the indicator  30  and inhibitor  32  from leaking from the container  110  during storage. 
     Where the container  110  is constructed of glass, the seal  124  is preferably formed by fusing an area of the tube at a high temperature. A sleeve  126  tightly surrounds the area  122  of the tube  118  containing the seal  124 . The sleeve  126  is preferably constructed of a flexible material, such as silicon rubber, such that the seal  124  may be broken by bending the tube  118  within the area  122  under the sleeve  126 . The seal  124  is broken by breaking the area  122  of the tube  118 . The sleeve  126  maintains the integrity of the tube  118  by sealing around the broken area and prevents shards of glass from escaping. 
     The wet chemical indicator is preferably assembled for use by drawing the required quantities of indicator  30  and inhibitor  32  into the container  110  through the tube  118  into the open area  116 , then drawing a vacuum in the container  110 , sealing the tube  118  with seal  124 , and surrounding the sealed area  122  of the tube with the flexible sleeve  126 . 
     A sample of the peracetic acid solution to be tested is drawn into the container  110  by first inserting the tube  118  into the solution, then breaking the seal  124 . The vacuum in the container  110  draws a reproducible quantity of the solution into the container to mix with the indicator  30  and inhibitor  32 . The container  110  is inverted and shaken to mix the three components together. When peracetic acid is present in the solution at concentrations above the minimum effective concentration, a color change is rapidly visible through the side  112  of the container  110 . Thus, an accurate determination of whether sufficient peracetic acid is present in the sterilizing or disinfecting solution is obtained within a few seconds. 
     Optionally, a cap  128  is attached to the opening  120  after drawing the peracetic acid to seal the opening and prevent leakage from the container  110 . The cap also limits the entry of oxygen into the tube, which is beneficial to the accuracy of some indicator/inhibitor systems. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.