Abstract:
A basic unit for a mobile water analyzing system includes a photometer comprising a light source configured to generate a measurement beam and a light detector configured to receive the measurement beam, a test element receptacle configured to allow a test element to be inserted therein, and a photometric measuring track arranged between the light source and the light detector. The measurement beam is coincident with the photometric measuring track during a photometric measurement, and not in a cross direction thereto.

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 13/122,561, filed on May 10, 2011. U.S. application Ser. No. 13/122,561 is a U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2009/062535, filed on Sep. 28, 2009 and which claims benefit to German Patent Application No. 10 2008 050 092.5, filed on Oct. 6, 2008. The International Application was published in German on Apr. 15, 2010 as WO 2010/040657 A1 under PCT Article 21(2). 
     
    
     FIELD 
       [0002]    The present invention provides a method for determining an analyte in a water-sample with a mobile water-analyzing system. The present invention also provides a mobile water-analyzing system for determining the aforementioned method as well as a basic unit and a test-element of the respective mobile water-analyzing system. “Mobile” means that the water-analyzing system of the present invention is not stationary as a continuously working process analyzing-device. 
       BACKGROUND 
       [0003]    The prior art in the field of mobile water-analysis is currently represented by so-called photometric cuvette tests as described in DE 41 09 118 A1. The use of these tests is performed manually. First, a water-sample is taken by a pipette and given into the cuvette. For determining the self-absorption of the water-sample, i.e., the so-called background signal, the water-sample is inserted into the photometer and is measured. A reagent is then added to the water-sample in the cuvette. The cuvette is closed and shaked to mix the water-sample with the key-reagent. It is subsequently inserted into a photometer and measured. 
         [0004]    The handling of such a cuvette test is inconvenient and extremely susceptible to errors. The used reagent can be dangerous for health and the environment, so that the used cuvette tests must be disposed in an appropriate way. The inconvenient handling makes the cuvette tests only applicable for use in a laboratory. Such water-sample analyzation, including the determination of the self-absorption of the water-sample, can be performed automatically in a non-mobile water-analyzing system. 
       SUMMARY 
       [0005]    An aspect of the present invention is to provide a method or a device for determining an analyte in a water-sample including the determination of the self-absorption of the water-sample with a mobile water-analyzing system with improved handling. 
         [0006]    In an embodiment, the present invention provides a basic unit for a mobile water analyzing system which includes a photometer comprising a light source configured to generate a measurement beam and a light detector configured to receive the measurement beam, a test element receptacle configured to allow a test element to be inserted therein, and a photometric measuring track arranged between the light source and the light detector. The measurement beam is coincident with the photometric measuring track during a photometric measurement, and not in a cross direction thereto. The present invention also provides a test element for a mobile water analysis system which includes a microfluidic sample line. The microfluidic sample line comprises an inlet opening disposed at a first end, a pump port disposed at a second end of the microfluidic sample line, an inlet section disposed between the inlet opening and the pump port, a measuring section comprising at least one window arranged at an end of the measuring section, a first reagent section disposed either between the inlet opening and the measuring section or between the measuring section and the pump port, and a reagent disposed in the first reagent section of the microfluidic sample line. The inlet opening is configured to receive a water sample. The measuring section is arranged between the inlet section and the pump port and is coincident with a sample pathway. The measuring section forms a photometric measuring track for a photometer so that a measurement beam of the photometer and the photometric measuring track are aligned during a photometric measurement so that the photometric measurement occurs along a length of each of the photometric measuring track and the measuring section, and not in a cross direction thereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention is described in greater detail below on the basis of embodiments and of the drawings in which: 
           [0008]      FIG. 1  shows a schematic drawing of a mobile water-analyzing system comprising a basic unit and a test-element, to determine the method according to the present invention; 
           [0009]      FIG. 2  shows the test-element of the water-analyzing system of  FIG. 1 ; 
           [0010]      FIG. 3  shows an embodiment of a mobile water-analyzing system including a removable cartridge with several test-elements, to determine the method according to the present invention; 
           [0011]      FIG. 4  shows the removable cartridge of  FIG. 3 ; 
           [0012]      FIG. 5  shows an embodiment of an electrochemical-optical test-element in a front view; 
           [0013]      FIG. 6  shows a rear view of the test-element of  FIG. 5 ; 
           [0014]      FIG. 7  shows a side view of an embodiment of a test-element with a pump membrane, which is controlled via a pump actuator from the basic unit. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The method for determining an analyte in a water-sample refers to a mobile water-analyzing system with a mobile basic unit and a removable disposable test-element which is inserted or is insertable, respectively, into the basic unit for determining the analyte. 
         [0016]    The test-element is a complex prefabricated part, whereby the test-element is provided with a sample-line with an inlet opening which is positioned at the distal end of the test-element. A measuring section is provided in line of the sample-line, the sample-line being provided with a measuring track for an analyzer which is provided at the basic unit. All other parts of the analyzer are positioned in the basic unit and not in the test-element. The basic unit is further provided with a receptacle for holding the inserted test-element. 
         [0017]    In the further course of the sample-line, a reagent section is provided, for example, in a section after the measuring section as seen from the inlet opening. The reagent section is provided with a reagent which can be, for example, a key-reagent, an auxiliary-reagent or an auxiliary-agent. A key-reagent reacts with the analyte of the water-sample by changing the color of the analyte, or the key-reagent reacts in such a way that an electrochemical analysis is possible. An auxiliary-reagent also reacts with the water-sample, but not with the aim of a quantitative determination of the analyte in the water-sample, but rather with another aim. An auxiliary-agent does not react chemically with the water-sample but influences the water-sample. 
         [0018]    According to the present invention, the inserting of the test-element into the test-element receptacle of the basic unit is first provided. The inserting of the test-element can be performed manually or automatically. The inlet opening of the test-element is then immersed manually or automatically into the water to be analyzed and a defined portion or a defined volume, respectively, of the water-sample is segregated by transporting the water-sample forward so that the water-sample is transported from the inlet opening to the measuring section. 
         [0019]    The creation of a defined portion of the water-sample can be realized automatically or semi-automatically: The insertion of the test-element into the test-element receptacle of the basic unit activates the basic unit for an analyte determination. In a next step, the inlet opening of the test-element is immersed manually or automatically into the water-reservoir to be analyzed. After the immersion of the inlet opening into the water-reservoir, the pump actuator is activated, whereby the immersion can be detected, for instance, through a small pressure impulse in the sample-line. The activating of the pump actuator can also be performed manually. By activation of the pump actuator, a water-sample is pumped actively through the inlet opening toward the measuring section. 
         [0020]    A defined volume of the water-sample is sucked and is segregated as a sample-column, whereby the sample column at both ends is limited by air. By limiting the sample-column to a defined volume, a defined ratio between the water-sample and the key-reagent is provided. 
         [0021]    The limitation of the water-sample portion to a defined portion can be realized by giving the customer a signal after the pump actuator has stopped after a defined volume of the water-sample has been sucked so that the customer obtains the information that the sampling is finished and that the inlet opening can be taken out of the water to be analyzed. The segregation of the water-sample can also be made automatically by using an appropriate valve which conducts air into the sample-line after the defined water-sample volume is sucked. 
         [0022]    “Pumping forward” means a transporting in the sample-line upstream from the inlet opening. The forward pumping can be performed in any known way, for example, by a pump in the basic unit, whereby the pump is connected with the sample-line, or by a relative large pump volume which is formed by a pump-membrane at the disposable test-element, whereby the pump-membrane is controlled via a pump actuator of the basic unit, for example, by a push rod. 
         [0023]    In the measuring section, a first analysis of the water-sample is performed with the analyzer, for example, a sample background signal is determined. The first analysis can also basically be a quantitative determination of an analyte in the water-sample when a key-reagent is positioned in the sample-line between the inlet opening and the measuring section. 
         [0024]    The analyzer can be an electrochemical analyzer which determines an electrical parameter of the water-sample. Alternatively or additionally, the analyzer can be a photometer with a light source and a light detector. 
         [0025]    As soon as the first analysis is finished, the water-sample is transported forward from the measuring section to the first reagent section. The water-sample is mixed in the first reagent section with the first reagent. The first reagent can be a key-reagent, for example, a reagent which colors the analyte of the water-sample. The first reagent can alternatively be an auxiliary-reagent or an auxiliary-agent of another nature, for example, if a key-reagent is positioned between the inlet opening and the measuring section. 
         [0026]    The first reagent can, for example, be a key-reagent which, for example, reacts with the analyte of the water-sample, whereby the reaction changes the optical or the electrochemical properties of the water-sample. Referring to the example of a photometrical analyzer, the absorption spectra of the water-sample changes particularly at defined spectra lines or defined spectra areas, respectively. The first reagent can also be an auxiliary-reagent which is not a detection-reagent or another auxiliary-agent. 
         [0027]    The prepared and homogenized water-sample, whereby the mixing of the water-sample is performed, for example, by repeatedly pumping back and forth, can be transported backward from the first reagent-section to the measuring section, whereby the water-sample can be analyzed a second time by the analyzer electrically and/or optically. The result of the second analysis can, for example, be a gross-value. For example, the background signal of the sample of the first analysis can be subtracted from the gross-value and lead to a net value of the analyte in the water-sample. The net-value can be displayed and/or saved. As soon as the net-value is displayed, the disposable test-element can be removed manually or automatically. 
         [0028]    All steps which are relevant for the quality of the measurement results of the water analysis, such as the analysis, for example, determining the background signal of the sample, the dosage of the first reagent, for example, the mixing of the first reagent with the water-sample, the waiting for the reaction time etc., can be performed semi-automatically or fully-automatically, respectively, and air-tight. Errors and hazards resulting from inaccurate handling can therefore be almost totally excluded. 
         [0029]    In an embodiment, the present invention provides a method for a disposable test-element, whereby the test-element is provided with a sample-line and a measuring section, whereby the water-sample is pumped forward after a first analysis of the water-sample in the measuring section to a first reagent-section in order to be mixed with the reagent and pumped backward to the measuring section to be analyzed again. The water-sample can be mixed with the key-reagent before the first analysis or can be transported free of reagent to the measuring section to perform a first analysis, for example, to determine a background signal of the sample. The positioning of a reagent in a first reagent section which is behind the measuring section allows numerous options to perform one or several analysis, respectively, and to improve further properties of the disposable test-element. 
         [0030]    According to an embodiment of the method of the present invention, the sample-line can, for example, be provided with a second reagent section with a second reagent which can, for example, be an auxiliary-agent, whereby the second reagent section can be positioned beyond the first reagent section. The following method steps are performed subsequently to the water-sample analysis: 
         [0031]    transporting the analyzed water-sample forward to the second reagent section. 
         [0032]    The water-sample reacts in the second reagent-section with the second reagent, for example, with an auxiliary-agent. A second reagent behind and separated from the first reagent or first reagent-section, respectively, as seen from the measuring section, allows the realization of numerous additional functions. For example, the second reagent can be an auxiliary-agent which is formed as a gelling agent and/or color agent so that the water-sample is gelatinized and/or colored after the analysis of the water-sample. Gelatinizing causes a fixation of the water-sample in the sample-line so that leakage is avoided. Changing the color of the water-sample shows the customer and/or the analyzer which detects the change of the optical properties that the disposable test-element is already used. 
         [0033]    The water-sample can, for example, be transported backwards from the second reagent section to measuring section, so that the water-sample can be analyzed a third time by the analyzer. A third reagent section can be positioned behind the second reagent section, for example, with a second auxiliary-agent, whereby the following method steps are performed subsequently to the third analyzing of the water-sample: 
         [0034]    transporting the water-sample forward to the third reagent section; 
         [0035]    transporting the water-sample backward from the third reagent section to the measuring section; and 
         [0036]    performing a fourth analyzing of the water-sample with the analyzer. 
         [0037]    Both auxiliary-agents in both auxiliary-agent sections, for example, the second and the third reagent section, can, for example, be analyte-standards with different respective quantities or concentrations. The described method allows the performance of a standard-addition so that a calibration can be performed at each measuring sequence. The actual and exact concentration of the analyte in the water-sample can be determined by calculating the regression of the result of the key-reagent analysis of the water-sample. 
         [0038]    It is also possible to provide more than two reagent sections with respective auxiliary-agents or analyte-standards, respectively, so as to increase the accuracy of the calibration, for example, with non-linear characteristics. 
         [0039]    The test-element can physically have a size of a flat match stick so that the sample-line can have a corresponding small cross-section, which is in a range between 0.01 mm 2  and some square millimeters. The photometrical section or the measuring track, respectively, should be as long as possible, for example, in the range of some millimeters to some centimeters. The volume of the water-sample in the photometrical section is therefore in the range of one to about one hundred cubic millimeters. According to the dimensions, the amount of the reagent is small so that the potential danger for health and the environment is low as well. The need for an appropriate disposure can therefore be avoided so that a considerable effort for the appropriate disposure or recycling to the distributor or producer, respectively, can be avoided. 
         [0040]    All steps which are relevant for the quality of the measurement results of the water analysis, such as the dosage of the analyte, the mixing of the analyte with the water-sample and the waiting for the reaction time etc., are performed semi-automatically or fully-automatically, respectively, and air-tight. Errors and hazards resulting from inaccurate handling can therefore be almost completely excluded. 
         [0041]    The analyzer can, for example, be a photometer with a light source for generating a measurement beam and a light detector for receiving the measurement beam, whereby the measuring section which is passed through by the beam can be formed by a photometer section. The photometer can, for example, be provided as a transmission-photometer. A transmission-photometer has, compared with a reflection-photometer, a better information signal. The transmission-photometer allows for a more precise quantitative determination of an analyte at a relative short measuring track. The measuring section can be provided with at least one photometrical-window for the inlet and the outlet of the measuring beam. With the photometrical method, different ions, for example, chlorine, phosphate and ammonium, can be measured. 
         [0042]    The analyzer can alternatively be an electrochemical analyzer which determines an electrical parameter in the measuring section. The measuring section can be provided with at least one electrode or sensor area, respectively, which is connected through electric lines with contacts or contact areas of the test-element, respectively. The contacts of the test-element can be connected with the basic unit through contacts or through contact areas, respectively, and therefore with the analyzer of the basic unit. The electrode or sensor area, respectively, has an area of some square micrometers to some millimeters. Different parameters, for example, conductivity, redox potential, pH-value and oxygen content oxygen, can be measured with the electrochemical method. 
         [0043]    The basic unit can alternatively be provided with a photometrical, an optical or an electrochemical analyzer, respectively, so that photometrical, optical and also electrochemical test-elements can alternatively be used at the basic unit. 
         [0044]    The test-element can be provided with a positioning element which can provide an exact positioning of the test-element in the basic unit. The measuring section of the test-element must be exactly in line with the photometer of the basic unit to provide a photometrical measurement without errors. The test-element can therefore be provided with at least one separate positioning element, which provides the positioning in addition to the lateral surface of the test-element in the basic unit. For example, the test-element can comprise a groove, a cavity or an opening in which a respective snap element of the basic unit snaps in so as to position and fix the test-element into the basic unit. 
         [0045]    The reagent or the reagents, respectively, can, for example, be provided in a dry state in the sample-line. It is also possible to arrange numerous different key-reagents or detection-reagents in series in the sample-line behind the measuring section. Once the water-sample arrives at the respective key-reagent, the reagent is dissolved in the water-sample and reacts with the analyte, for example, by changing the color. 
         [0046]    The test-element can, for example, be provided with a pump-membrane which can be controlled via a pump actuator of the basic unit, whereby the pump actuator can be provided, for example, with an electromotive rod. The flexible pump-membrane can be positioned at the opposite end as seen from the inlet opening on the sample-line and seals the pump opening of the sample-line air-tight so that air is pushed and the water-sample is transported in the sample-line by a deformation of the pump-membrane. Pushing or deformation, respectively, of the pump-membrane causes a transporting of the fluid backward, while releasing of the pump-membrane causes a transporting of the fluid forward in the sample-line. The releasing of the membrane can be performed by the resilience of the membrane or by pulling the membrane with the pump actuator. Both the pump-membrane and the pump actuator can form a membrane-pump, whereby the test-element can be provided with the pump-membrane. The electromotive pump actuator or the rod, respectively, can be positioned in the basic unit. 
         [0047]    The test-element can alternatively be provided with a pump-opening in the sample-line. The pump-opening can be positioned at the opposite end as seen from the inlet opening on the sample-line. The basic unit can be provided with sample pump which is connected with the inserted test-element as soon as the test-element is inserted in the basic unit. 
         [0048]    In an embodiment of the present invention, an absorption body can, for example, be arranged between the measuring section and the pump opening. The absorption body serves to absorb the water-sample after the water-sample has been finally analyzed and is transported forward to the absorption body. The water-sample can thereby be immobilized and a leakage of the water-sample avoided. For example, the absorption body can be a fleece body, a clay body like bentonite or can be a so called super-absorber. The body can additionally comprise a neutralizing reagent. 
         [0049]    In an embodiment of the present invention, the sample-line can, for example, be provided with a sample filter which filters the water-sample which is sucked through the inlet opening of the test-element. The filtration can be performed before the water-sample is transported to the measuring section to be analyzed, for example, analyzed photometrically. The sample filter can be made of mineral wool. 
         [0050]    According to an embodiment of the present invention, the sample-line can, for example, be provided with a dose capillary which can be arranged between the inlet opening and the measuring section. The dose capillary can, for example, be arranged adjacent to the inlet opening. By using a dose capillary, the water-sample can be sucked, basically by the capillary force, into the test element so that a defined water-sample volume can be segregated. The water-sample can thereafter be pumped repeatedly forth and back inside the sample-channel by using the pump actuator. 
         [0051]    The test-element can be provided with a drying agent to protect the key-reagent against humidity. For example, the drying agent can be separated in the sample-line by using a hydrophobic stopper capillary, whereby the humidity can flow through the stopper capillary to the drying agent. 
         [0052]    The inlet opening and/or the pump opening can be sealed with a humidity-tight transport-seal which can be opened manually or automatically by inserting the test-element into the basic unit, for example, by piercing. The single test-element can alternatively or additionally be sealed in a humidity-tight package. 
         [0053]      FIGS. 1 and 3  schematically show a mobile water-analyzing system  10 ,  10 ′ for a quantitative determination of an analyte in a water-sample. With the described embodiment of a photometrical analyzing system  10 ,  10 ′, chlorine, phosphate or ammonium can be determined. Alternatively or additionally, the analyzing system can be provided as an electrochemical analyzing system with an electrical analyzer. 
         [0054]    The analyzing system of  FIG. 1  comprises a basic unit  14  and a removable disposable test-element  16  which is presently inserted into the basic unit  14 . 
         [0055]    The test-element  16  is provided with a test-element body  18  made of plastic. The test-element body  18  is provided with a sample-line  20  which is formed as a groove. The side of the groove opening of the test-element body  18  is closed with a transparent plastic film or aluminum cover, respectively (not shown). 
         [0056]    The sample-line  20  is provided with an inlet opening  22  which is positioned at the distal end, referring to the basic unit  14 , and through which a water-sample is sucked from a water-reservoir  12 . Adjacent to and, in the flow direction, behind the inlet opening  22 . is a meander-like mix section  26  of the sample-line  20  in which a key-reagent, an auxiliary-reagent or an auxiliary-agent and the sucked water-sample are homogeneously mixed. 
         [0057]    A measuring section  28  is arranged adjacent to the mix section  26  in which a quantitative determination can be performed. The present measuring section  28  is a photometrical section, whereby the measuring section  28  forms a measuring track for the respective photometrical analyzer  30  of the basic unit  14 . Both sides of the photometer section  28  comprise a clear-transparent photometrical window  44 ,  46  as shown in  FIG. 2 . The test-element body  18  can be completely made out of a clear transparent plastic which allows the measuring beam  35  to pass through the measuring section  28 . 
         [0058]    Proximal to the measuring section  28 , i.e., behind the measuring section  28  as seen from the inlet opening  22 , is a reagent section  23  with a dry reagent  24  which is a key-reagent. At the sample-line end opposite to the inlet opening  22 , i.e., behind the first reagent section  23 , a pump opening  40  as a pump element is provided which is connected with a pump actuator of the sample pump of the basic unit when the test-element is inserted into the basic unit  14 . 
         [0059]    The basic unit  14  is provided with an analyzer  30  which is a transmission photometer with two light sources  32 ,  33  and a light detector  34 . The light-sources  32 ,  33  emit light of different wavelengths and the light detector detects both wavelengths. 
         [0060]    The test-element  16  is provided with a positioning element  48  which is realized as an opening. The positioning element  48  cooperates with a respective snap element of the basic unit  16  so that the test-element  16  is fixed reproducibly and exactly. This provides that the measuring beam  35  generated by the light source  32 ,  33  is exactly in line with the photometrical measuring section  28 . The test-element receptacle of the basic unit  14  is formed as a slot  15  in which the test-element  16  fits without any clearance. 
         [0061]      FIG. 3  shows a second embodiment of a mobile water-analyzing system  10 ′ including a removable cartridge  60  which is formed as a drum with 15 drum chambers  62 , whereby each drum chamber  62  carries a test-element  16 . The plastic drum body  64  is sealed axially with a circular sealing foil  66  so that the chambers  62  are sealed gas-tight and fluid-tight. 
         [0062]    As shown in  FIG. 3 , the removable cartridge  60  is inserted into a respective cartridge slot of the basic unit  14 ′. The basic unit  14 ′ is provided with a cartridge rotary actuator  67  and a test-element shifter  70 . The shifter  70  can shift a test-element  16  automatically from a chamber  62  into the measuring position as shown in  FIG. 3 . 
         [0063]    As soon as the measurement is finished, the shifter  70  moves the test-element  16  out of the measuring position and ejects it out of the basic unit  14 ′. In a next step, the shifter  70  is completely removed from the cartridge  60 ; subsequently the rotary actuator  67  turns the cartridge  60  by one chamber angle so that the next chamber  62  with the test-element  16  is in line with the shifter  70 . As soon as a measurement request is signalized by the customer, the shifter  70  moves the test-element  16  from the chamber  62  into the measuring position so that the measurement can start. 
         [0064]      FIG. 5  and  FIG. 6  show the front view and the rear view of embodiments of a test-element  80 . The test-element  80  is an electrochemical-optical test-element which is provided with an electrochemical-optical measuring section  82  in the sample-line  84 . The measuring section  82  is provided with two electrodes  86 ,  88  positioned opposite to each other, the electrodes being connected through electric lines  90 ,  92  with contacts  94 ,  96 . The contacts  94 ,  96  are arranged in line to respective contacts of the basic unit whereby the contacts are connected with the electrochemical analyzer of the basic unit. The measuring section  82  is furthermore provided with an optical measuring track for the photometer. 
         [0065]    Behind the measuring section  82 , as seen from the inlet opening  22 , is a first reagent section  23  with a first reagent  24  which is a key-reagent  24 . Behind the first reagent section  23  is a second reagent section  25  with a second reagent  27  which is a first auxiliary-agent  27 . Adjacent to the second reagent section  25  is a third reagent section  29  with a third reagent  31  which is a second auxiliary-agent  31 . Both auxiliary-agents  27 ,  31  are analyte-standards with different quantities or concentrations, respectively. 
         [0066]    The backside of the test-element of  FIG. 6  is provided with the pump opening  40  as a pump element of the test-element  80 . A circular seal element  41  is provided surrounding the pump opening  40  to provide a gas-tight connection between the sample-line  84  and a sample-pump realized as a pump actuator  42 . 
         [0067]      FIG. 7  schematically shows a side view of a part of an embodiment of a mobile water-analyzing system  10 ″. The water-analyzing system  10 ″ is provided with a disposable test-element  116  which is provided with a convex and vesicular pump membrane  118  above the pump opening  40 . The pump element, which is formed as a pump membrane,  118  has a pump volume which is higher than the total volume of the sample-line  84 . 
         [0068]    The basic unit  114  is provided with a pump actuator  120  with a motor  122  whereby the pump actuator  120  controls a rod  124 . The rod  124  pushes the pump membrane  118  of the inserted disposable test-element  116 . The water-sample can be moved over the complete length of the sample-line forward and backward by the rod  124  deforming the pump membrane  118 . 
         [0069]    To determine an analyte in a water-sample, a test-element  16  is first inserted into the test-element receptacle of the basic unit  14 . The basic unit  14  can, for example, be activated thereby. In a next step, the inlet opening  22  of the test-element is immersed manually into the analyzing water-reservoir  12  so that the sample-pump  42  sucks a water-sample into the measuring section  28  of the sample-line  20 . The analyzer  30  determines in a first analysis the background signal of the water-sample in the measuring section. 
         [0070]    As soon as the background signal determination is finished, the water-sample is pumped forward from the measuring section  28  into the first reagent section  23 . The water-sample meets the key-reagent as the first reagent  24  in the first reagent section  23  so that the key-reagent is mixed with the water-sample. The key-reagent reacts with the analyte in the water-sample so that the optical properties of the water-sample change. 
         [0071]    By pumping backwards, the water-sample flows back from the first reagent section  23  to the measuring section  28 . The water-sample is analyzed a second time photometrically by the analyzer  30 . The result of the second measurement is a gross-value. Subtracting the background from the gross-value leads to a net concentration of the analyte in the water-sample. 
         [0072]    With the test-element  80  of  FIG. 5  and  FIG. 6 , an even more precise determination of the analyte in a water-sample can be realized by using standard addition. After the determination of the analyte in the measuring section  82 , the water-sample is again pumped forward to the second reagent section  23  which is provided with a second reagent  27 . The second reagent  27  is a first auxiliary-agent which is an analyte-standard. The water-sample is mixed with the second reagent  27  in the second reagent section  25 . The water-sample is then pumped backwards from the second reagent section  25  into the measuring section  82 , whereby the water-sample is analyzed a third time photometrically. The water-sample is again transported forward to the third reagent section  29 , whereby the water-sample is mixed with the third reagent  31  which defines a second analyte standard and which is a second auxiliary-reagent. Finally, the water-sample is again transported backwards from the third reagent section  29  to the measuring section  82 , whereby the water-sample is measured a fourth time by the photometer. 
         [0073]    Both photometrical analyte standard measurements lead to a concentration-absorption characteristic line which allows an exact determination of the analyte concentration of the water-sample by using the net concentration value. 
         [0074]    Reagents  24 ,  27 ,  31  can alternatively be auxiliary-reagents of another kind. 
         [0075]    The auxiliary-reagent can also be a neutralization-reagent. After the water-sample has been analyzed in the measuring section  28 , the water-sample is pumped to the auxiliary-reagent which reacts with the key-reagent in such a manner that the key-reagent is neutralized so that the test-element can be disposed of, for example, in the household trash. 
         [0076]    The auxiliary-reagent can also gelatinize and/or color the water-sample after the water-sample has been analyzed. A change of the color of the water-sample shows the customer that the test-element has been used. The change of the color can also be detected by the analyzer. Gelatinizing causes a fixation of the water-sample in the sample-line so that leakage is avoided. 
         [0077]    The auxiliary-reagent can also be an activator which activates the key-reagent. The auxiliary-agent and the key-reagent must be arranged in line. The auxiliary-reagent activates the key-reagent as soon as key-reagent is being mixed with the auxiliary-reagent in the water-sample. The key-reagent as well as the auxiliary-reagent can be positioned between measuring section  28  and pump opening  40  or alternatively between inlet opening  22  and measuring section  28 . 
         [0078]    The present invention is not limited to embodiments described herein; reference should be had to the appended claims.