JP3712716B2 - Method for detecting cadmium in food and apparatus therefor - Google Patents

Description

Background of the Invention

The present invention relates to a method and apparatus for detecting cadmium contained in food, particularly rice, using a diamond electrode.

Background Art In recent years, cadmium in rice has been regarded as a problem. When cadmium is absorbed or accumulated in the human body, it causes various diseases such as kidney damage. For this reason, the amount of cadmium in the US is regulated by the government. For example, when the cadmium concentration in rice is 0.40 ppm or less, it is used as edible rice, and when it is 0.40 to 1.00 ppm, it is used as industrial quasi-cadmium-contaminated rice, and when it is 1.00 ppm or more, cadmium is used. It is incinerated as contaminated rice.

  Cadmium in rice is measured by decomposing test rice using nitric acid, sulfuric acid, and water to prepare a sample solution, and subjecting this sample solution to atomic absorption method or dithizone / chloroform method. Yes. However, in these methods, the apparatus is large and it is difficult to say that it is a simple measurement method. Accordingly, there is a need for a measuring method that can detect and quantify cadmium in rice easily and accurately.

On the other hand, diamond is originally an insulating material having a resistivity of about 10 ¹³ Ωcm, but acquires conductivity by doping with a small amount of impurities. Various uses are expected for this conductive diamond. One of them is the use as an electrode for electrochemical use. When the conductive diamond is viewed as an electrode for electrochemical use, it has excellent features of having a wide potential window and extremely low background current. Furthermore, it has the characteristics that it is physically and chemically stable and has excellent durability. Electrodes having conductive diamond (preferably a thin film thereof) have been commonly referred to as diamond electrodes.

  Pioneering research on diamond electrodes was performed by Iwaki et al. (See, for example, Iwaki et al., Nuclear Instruments and Methods, 209-210, 1129 (1983) (Non-patent Document 1)). They studied the properties of single-crystal diamond that was given surface conductivity by implanting argon or nitrogen ions as an electrically conductive material. At the same time, a cyclic voltammogram in the electrolyte solution is also shown. Thereafter, the characteristics of a polycrystalline diamond electrode synthesized by vapor phase using a hot filament have been reported (see Pleskov et al., J. Electroanal. Chem., 228, 19 (1993) (non-patent document 2)). .

  Some of the inventors have previously reported the reduction of nitrogen oxides using gas phase synthesized diamond electrodes (see, for example, Tenne et al., J. Electroanal. Chem., 347, 409 (1993)). (Refer nonpatent literature 3). In this study, p-type semiconductor diamond introduced with boron as a dopant was used as an electrode. After that, as diamond electrodes, the use of p-type semiconductors with boron as a dopant or metal-like conductive diamond having higher conductivity has become mainstream. In the 1990s, research on diamond electrodes was conducted by a plurality of groups. Since 1995, research on diamond electrodes obtained using a plasma CVD (PCVD) apparatus capable of obtaining a diamond thin film with a larger area has been conducted. It has also been found in the electrochemical field.

Some of the present inventors have reported that cadmium ions in an aqueous solution can be easily detected using this conductive diamond electrode (for example, JP-A-2001-91499 (Patent Document 1). )reference). In this method, a conductive diamond electrode is used as a working electrode, and cadmium in an aqueous solution is first electrodeposited on the electrode, and then the potential is swept to elute the cadmium from the electrode surface, and the current flowing at this time is detected. , Which detects cadmium ions. However, this method does not test cadmium contained in food, particularly rice.
JP 2001-91499 A Iwaki et al., Nuclear Instruments and Methods, 209-210, 1129 (1983) Pleskov et al., J. Electroanal. Chem., 228, 19 (1987) Tenne et al., J. Electroanal. Chem., 347, 409 (1993)

Summary of the Invention

The present inventors have recently found that cadmium ions in a sample solution obtained by decomposing food with a strong acid can be detected or quantified easily and accurately using a conductive diamond electrode. Then, by knowing the presence or concentration of cadmium ions in the sample solution, the presence or concentration of cadmium in the food can be easily known.
Accordingly, an object of the present invention is to provide a method and an apparatus therefor that can easily and accurately detect or quantify cadmium in foods, particularly cadmium ions in a sample solution obtained by decomposing foods with a strong acid.

And the detection method of cadmium in the foodstuff by this invention is the following.
Prepare a sample solution that decomposes food with strong acid,
The sample liquid is brought into contact with a conductive diamond electrode as a working electrode and a counter electrode together,
Between the working electrode and the counter electrode, a voltage at which cadmium deposition occurs on the working electrode is applied, cadmium is deposited on the working electrode, and then
A voltage that causes elution of cadmium deposited on the working electrode is applied between the working electrode and the counter electrode, and a current flowing between the two electrodes is detected.

Further, a detection apparatus for carrying out the above method according to the present invention comprises:
A container for storing a sample solution obtained by decomposing food with a strong acid;
Means for bringing a conductive diamond electrode as a working electrode into contact with the sample solution and a counter electrode;
Means for applying a voltage that causes cadmium deposition on the working electrode between the working electrode and the counter electrode;
Means for applying a voltage at which elution of cadmium deposited on the working electrode occurs between the working electrode and the counter electrode;
And at least means for detecting a current flowing between the working electrode and the counter electrode under the applied voltage.

Detailed description of the invention

Subject to be tested The subject to be tested in the method according to the present invention is a sample solution obtained by decomposing food (for example, rice) with a strong acid. When the food is treated with a strong acid, organic substances such as carbohydrates are decomposed, and when cadmium is present in the food, cadmium is present in the sample solution as cadmium ions (Cd ²⁺ ). In the present invention, cadmium ions in the sample solution are detected or quantified. Further, by knowing the presence or concentration of cadmium ions in the sample solution, the presence or concentration of cadmium in rice can be easily known.

  The food provided to the test subject in the present invention is not limited as long as it can dissolve cadmium ions by decomposing with a strong acid, and specific examples thereof include cereals, beans, fruits, vegetables, seeds, tea , And hops. Preferable examples of the food in the present invention include cereals such as rice, barley, wheat, rye, corn, corn, mushrooms, whey, corn, acne, and more preferably rice. Moreover, the form of rice may be either brown rice or polished rice.

  In the present invention, a sample solution obtained by decomposing food with a strong acid can be prepared according to a known method. For example, the decomposition treatment of foods using strong acids is a technique that has been conventionally performed when measuring the cadmium concentration in foods. For example, “Food, additives, etc. Standards (December 28, 1959, Ministry of Health and Welfare Notification No. 370) “First Food D Each Article ○ Cereals, Beans, Fruits, Vegetables, Seeds, Tea and Hops” “2 Cereals, Beans, Fruits, This is explained in the section “Testing methods for component standards of vegetables, seeds, tea and hops (36) Cadmium testing method”.

  The strong acid used in the preparation of the sample solution in the present invention is not limited as long as it can elute cadmium in food into the sample solution, and preferred examples include nitric acid and / or sulfuric acid, more preferably Both. The nitric acid may be concentrated nitric acid, dilute nitric acid, or fuming nitric acid, and the sulfuric acid may be concentrated sulfuric acid, dilute sulfuric acid, or fuming sulfuric acid. In addition, when a food is decomposed using a strong acid, heating may be appropriately performed. Thus, the sample solution used in the method according to the present invention has strong acidity because it contains a strong acid, and its pH is less than 1.00, 0.90 or less, and further 0.10 or less. It may be. Therefore, in the method according to the present invention, the sample solution having strong acidity can be subjected to measurement without performing a special treatment for pH adjustment.

  According to a preferred embodiment of the present invention, when both concentrated nitric acid and concentrated sulfuric acid are used as strong acids, the amount of concentrated nitric acid added per 10 g of cereal is preferably about 20 to 40 ml, and concentrated nitric acid added per 10 g of cereal. The amount is preferably about 30 to 40 ml. Further, water may be added simultaneously with the addition of the strong acid, and the addition amount is preferably such that the total amount of the sample solution after the addition is 40 to 200 ml, preferably 100 ml.

According to a preferred embodiment of the present invention, it is preferable to add a complexing agent that easily forms a complex with Fe ³⁺ and / or Cu ²⁺ to the sample solution prior to measurement. This is because foods such as rice may contain iron or copper, and if this content is high, electrochemical detection of cadmium ions may be inhibited. That is, when cadmium is deposited on the diamond electrode and then cadmium on the diamond electrode is eluted by potential sweep, iron and / or copper tends to generate a current peak at the same potential as cadmium. Therefore, when a relatively large amount of iron and / or copper is present in the test substance, a current higher than that due to cadmium alone is detected, and accurate detection of cadmium can be hindered. Such an increase in current value can be reduced or eliminated by adding a complexing agent that easily forms a complex with Fe ³⁺ and / or Cu ²⁺ to the sample solution. That is, by adding the complexing agent to the sample solution, it is possible to reduce or eliminate the influence of Fe ³⁺ and / or Cu ²⁺ that may interfere with the measurement.

The complexing agent that can be used in the present invention is not limited as long as it is easier to complex with Fe ³⁺ and / or Cu ²⁺ than cadmium ions, but preferred examples include potassium thiocyanate (KSCN). It is done. It should be noted that the evaluation as to whether or not complex formation with Fe ³⁺ and / or Cu ²⁺ is easier than cadmium ions is based on, for example, the magnitude of the complex formation constant between cadmium ions and Fe ³⁺ and / or Cu ²⁺ and the complexing agent. Can do.

In the present invention, the complexing agent may be added in an amount sufficient to form a complex with Fe ³⁺ and / or Cu ²⁺ that is expected to be contained in the sample solution. According to a preferred embodiment of the present invention, the concentration of the complexing agent in the sample solution is preferably equal to or more than the molar concentration of Fe ³⁺ expected to be generally contained in the sample solution, preferably 10 Double or more, more preferably 100 times or more, and further preferably 200 times or more. In general, the iron content in 100 g of brown rice is known to be about 2.1 mg, and the amount of Fe ³⁺ expected to be contained in the sample solution based on such known data. Can be predicted to some extent.

  According to a preferred aspect of the present invention, the cadmium ion concentration in the sample solution can be measured on the order of nM or ppb.

  According to a preferred embodiment of the present invention, an electrolyte may be further added to the sample solution. Examples of preferable electrolytes include potassium nitrate, potassium chloride, sodium chloride, sodium sulfate and the like. The electrolyte may be added to the sample solution in the form of an aqueous solution, whereby the sample solution may be diluted to a concentration suitable for measurement. Further, according to a preferred embodiment of the present invention, a buffer solution may be further added to keep the pH of the sample solution constant, and a preferable buffer solution includes an acetate buffer solution and the like.

Measurement System and Measurement Method In the measurement method according to the present invention, the electrochemical system for cadmium ion measurement can be a general electrochemical system except that a diamond electrode is used as a working electrode. That is, in the method according to the present invention, the diamond electrode is used as a working electrode, the sample electrode is brought into contact with the counter electrode, and a voltage at which cadmium is deposited on the working electrode is applied between the two electrodes. Cadmium is deposited on the electrode. A schematic representation of this system is shown in FIG. That is, the conductive diamond electrode 3 and the counter electrode 4 are provided in a container 2 containing the sample solution 1 so as to be in contact with the sample solution 1. According to a preferred aspect of the present invention, it is preferable that the reference electrode 5 is provided in the system so that the potential of the working electrode 3 can be accurately controlled with reference to the reference electrode 5.

  In the present invention, the voltage applied between the conductive diamond electrode, which is a working electrode, and the counter electrode is appropriately determined in consideration of the type of the electrode as long as the cadmium is deposited on the diamond electrode. Although good, it will generally be such that the potential of the working electrode is negative with respect to the counter electrode. According to a preferred aspect of the present invention, when an Ag / AgCl electrode is provided in the system as a reference electrode, it is preferable to apply a voltage of −1.00 to −0.40 V (vs. Ag / AgCl) to the working electrode, More preferably, it is −0.70 to −0.45 V (vs. Ag / AgCl), and further preferably −0.55 to −0.45 V (vs. Ag / AgCl). Moreover, according to the preferable aspect of this invention, it is preferable to set the application time of the voltage which precipitates cadmium to 5 minutes or more, More preferably, it is 5 to 30 minutes, More preferably, it is 10 to 20 minutes.

  After cadmium is deposited on the working electrode, a voltage at which elution of cadmium deposited on the working electrode is applied between the two electrodes. This voltage application is preferably performed, for example, while sweeping the potential of the working electrode from the fixed potential of cadmium in the positive potential direction. According to a preferred embodiment of the present invention, the potential sweep rate is preferably 300 to 1200 mV / s, more preferably 500 to 1000 mV / s, and still more preferably 600 to 900 mV / s. In this case, it is preferable to obtain a current value at a voltage that gives a peak current and use it as an index for measurement of the cadmium ion concentration. For example, the current value at a voltage other than the voltage that gives this peak current value can also be used as an index. It is not excluded from the invention. It is also possible to use the accumulated amount of electricity in the sweep range as an index. Further, according to the knowledge obtained by the present inventors, the potential of the working electrode that gives this peak current also depends on the pH of the sample solution, but in the case of pH 0.01, −0.1 V (vs. Ag / AgCl). ) It tends to exist in the vicinity. Therefore, it is of course possible to immediately apply this voltage to the conductive diamond electrode as the working electrode after cadmium deposition, and use the current value or the accumulated electric quantity as an index for measuring the cadmium ion concentration.

  The current value and the quantity of electricity measured as described above are directly proportional to the cadmium ion concentration in the sample solution. Therefore, if the relationship between the current value or quantity of electricity and the cadmium ion concentration at the same voltage value as when the current value or quantity of electricity was obtained, the current value or quantity of electricity obtained can be determined from that relationship. The cadmium ion concentration in the sample solution to be obtained can be easily known. That is, in a preferred embodiment of the present invention, a calibration curve between a cadmium ion concentration and a current value or an electric quantity prepared in advance is compared with the obtained current value or electric quantity to thereby obtain a sample solution. You can know the cadmium ion concentration.

  Furthermore, according to the preferable aspect of this invention, it is preferable to calculate the cadmium density | concentration in a foodstuff from the cadmium ion density | concentration in the obtained sample liquid. If the cadmium ion concentration in the sample solution is known, the mass of cadmium ions contained in the entire sample solution can be calculated. And the cadmium density | concentration in rice can be easily calculated based on the mass of the cadmium ion contained in the whole sample liquid, and the mass of the food provided to the sample liquid.

  In the present invention, the shape or the like of the working electrode is not particularly limited, but may be a plate-like one as shown in FIG. 2 to be described later, or a so-called rotating electrode used while rotating the electrode on the disk. . In particular, the use of a rotating electrode is advantageous because it improves measurement sensitivity and eliminates measurement errors.

In the present invention, as the counter electrode, it is preferable to use a diamond electrode similar to the working electrode, platinum, carbon, stainless steel, gold, diamond, SnO _2, etc., but it has excellent resistance to the strong acidity of the sample solution. Thus, it is particularly preferable to use a diamond electrode similar to the working electrode.

  In the present invention, a known electrode can be used as the reference electrode, and a saturated calomel electrode (SCE), a standard hydrogen electrode, a silver-silver chloride electrode, a mercury mercury chloride electrode, a hydrogen palladium electrode, and the like can be used.

  According to a preferred aspect of the present invention, it is preferable that the reference electrode is electrically connected to the sample solution without directly contacting the sample solution. This prevents the function of the reference electrode and the salt bridge that can be connected to it from being damaged by the strong acidity of the sample solution, thereby enabling more accurate and stable measurement. An example of a reference electrode system for carrying out this embodiment is as shown in FIG. In the system shown in FIG. 1, the communication pipe 6 through which the sample solution 1 communicates, the electrolyte solution 7 in which the end of the communication pipe 6 is immersed, and the salt bridge 9 in which the electrolyte solution 7 is immersed, The sample solution 1 and the reference electrode 5 are electrically connected. The electrolyte solution 7 is accommodated in the electrolyte solution tank 8. According to this reference electrode system, since the strongly acidic sample solution 1 and the salt bridge 9 are prevented from coming into direct contact, it is effective that the salt bridge 9 is damaged by the strong acid and the function of the reference electrode 5 is lowered. Can be prevented. Therefore, more accurate and stable measurement is possible.

  The communication pipe that can be used in the present invention is not limited as long as it can contain a sample solution therein, and may be a glass pipe. The salt bridge used in the present invention may be a Lugin tube filled with agar containing an electrolyte such as KCl, as long as the solution in the tube does not mix with the solution immersed at both ends.

Diamond electrode and production thereof In the present invention, the diamond electrode described in Japanese Patent Application Laid-Open No. 2001-91499 can be used. The contents are as follows.

  Diamond is inherently an excellent insulator. However, by adding Group 3 or Group 5 impurities, semiconductor-to-metal conductivity is exhibited. In the present invention, diamond having a semiconductor to metal-like conductivity is used as an electrode.

Examples of the substance added to impart such conductivity include the elements of Group 3 and Group 5 as described above, more preferably boron, nitrogen, phosphorus, and sulfur, and most preferably boron. is there. The amount of the substance added for imparting conductivity may be appropriately determined within a range in which conductivity can be imparted to diamond. For example, conductivity of about 1 × 10 ⁻² to 10 ⁻⁶ Ωcm is imparted. An amount is preferably added. In general, the amount of a substance added for imparting conductivity is controlled by the amount added in the manufacturing process of conductive diamond.

According to a preferred embodiment of the present invention, it is preferable that a conductive diamond thin film is formed on a substrate, and a conductive wire is connected to the conductive diamond thin film to form an electrode. As a base material, Si (for example, single crystal silicon), Mo, W, Nb, Ti, Fe, Au, Ni, Co, Al ₂ O ₃ , SiC, Si ₃ N ₄ , ZrO ₂ , MgO, graphite, single Crystal diamond, cBN, quartz glass and the like can be mentioned, and it is particularly preferable to use single crystal silicon, Mo, W, Nb, Ti, SiC, or single crystal diamond. Moreover, according to the preferable aspect of this invention, you may use the diamond thin film 0.5 mm or more obtained by peeling a diamond thin film from the base material in which the electroconductive diamond thin film was formed as an electrode.

  The electrode of this aspect will be further described with reference to FIG. FIG. 2A is a cross-sectional view of the diamond electrode 21, which is composed of a conductive diamond thin film 23 formed on a base material 22, and further, a conductive wire 24 is provided on the conductive diamond thin film 23. For example, it is connected via a gold coating 25. FIG. 2B is a perspective view of the diamond electrode 21, which is composed of a conductive diamond thin film 23 formed on the base material 22, and a gold coating 25 for using the conductive diamond thin film 23 as an electrode. The conducting wire 24 is connected via the via. Further, as shown by a dotted line in FIG. 2 (b), the outermost edge and end side surface of the diamond electrode 21 may be covered with a protective film 26. The material of the protective film 26 is not limited as long as it is an insulating resin having resistance to a strongly acidic sample solution, but a preferable example is polytetrafluoroethylene. As a result, the end side surface of the diamond electrode 21, the gold coating 25, and the conductive wire 24 are protected so as to ensure an electrochemically stable state regardless of strong acidity, thereby enabling more stable and accurate measurement. To do.

  When used as an electrode with a substrate, the thickness of the conductive diamond thin film is not particularly limited, but is preferably about 1 to 100 μm, more preferably 5 to 50 μm, which is a thickness where there is no pinhole and the doping is stable. Degree.

  Further in accordance with a preferred embodiment of the present invention, the diamond electrode according to the present invention can take the form of a microelectrode. The concept of a microelectrode is already known, and in the present invention, a diamond electrode in the form of a microelectrode is obtained by sharply cutting the end of a fine wire such as Pt, W, Mo, etc. It means a structure in which a conductive diamond thin film is formed on the end surface.

  According to a preferred embodiment of the present invention, the conductive diamond thin film is preferably produced by a chemical vapor deposition method. The chemical vapor deposition method is a method of synthesizing substances by chemically reacting gas raw materials in a gas phase, and is generally called a CVD (Chemical Vapor Deposition) method. This method is widely used in the semiconductor manufacturing process, and can be used for manufacturing the conductive diamond thin film in the present invention under a purposeful modification.

  The chemical vapor synthesis of diamond is performed by using a mixture of a carbon-containing gas such as methane and hydrogen as a source gas, exciting it with an excitation source, supplying it on a substrate, and depositing it.

  Examples of the excitation source include a hot filament, microwave, high frequency, direct current glow discharge, direct current arc discharge, and combustion flame. It is also possible to adjust the nucleation density by combining a plurality of these, and to increase the area and make it uniform.

As raw materials, many kinds of carbon containing carbon are decomposed and excited by an excitation source, and active carbon such as C and C ₂ and carbonization such as CH, CH ₂ , CH ₃ and C ₂ H ₂ are used. Compounds that generate hydrogen radicals can be used. Preferred examples include CH ₄ , C ₂ H ₂ , C ₂ H ₄ , C ₁₀ H ₁₆ , CO, CF ₄ as gas, CH ₃ OH, C ₂ H ₅ OH, (CH ₃ ) ₂ CO as liquid, Examples of the solid include graphite and fullerene.

In the gas phase synthesis method, for example, a method for adding a substance that imparts conductivity to diamond is a method of introducing an additive substance into a carbon gas phase by, for example, placing a disk of the additive substance in the system and exciting the same as the carbon source material. Alternatively, an additive substance may be added in advance to the carbon source, introduced into the system together with the carbon source, excited by the excitation source, and introduced into the carbon gas phase. According to a preferred embodiment of the present invention, the latter method is preferred. In particular, when a liquid such as acetone or methanol is used as the carbon source, the method of dissolving boron oxide (B ₂ O ₃ ) in the liquid to form a boron source is easy to control the boron concentration and simple. To preferred. For example, in the gas phase synthesis method, when boron is added to the carbon source, it is generally about 10 to 12,000 ppm, and preferably about 1,000 to 10,000 ppm.

  According to a preferred embodiment of the present invention, the conductive diamond thin film is preferably produced by a plasma chemical vapor synthesis method. This plasma chemical vapor synthesis method has the advantage that the activation energy causing a chemical reaction is large and the reaction is fast. Furthermore, according to this method, a chemical species that does not exist unless it is thermodynamically high is generated, and a reaction at a low temperature is possible. There have already been several reports on the production of conductive diamond thin films by the plasma chemical vapor synthesis method, including some of the present inventors (eg, Yano et al., J. Electrochem. Soc., 145 (1998)). 1870), preferably according to the methods described in these reports.

Measuring device According to yet another aspect of the present invention, an apparatus for measuring the cadmium ions concentration in the sample liquid is provided. The basic configuration of this apparatus is as shown in FIG. In the apparatus of FIG. 3, a power source / ammeter (potentiostat) 31 is connected to a conductive wire A connected to the conductive diamond electrode 3, a conductive wire B connected to the counter electrode 4 from the system shown in FIG. 1, and A conductive wire C connected to the reference electrode 5 is connected. Accordingly, the apparatus according to the present invention comprises a combination of the system of FIG. 1 and the basic configuration of FIG. The power source / ammeter 31 includes a means for applying a voltage for precipitating cadmium on the diamond electrode between the diamond electrode and a counter electrode, a means for applying a voltage for eluting the precipitated cadmium, It also serves as a means for measuring a current value or an electric quantity under an applied voltage. That is, the power source / ammeter 31 applies a voltage at which cadmium is deposited on the working electrode between the working electrode and the counter electrode to deposit cadmium on the working electrode, and then the working electrode and the counter electrode. In between, a voltage at which elution of cadmium deposited on the working electrode is applied, and the current value or the amount of electricity flowing between the two electrodes is measured. This current value or quantity of electricity is sent to the concentration calculation device 32. The calibration curve data 33 is sent to the device 32. In this device, the calibration curve data is compared with the current value or the electric quantity sent from the power source / ammeter 31, and the cadmium ion concentration in the sample solution is determined. calculate. That is, the device 32 is a means for calculating the cadmium ion concentration in the sample solution from the obtained current value or electric quantity. The obtained cadmium ion concentration is displayed by the display device 34.

  According to a preferred embodiment of the present invention, the preparation condition of the sample solution is input to the concentration calculation device 32, and the cadmium concentration in the food is calculated from the preparation condition and the cadmium ion concentration in the obtained sample solution. calculate. The cadmium concentration in the obtained food is displayed by the display device 34.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1: Preparation of diamond electrode According to a known method, a diamond thin film in which boron is introduced at a high concentration (10,000 ppm, B / C ratio) on an n-type silicon (111) surface is used using a microwave plasma CVD apparatus. Formed.
A diamond electrode having an effective area of 0.13 cm ² was obtained from the obtained silicon substrate with a diamond thin film.

Example 2: Preparation of sample solution A sample solution was prepared as follows using commercially available brown rice. The method shown below is based on “2 Cereals, Beans, Fruits, Standards for Foods, Additives, etc. (December 28, 1959, Ministry of Health and Welfare Notification No. 370) established based on the Food Sanitation Law. This is a method based on the description of “Testing methods for component standards of vegetables, seeds, tea and hops (36) Cadmium testing method”. First, about 10-30 g of brown rice as a sample was accurately weighed and placed in a 300 ml Kjeldahl flask. 10-40 ml of water and 40 ml of concentrated nitric acid were further added to the Kjeldahl flask, mixed well, and then gently heated. The contents were heated for a while and then allowed to cool, 20 ml of concentrated sulfuric acid was added and heated again. During that time, concentrated nitric acid was added in small portions as needed. It was judged that the decomposition was complete when the content turned from pale yellow to a colorless transparent liquid. After the contents were allowed to cool, water was added to bring the total volume to 100 ml to obtain a sample solution. The pH of the obtained sample solution was 0.01.

The obtained sample solution was subjected to ICP-AES analysis. As a result, the concentrations of cadmium, iron, and copper in the sample solution were as follows:
Cd: less than 0.002 ppm (less than detection limit),
Fe: 2.344 ppm,
Cu: 0.212 ppm.
Thus, although the obtained brown rice did not contain cadmium substantially, it confirmed that it contained iron comparatively much.

Example 3: Detection of cadmium A cadmium standard solution was added to the brown rice-containing sample solution prepared in Example 2 to prepare a brown rice sample solution containing cadmium in a pseudo manner with cadmium ion concentrations of 40 ppb and 50 ppb. The pH of this sample solution was 0.02. To this sample solution, 2 ml of a 0.1 M (1/10 N) KSCN aqueous solution was added so that the molar concentration of KSCN in the sample solution was about 100 times the molar concentration of Fe ³⁺ . The cadmium standard solution is a commercially available solution containing 100 ppm of Cd (NO ₃ ) ₂ in a 0.1 M HNO ₃ aqueous solution.

  In this sample solution, the diamond electrode prepared in Example 1 was immersed as a working electrode, and a similar diamond electrode was immersed as a counter electrode. In addition, a tank containing an electrolyte solution and an Ag / AgCl electrode as a reference electrode were disposed apart from the sample solution. Both ends of the communication tube were immersed in the vicinity of the working electrode and the electrolyte solution in the sample solution, respectively, and the sample solution was connected to the communication tube. Moreover, both ends of the salt bridge were immersed in the electrolyte solution and the reference electrode, respectively. Thus, an electrochemical measurement cell as shown in FIG. 1 was constructed. In addition, a Lugin tube containing KCl was used as the salt bridge, and a 0.2M KCl solution was used as the electrolyte solution.

  A voltage of -1.00 V (vs. Ag / AgCl) was applied to the working electrode for 15 minutes to deposit cadmium on the working electrode. Thereafter, the potential was swept in the positive direction at 300 mV / s to elute cadmium, and the current change at that time was measured. The result was as shown in FIG. As shown in FIG. 4, a peak considered to be caused by cadmium ions was detected around −0.20 V (vs. Ag / AgCl).

Example 4: Preparation of calibration curve A cadmium standard solution, an iron standard solution, and a copper standard solution were added to a 0.2 M KCl solution, and cadmium ion concentrations of 40, 80, 100, and 150 ppb, an iron ion concentration of 2.0 ppm, copper A cadmium-containing sample solution having an ion concentration of 0.3 ppm was prepared. Concentrated nitric acid was added to the sample solution to adjust the pH of the sample solution to 0.01. To this sample solution, 6 ml of a 0.1M (1 / 10N) KSCN solution was added so that the molar concentration of KSCN in the sample solution was about 300 times the molar concentration of Fe ³⁺ . Incidentally, ^{Fe 3+} concentration and ^{Cu 2+} concentration in the sample solution as prepared is a ^{Fe 3+} concentration and ^{Cu 2+} concentration and the concentration of the same order contained in the sample liquid obtained from brown rice in the example 2.

The iron standard solution is a commercially available solution containing 2.0 ppm of Fe (NO ₃ ) ₂ in an aqueous nitric acid solution. The copper standard solution is a commercially available solution containing 0.3 ppm of Cu (NO ₃ ) ₂ in an aqueous nitric acid solution.

  An electrochemical measurement cell similar to that in Example 3 was constructed using this sample solution. A voltage of −0.50 V (vs. Ag / AgCl electrode) was applied to the working electrode for 15 minutes to deposit cadmium on the working electrode. Thereafter, the potential was swept in the positive direction at 900 mV / s to elute cadmium, and the current change at that time was measured. Some of the results were as shown in FIG.

  A calibration curve shown in FIG. 6 was obtained from the integrated electric quantity calculated from the area of the obtained current peak and the cadmium ion concentration. Therefore, by using this calibration curve, the cadmium ion concentration in the sample solution obtained by degrading brown rice with a strong acid can be measured.

Example 5: Effect of addition of complexing agent A cadmium standard solution, an iron standard solution, and a copper standard solution were added to a 0.2 M KCl solution to obtain a cadmium ion concentration of 0.08 ppm, an iron ion concentration of 2.0 ppm, and a copper ion concentration. A sample solution containing 0.3 ppm of cadmium was prepared. Concentrated nitric acid was added to the sample solution to adjust the pH of the sample solution to 0.01. To this sample solution, 200 μl of a 0.1 M (1/10 N) KSCN solution was added so that the molar concentration of KSCN in the sample solution was about 10 times the molar concentration of Fe ³⁺ .

  A voltage of −0.60 V (vs. Ag / AgCl) was applied to the working electrode for 15 minutes to deposit cadmium on the working electrode. Thereafter, the potential was swept in the positive direction at 300 mV / s to elute cadmium, and the current change at that time was measured. The result was as shown in FIG.

  For comparison, measurement was performed in the same manner as above except that KSCN was not added as a complexing agent. The result was as shown in FIG.

The following can be understood from the results of FIG. As shown in FIG. 7, when KSCN is not added, overlapping high peaks due to Cd ²⁺ , Fe ³⁺ , and Cu ²⁺ are observed around −0.05 V (vs. Ag / AgCl). On the other hand, when KSCN is added, the peak around −0.05 V (vs. Ag / AgCl) is lowered, and Fe ³⁺ and Cu ²⁺ complexed around +0.10 V (vs. Ag / AgCl). A new peak that was attributed to was observed. From this result, it can be seen that by adding the complexing agent, the peak caused by Fe ³⁺ and Cu ²⁺ can be moved, and the peak current caused by cadmium ions can be detected more accurately.

Example 6 Influence of Complexing Agent Addition Amount The 0.1 M (1 / 10N) KSCN solution addition amount was 2, 4, and 6 ml, and the KSCN molar concentration in the sample solution was about 100 times the molar concentration of Fe ^3+. Measured in the same manner as in Example 5 except that the molar concentration was about 200 times, about 300 times, and about 300 times, and the cadmium fixed potential was -0.5 V (vs. Ag / AgCl electrode). Went. The result was as shown in FIG. From the results of FIG. 8, it can be seen that when the amount of KSCN added is about 200 times or more that of iron ions, the influence of Fe ³⁺ and Cu ²⁺ can be greatly reduced.

Example 7: Effect of cadmium fixed potential The amount of 0.1M (1 / 10N) KSCN solution added was 2 ml, and the molar concentration of KSCN in the sample solution was about 100 times the molar concentration of Fe ^3+. The cadmium fixed potential was −0.40, −0.50, −0.60, and −0.70 V (vs. Ag / AgCl), and the measurement was performed in the same manner as in Example 5. The result was as shown in FIG. As shown in FIG. 9, when the cadmium fixed potential is −0.50 V (vs. Ag / AgCl electrode), the influence of Fe ³⁺ ions and Cu ²⁺ ions is greatly reduced, and it is considered to be caused by cadmium ions. A high peak was observed.

Example 8: Influence of application time of cadmium fixed potential A cadmium standard solution was added to a 0.2 M KCl solution to prepare a cadmium-containing sample solution having a cadmium ion concentration of 10 ppb. Concentrated nitric acid was added to the sample solution to adjust the pH of the sample solution to 0.01. Using this sample solution, an electrochemical measurement cell was constructed in the same manner as in Example 3.

  A voltage of -1.00 V (vs. Ag / AgCl) was applied to the working electrode for 0, 5, 10, 15, 20, and 30 minutes to deposit cadmium on the working electrode. Thereafter, the potential was swept in the positive direction at 400 mV / s to elute cadmium, and the change in the current peak area at that time was measured. The result was as shown in FIG. As shown in FIG. 10, when the application time of the cadmium precipitation potential reaches 10 to 15 minutes, it can be seen that the peak electric quantity is almost saturated.

Example 9: Effect of potential sweep rate The amount of 0.1 M (1 / 10N) KSCN solution added was 6 ml, and the KSCN molar concentration in the sample solution was about 300 times the molar concentration of Fe ³⁺ , cadmium Measurement was performed in the same manner as in Example 5 except that the fixed potential was set to -0.50 V (vs. Ag / AgCl) and the potential sweep rate at the time of elution of cadmium was set to 300, 600, and 900 mV / s. It was. The result was as shown in FIG. As shown in FIG. 11, it can be seen that when the potential sweep rate is increased, the current peak increases and the detection accuracy increases.

2 is an electrochemical system for cadmium ion measurement according to the present invention. A conductive diamond electrode is added so that a complexing agent that is easier to complex with copper ions than cadmium ions is added, or a container 2 containing a sample solution 1 having a strongly acidic pH is brought into contact with the sample solution 1. 3 and the counter electrode 4. The reference electrode 5 may be provided in the system via the communication pipe 6, the tank 8 containing the electrolyte solution 7, and the salt bridge 9. It is a figure which shows the structure of a diamond electrode, (a) is sectional drawing of the diamond electrode 21, and this electrode consists of the electroconductive diamond thin film 23 formed on the base material 22. FIG. (B) is a perspective view of the diamond electrode 21. It is a figure which shows the basic composition of the cadmium ion concentration measuring apparatus by this invention. FIG. 6 is a diagram showing the cadmium ion concentration and peak current obtained in Example 3. FIG. 6 is a diagram showing the cadmium ion concentration and peak current obtained in Example 4. It is a calibration curve of the cadmium ion concentration and the peak electric quantity created in Example 4. It is a figure which shows the peak electric current in the case where KSCN is added to the sample solution obtained in Example 5 and when it is not added. It is a figure which shows the peak electric current when the addition amount of the KSCN solution obtained in Example 6 is 2, 4 and 6 ml, respectively. It is a figure which shows the peak current when the cadmium fixed potential obtained in Example 7 is −0.40, −0.50, −0.60, and −0.70 V (vs. Ag / AgCl), respectively. It is a figure which shows the relationship between the application time of the cadmium fixed potential obtained in Example 8, and a peak electric quantity. It is a figure which shows the peak current when the potential sweep rate obtained in Example 9 is 300, 600, and 900 mV / s, respectively.

Claims (21)

A method for detecting cadmium in food,
Prepare a sample solution having a pH of less than 1.00, which is obtained by decomposing food with a strong acid,
The sample liquid is brought into contact with a conductive diamond electrode as a working electrode and a counter electrode together,
Between the working electrode and the counter electrode, a voltage at which cadmium deposition occurs on the working electrode is applied, cadmium is deposited on the working electrode, and then
Applying a voltage between the working electrode and the counter electrode to cause elution of cadmium deposited on the working electrode, and detecting a current flowing between the two electrodes.   The step of detecting the current further comprises measuring a current value or an electric quantity, and calculating a cadmium ion concentration in the sample solution from the obtained current value or electric quantity. Method.   The step of calculating the cadmium ion concentration in the sample solution from the obtained current value or electric quantity includes a calibration curve of the cadmium ion concentration and the current value or electric quantity prepared in advance, and the obtained current value or electric quantity. The method according to claim 2, wherein the method is carried out by contrasting.   The method according to claim 2 or 3, further comprising calculating the cadmium concentration in the food from the cadmium ion concentration in the obtained sample solution.   The method according to any one of claims 1 to 4, wherein the food is rice.   The method according to any one of claims 1 to 5, wherein the strong acid is nitric acid and / or sulfuric acid. Prior to detection, the sample solution, than cadmium ions, further comprising adding a Fe ³⁺ and / or Cu ²⁺ and complexing easily complexing agent, in any one of claims 1 to 6 The method described. The method of claim 7 , wherein the complexing agent is potassium thiocyanate. The reference electrode is further provided, by controlling the potential of the working electrode with respect to the reference electrode, and controls the voltage which the voltage deposition of the cadmium occurs, and elution of the cadmium occurs, any claim 1-8 The method according to claim 1. The application of voltage to deposition of the cadmium occurs, carried out by applying a potential of -1.00~-0.40V (vs. reference electrode) to the working electrode, according to any one of claims 1-9 the method of. The method according to any one of claims 1 to 10 , wherein an application time of a voltage at which the cadmium deposition occurs is 5 minutes or more. The application of voltage to the elution of the cadmium occurs, carried out by applying a potential to provide a peak current value when the potential sweep potential to a positive potential direction of the working electrode, in any one of claims 1 to 11 The method described. The method according to any one of claims 1 to 12 , wherein the voltage at which elution of cadmium deposited on the working electrode is applied by sweeping the potential of the working electrode in the positive potential direction. The method of claim 13 , wherein the potential sweep rate is 300 to 1200 mV / s. It said counter electrode is a conductive diamond electrode, The method of claim 1-14. The method according to claim 15 , wherein the reference electrode is electrically connected to the sample solution without being in direct contact with the sample solution. The reference electrode is electrically connected to the sample solution via a communication tube that communicates with the sample solution, an electrolyte solution that immerses the end of the communication tube, and a salt bridge that is immersed in the electrolyte solution. The method of claim 16 , wherein: An apparatus for carrying out the method for detecting cadmium in food according to any one of claims 1 to 17 ,
A container containing a sample solution having a pH of less than 1.00, which is obtained by decomposing food with a strong acid;
Means for bringing a conductive diamond electrode as a working electrode into contact with the sample solution and a counter electrode;
Means for applying a voltage that causes cadmium deposition on the working electrode between the working electrode and the counter electrode;
Means for applying a voltage at which elution of cadmium deposited on the working electrode occurs between the working electrode and the counter electrode;
An apparatus comprising at least means for detecting a current flowing between the working electrode and the counter electrode under the applied voltage. 19. The apparatus according to claim 18 , wherein the means for detecting current further comprises means for measuring a current value or an electric quantity and calculating a cadmium ion concentration in the sample solution from the obtained current value or electric quantity. . 20. An apparatus according to claim 18 or 19 , further comprising a reference electrode, whereby the potential of the working electrode relative to the reference electrode is controlled. An electrolyte solution tank for storing the electrolyte solution;
Connecting the container and the electrolyte solution tank, and a communication pipe through which the sample solution communicates;
21. The apparatus according to claim 20 , further comprising a salt bridge that connects the electrolyte solution tank and the reference electrode and electrically connects the electrolyte solution and the reference electrode.

Images