JP6871088B2 - How to clean carbon electrodes - Google Patents

Description

The present invention relates to a carbon-containing electrode when the heavy metal is lead and at least one of the electrodes is a carbon-containing electrode in a method for quantifying a heavy metal by concentration on an electrode by stripping and plasma emission. Regarding the cleaning method.

As shown in Patent Document 1 below, following the concentration of heavy metal ions in the sample on the electrode by stripping, a large current is applied to generate plasma emission from the heavy metal ions, and the heavy metal in the sample is generated according to the amount of light emitted. Methods for quantifying ions are disclosed.

Japanese Unexamined Patent Publication No. 2016-130734

When quantifying lead as a heavy metal ion in the sample, if at least one of the pair of electrodes is an electrode containing carbon as a material, for example, a carbon electrode, the quantification may not be performed accurately. It is considered that this is because the carbon electrode used contains lead. That is, it is considered that accurate quantification of lead to be measured in the sample is hindered by the presence of lead contained in the carbon electrode. Therefore, it is desirable to remove the lead contained in advance from the carbon electrode by washing before quantifying the sample.

However, since carbon electrodes have a physically brittle property, cleaning with chemical substances such as acids and alkalis should be avoided as much as possible. Also, cleaning in the form of physical force is not desirable.

An object of the present invention is to make it possible to remove lead from a carbon-containing electrode as easily as possible and without physical damage before measuring a sample.

In the method for cleaning a carbon-containing electrode of the present invention, a sample presumed to contain lead, which is a metal species to be analyzed, is transferred to a pair of electrodes placed in a measuring container, one of which is an electrode containing carbon as a material. It is characterized by including a cleaning step of introducing a cleaning electrolyte solution into the measuring container and applying an electric current to the pair of electrodes prior to performing a quantitative analysis of the lead through the application of an electric current.

When conducting quantitative analysis of lead using electrolysis, by cleaning and removing lead contained in carbon-containing electrodes in advance, it is possible to accurately quantify lead without interfering with the quantification of lead in the sample. Become.

It is a schematic perspective perspective view (A) and the schematic cross-sectional view (B) seen from the I-I direction of the main part in the embodiment of the measuring container used in this invention. It is a schematic cross-sectional view which shows the outline of the cleaning process using the measuring container of FIG. FIG. 5 is a schematic cross-sectional view showing an outline of a concentration step (A) and a detection step (B) in plasma spectroscopic analysis using the measuring container of FIG. It is a schematic diagram of the emission spectrum obtained in the detection step. The emission spectrum of lead is shown.

As described above, in the mode of the method for cleaning the carbon-containing electrode of the present invention, at least one of the samples, which is supposed to contain lead, which is the metal species to be analyzed, is placed in the measuring container and contains carbon as a material. In an electrolyzer that quantitatively analyzes the lead by applying a current to the pair of electrodes, the cleaning electrolyte solution is introduced into the measuring container prior to the quantitative analysis, and the current is applied to the pair of electrodes. It is characterized by including a cleaning step of performing the above.

The quantitative analysis to which the method of the present invention is applied is, for example, a so-called plasma spectroscopic analysis method, in which a pair of electrodes are installed in a liquid sample introduced into a measuring container, and a predetermined current is applied to these electrodes. Is applied (stripping), first, lead as a metal species to be analyzed is concentrated (concentration step) in the vicinity of one of the electrodes, and then, for example, a larger current is applied during this stripping. Plasma emission is generated from the concentrated metal species to be analyzed, this is detected (detection step), and the metal species to be analyzed is quantified by the amount of emission of the plasma emission.

The sample referred to here is a liquid, but may be a diluted solution in which a solid is suspended, dispersed, or dissolved in a liquid medium, for example. As the sample which is the liquid, for example, the stock solution of the sample may be used as it is, or when the concentration is too high, the stock solution is suspended, dispersed or dissolved in a liquid medium, for example. It may be used as a diluent. The liquid medium is not particularly limited as long as it can suspend, disperse or dissolve the sample, and examples thereof include water and a buffer solution. Examples of the sample include a biological sample (sample), an environment-derived sample (sample), a metal, a chemical substance, a pharmaceutical product, and the like. The biological sample is not particularly limited, and examples thereof include urine, blood, hair, saliva, sweat, and nails. Examples of the blood sample include red blood cells, whole blood, serum, plasma and the like. Examples of the living body include humans, non-human animals, plants and the like, and examples of the non-human animals include mammals other than humans, amphibian reptiles, fish and shellfish, insects and the like. The sample derived from the environment is not particularly limited, and examples thereof include food, water, soil, air, and air. Examples of the food include fresh foods and processed foods. Examples of the water include drinking water, groundwater, river water, seawater, domestic wastewater and the like.

The liquid may be, for example, a pH-adjusted liquid. The pH in such a case is not particularly limited as long as it contributes to the detection of the test substance. The pH of the liquid can be adjusted with a pH adjusting reagent such as an alkaline reagent or an acidic reagent.

Examples of the alkaline reagent include alkali or an aqueous solution thereof. The alkali is not particularly limited, and examples thereof include sodium hydroxide, lithium hydroxide, potassium hydroxide, and ammonia. Examples of the alkaline aqueous solution include those obtained by diluting alkali with water or a buffer solution. In the aqueous alkali solution, the concentration of the alkali is not particularly limited, and is, for example, 0.01 to 5 mol / L.

Examples of the acidic reagent include an acid or an aqueous solution thereof. The acid is not particularly limited, and examples thereof include hydrochloric acid, sulfuric acid, acetic acid, boric acid, phosphoric acid, citric acid, malic acid, succinic acid, nitric acid and the like. Examples of the aqueous solution of the acid include those obtained by diluting the acid with water or a buffer solution. In the aqueous solution of the acid, the concentration of the acid is not particularly limited, and is, for example, 0.01 to 5 mol / L.

The metal species to be analyzed is lead (Pb) in the present invention, and can exist in a charged state in a sample as a liquid, for example, in an ionic state.

Further, the liquid may contain, for example, a reagent for separating the metal species to be analyzed in the sample. Examples of the reagent include a chelating agent and a masking agent. The chelating agent includes, for example, dithizone, thiopronin, meso-2,3-dimercaptosuccinic acid (DMSA), 2,3-dimercapto-1-propanesulfonate sodium (DMPS), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid. Examples thereof include acetic acid (NTA), ethylenediamine-N, N'-dimercaptosuccinic acid (EDDS), and α-lipoic acid. In the present analytical method, "masking" means inactivating the reactivity of the SH group, which can be performed, for example, by chemically modifying the SH group. Examples of the masking agent include maleimide, N-methylmaleimide, N-ethylmaleimide, N-phenylmaleimide, maleimide propionic acid, iodoacetamide, iodoacetic acid and the like.

The concentration step is a step of introducing the sample into a measuring container and concentrating lead in the sample in the vicinity of one of the electrodes by applying a current to a pair of electrodes installed in the measuring container.

The pair of electrodes refers to a combination of an anode and a cathode in electrolysis. The electrode is a solid electrode, and specific examples thereof include a rod electrode and the like. The material of the electrode is not particularly limited as long as it is a carbon-containing electrode containing at least one carbon as a material, and may be a solid conductive material, and the other electrode can be appropriately determined. The carbon-containing electrode as the "at least one" electrode here may or may not constitute the "one electrode" on which the lead in the sample is concentrated. Either may be used. Examples of the carbon-containing electrode include so-called carbon electrodes. The material of the other electrode may be, for example, a non-metal, a metal, or a mixture thereof. Further, the other electrode may also be a carbon-containing electrode. When the material of the other electrode contains a non-metal, the material of the other electrode may contain, for example, one kind of non-metal or two or more kinds of non-metal. Examples of the non-metal include carbon and the like. When the material of the other electrode contains a metal, the material of the other electrode may contain, for example, one kind of metal or two or more kinds of metals. Examples of the metal include gold, platinum, copper, zinc, tin, nickel, palladium, titanium, molybdenum, chromium and iron. When the material of the other electrode contains two or more kinds of metals, the material of the other electrode may be an alloy. Examples of the alloy include brass, steel, Inconel (registered trademark), nichrome, stainless steel and the like.

The size of the electrode is not particularly limited as long as at least a part thereof is accommodated in the measuring container. When the measuring container is to be made into a cartridge that can be mass-produced, for example, it is desirable to reduce the size of the measuring container as much as possible. In that case, the electrode is also miniaturized according to the size of the measuring container. Further, one or both of the pair of electrodes may be provided as a unit in the measuring container in advance, or may be appropriately inserted into the measuring container at the time of measurement. May be good.

The electrode on which lead as the metal species to be analyzed is concentrated is the cathode in this concentration step.

As described above, the concentration step is a step of concentrating lead in the sample in the vicinity of the one electrode by applying a current to the pair of electrodes in the presence of the sample. The pair of electrodes are in contact with the sample. In the concentration step, the vicinity of the one electrode is not particularly limited, and examples thereof include a range in which plasma is generated in the detection step described later, for example, the surface of the one electrode.

In the concentration step, for example, a part of lead in the sample may be concentrated in the vicinity of the one electrode, or the entire lead may be concentrated in the vicinity of the one electrode.

In the concentration step, in the detection step described later, the electrode used for lead detection, that is, the electrode that generates plasma (hereinafter, also referred to as “plasma generation electrode”) becomes one of the electrodes, and this electrode It is preferable to set the charge conditions of the pair of electrodes so that lead is concentrated in the plasma. In the charge condition, since lead has a positive charge as a metal ion, the current direction may be set so that one of the electrodes (that is, the plasma generating electrode) becomes a cathode in the concentration step.

Lead enrichment can be adjusted, for example, by voltage. Therefore, those skilled in the art can appropriately set the voltage at which the concentration occurs (hereinafter, also referred to as "concentration voltage"). The concentration voltage is, for example, 1 mV or more, preferably 400 mV or more, and the upper limit thereof is not particularly limited. The enrichment voltage may be constant or variable, for example. Further, the concentration voltage may be, for example, a voltage at which plasma is not generated.

The time for applying the enrichment voltage is not particularly limited and can be appropriately set according to the enrichment voltage. The time for applying the concentration voltage is, for example, 0.2 to 40 minutes, preferably 5 to 20 minutes. The voltage applied to the pair of electrodes may be applied continuously or discontinuously, for example. Examples of the discontinuous application include pulse application. When the application of the concentration voltage is discontinuous, the time for applying the concentration voltage may be, for example, the total time of the time for applying the concentration voltage, or the time for applying the concentration voltage. It may be the total time with the time when the concentration voltage is not applied.

The voltage applying means as a means for applying a voltage to the pair of electrodes is not particularly limited, and for example, a predetermined voltage may be applied between the pair of electrodes, and a voltage device or the like can be used as a known means. .. In the concentration step, the current applied between the pair of electrodes can be set to, for example, 0.01 to 200 mA, preferably 10 to 60 mA, and more preferably 10 to 40 mA.

In the detection step, as described above, plasma is generated by applying, for example, a larger current than in the case of the concentration step to the pair of electrodes, and the emission of lead generated by the plasma is detected.

Here, the direction of the current in the detection step may be the same as the direction of the current in the concentration step. However, the voltage applying means is formed so that the direction of the current when applying the voltage can be switched, and the direction of the current when generating the plasma is opposite to the direction of the current when concentrating lead. Is desirable.

Specifically, since lead has a positive charge in the concentration step, the current direction from the voltage applying means should be set so that the one electrode as the plasma generating electrode serves as an anode in the detection step. Just do it.

The detection step may be performed continuously or discontinuously with the concentration step. In the former case, the detection step is performed at the same time as the completion of the concentration step. In the latter case, the detection step is performed within a predetermined time after the completion of the concentration step. The predetermined time is, for example, 0.001 to 1,000 seconds, preferably 1 to 10 seconds after the concentration step.

In the detection step, "generating plasma" means substantially generating plasma, and specifically, in detecting plasma emission, it means generating plasma exhibiting substantially detectable emission. To do. As a specific example, it can be said that plasma emission can be detected by a plasma emission detector.

Substantial plasma generation can be regulated, for example, by voltage. Therefore, a person skilled in the art can appropriately set a voltage for generating plasma exhibiting substantially detectable light emission (hereinafter, also referred to as “plasma generation voltage”). The plasma generation voltage is, for example, 10 V or more, preferably 100 V or more, and the upper limit thereof is not particularly limited. The voltage generated by the plasma is, for example, a voltage relatively higher than the voltage at which the enrichment occurs. Therefore, the plasma generation voltage is preferably a voltage higher than the concentration voltage. The plasma generation voltage may be constant or variable, for example.

The time for applying the plasma generation voltage is not particularly limited, and can be appropriately set according to the plasma generation voltage. The time for applying the plasma generation voltage is, for example, 0.001 to 0.02 seconds, preferably 0.001 to 0.01 seconds. The plasma generation voltage to the pair of electrodes may be applied continuously or discontinuously, for example. Examples of the discontinuous application include pulse application. When the application of the plasma generation voltage is discontinuous, the time of applying the plasma generation voltage may be, for example, the time of applying the plasma generation voltage once, or the time of applying the plasma generation voltage. It may be the total time during which the plasma is generated, or it may be the total time between the time when the plasma generation voltage is applied and the time when the plasma generation voltage is not applied.

In the detection step, the generated plasma emission may be detected continuously or discontinuously, for example. Examples of the light emission detection include detection of the presence or absence of light emission, detection of light emission intensity, detection of a specific wavelength, detection of a spectrum, and the like. The detection of the specific wavelength includes, for example, the detection of a specific wavelength emitted by the analysis object at the time of plasma emission. The method for detecting light emission is not particularly limited, and for example, a known optical measuring device such as a CCD (Charge Coupled Device) or a spectroscope can be used.

The application of the plasma generation voltage to the pair of electrodes in the detection step can be performed by the voltage application means used in the concentration step at a higher voltage, preferably with the current direction reversed. In the detection step, the current between the electrodes is relatively larger than that of the concentration step because the plasma generation voltage is relatively higher than the concentration voltage, for example, 0.01 to 100,000 mA, preferably 0.01 to 100,000 mA. It can be set to 50 to 2,000 mA.

Here, since the carbon-containing electrode, for example, a carbon electrode often contains lead, when an attempt is made to detect lead in a sample by using electrolysis, the lead in the electrode is contained in the sample. There is a possibility that the amount of lead will be quantified more than the actual amount due to elution. Therefore, prior to the above-mentioned quantitative analysis, it is necessary to wash the carbon-containing electrode to remove lead.

Specifically, a cleaning electrolyte solution is introduced into the measuring container in which the pair of electrodes are installed, and a current is applied (stripping) to the pair of electrodes in the same manner as in the concentration step. The cleaning step is carried out by performing the above.

At this time, an aqueous solution of the electrolyte is introduced into the measuring container as the cleaning electrolyte solution in order to enable energization between the pair of electrodes in the measuring container. The electrolyte referred to here is not particularly limited as long as it dissolves in water and is ionized. For example, lithium hydroxide, sodium hydroxide, calcium hydroxide, magnesium hydroxide, potassium hydroxide, aluminum hydroxide, ammonium hydroxide, manganese hydroxide, sodium chloride, sodium hypochlorite, sodium chloride, sodium hydrogen carbonate, Examples thereof include hydrochloric acid, potassium carbonate and potassium chloride, but among the above, hydroxide is more preferable.

It is desirable that the current application in this cleaning step is performed by repeating energization and power interruption for a predetermined time and then discarding the cleaning electrolyte solution as one set, and performing this one set or more and five sets or less.

Further, as one set of the current application, it is desirable that the voltage is 1 mV or more, the current is 10 to 40 mA for 2 seconds, and the power is cut off for 2 seconds for 0.2 to 40 minutes.

An example of the measuring container used in the embodiment of the present invention will be described with reference to the drawings. Further, in the drawings, for convenience of explanation, the structure of each part may be simplified as appropriate, and the dimensional ratio of each part may be shown schematically, which is different from the actual one.

In FIG. 1, (A) is a schematic perspective perspective view of the measuring container 10 used in the present embodiment, and (B) is a schematic cross-sectional view of (A) viewed from the I-I direction. As shown in FIGS. 1A and 1B, the measuring container 10 used in the present embodiment includes a pair of electrodes (plasma generating electrode 20 and plasma non-generating electrode 30) inside. The measuring container 10 has a substantially cylindrical shape in which a part of the side surface is scraped off in a plane shape, and the flat surface portion includes a circular translucent portion 11. Outside the measuring container 10, a light receiving unit 40 is arranged so that light emitted by applying a current to the plasma generating electrode 20 and the plasma non-generating electrode 30 can be received by lead light emitted through the translucent unit 11. There is. Further, the plasma generating electrode 20 is arranged parallel to the liquid surface 61 of the sample 60 which is a liquid, and its tip is arranged so as to be in contact with the translucent portion 11. A part of the side surface of the cylindrical plasma non-generating electrode 30 is arranged on the side surface of the measuring container 10 on the side facing the translucent portion 11 so as to intersect the vertical direction at right angles, and the inside of the measuring container 10 is provided. A part of it is exposed. That is, the longitudinal direction of the plasma non-generating electrode 30 and the longitudinal direction of the plasma generating electrode 20 are in twisted positions. The plasma generating electrode 20 is covered with an insulator 22. The sample 60, which is presumed to contain lead, is introduced into the cylinder of the measuring container 10 so as to be in contact with the plasma generating electrode 20 and the plasma non-generating electrode 30.

In the present embodiment, most of the surface of the plasma generating electrode 20 is covered with the insulator 22. The portion not covered with the insulator 22 is the wetted portion 21. On the other hand, the plasma non-generating electrode 30 is a carbon electrode as a carbon-containing electrode.

In the present embodiment, the plasma generating electrode 20 and the translucent portion 11 are in contact with each other, but the present invention is not limited to this, and for example, the plasma generating electrode 20 may be arranged away from the translucent portion 11. The distance between the plasma generating electrode 20 and the translucent portion 11 is not particularly limited, and is, for example, 0 to 0.5 cm.

The material of the translucent portion 11 is not particularly limited, and may be, for example, a material that transmits light emitted by applying a current to the plasma generating electrode 20 and the plasma non-generating electrode 30, depending on the wavelength of the light emission. Can be set as appropriate. Examples of the material of the translucent part 11 include quartz glass, acrylic resin (PMMA), borosilicate glass, polycarbonate (PC), cycloolefin polymer (COP), and methylpentene polymer (TPX (registered trademark)). The size of the translucent portion 11 is not particularly limited as long as it can transmit light emitted by applying a current to the plasma generating electrode 20 and the plasma non-generating electrode 30.

In the present embodiment, the measuring container 10 has a bottomed cylindrical shape in which a part of the side surface is cut flat along the longitudinal direction, but the shape of the measuring container 10 is not limited to this, and is an arbitrary shape. May be. The material of the measuring container 10 is not particularly limited, and examples thereof include acrylic resin (PMMA), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polystyrene (PS). Be done. When the measuring container 10 has a bottomed tubular shape, the diameter of the measuring container 10 is, for example, 0.3 to 1 cm, and the height thereof is, for example, 0.9 to 5 cm. A sample 60 having a size of 0.3 to 0.8 cm ^{3 is introduced into the measuring container 10.}

The light receiving unit 40 is not particularly limited, and examples thereof include known optical measuring devices such as a CCD and a spectroscope. The light receiving unit 40 may be, for example, a transmission means for transmitting the light emission to the optical measuring device. Examples of the transmission means include a transmission line such as an optical fiber.

The manufacturing method of the measuring container 10 is not particularly limited, and for example, a molded body may be manufactured by injection molding or the like, or a recess may be formed in a base material such as a plate. In addition, the manufacturing method of the measuring container 10 and the like is not particularly limited, and examples thereof include lithography, cutting, and the like.

The case of quantitatively analyzing the lead ions present in the sample 60 as an aqueous solution will be described.

First, before introducing the sample into the measuring container 10, as the cleaning step, as shown in FIG. 2, in a state where the cleaning electrolyte solution 70 is introduced into the measuring container 10, the plasma generating electrode is used by the voltage applying means 50. A voltage is applied so that 20 serves as a cathode and the plasma non-generating electrode 30 serves as an anode. Then, if lead is contained in the carbon-containing electrode constituting the plasma non-generating electrode 30, the lead is eluted as lead ions in the cleaning electrolyte solution 70, and then the application of voltage is stopped and the measurement container 10 is used. Discard this cleaning electrolyte solution 70. Repeat this procedure several times.

After the completion of the cleaning step, with the sample 60 introduced into the measuring container 10, the plasma generating electrode 20 becomes a cathode by the voltage applying means 50 as shown in FIG. 3A as the concentration step. , A voltage is applied so that the plasma non-generating electrode 30 serves as an anode. Then, the lead ions present in the sample 60 are attracted to the wetted portion 21 of the plasma generating electrode 20 which is a cathode. At this time, the lead eluted from the plasma non-generating electrode 30 is reduced as much as possible by the cleaning step.

Next, as the detection step, as shown in FIG. 3B, a voltage is applied by the voltage applying means 50 so that the plasma generating electrode 20 becomes an anode and the plasma non-generating electrode 30 becomes a cathode this time. .. Then, plasma emission is generated from the lead ions attracted to the periphery of the wetted portion 21 of the plasma generating electrode 20 by the previous concentration step, and this is passed through the translucent portion 11 and received and detected by the light receiving portion 40. It will be. The light detected here is due to the lead contained in the sample 60, and the influence of lead contained in the plasma non-generating electrode 30 as the carbon-containing electrode is eliminated as much as possible by the cleaning step. It is supposed to be done.

Here, it is assumed that the emission spectrum obtained in this detection step is as shown in FIG. In the figure, W is the analysis wavelength, P is the peak emission amount at the wavelength W, and B is the base emission amount corresponding to the peak emission amount P.

In this case, the analytical luminescence amount E of lead can be defined as, for example, the following equation.

E = (P-B) / B = P / B-1

(1) Measuring container and electrode As the measuring container 10, a bottomed tubular transparent PMMA cell (height 28 mm × diameter (maximum diameter portion) φ7 mm) was prepared. Quartz glass (diameter 4.5 mm, thickness 0.3 mm) as the translucent portion 11 was arranged near the lower end of the flat portion on the side surface of the measuring container 10. The plasma generating electrode 20 and the plasma non-generating electrode 30 were arranged in the measuring container 10. The plasma generating electrode 20 was arranged parallel to the liquid level 61. Then, the tip of the plasma generating electrode 20 was arranged so as to come into contact with the translucent portion 11. As the plasma generating electrode 20, a nichrome wire having a diameter of 0.1 mm was used. On the other hand, as the plasma non-generating electrode 30, a carbon rod having a diameter of 4.0 mm was used. A part of the side surface of the plasma non-generating electrode 30 is arranged on the side surface of the measuring container 10 on the side facing the translucent portion 11 so as to intersect the vertical direction at right angles, and the inside of the measuring container 10 is provided. Part of it is exposed. That is, the longitudinal direction of the plasma non-generating electrode 30 and the longitudinal direction of the plasma generating electrode 20 are in twisted positions. Further, an optical fiber as a light receiving unit 40 is arranged so as to face the tip of the plasma generating electrode 20 via the translucent unit 11. The optical fiber used was a single core having a diameter of 400 μm. Further, the optical fiber was connected to a concave grating spectroscope (not shown).

(2) Cleaning Step 900 μL of a lithium hydroxide aqueous solution adjusted to 2 mol / L as the cleaning electrolyte solution 70 was introduced into the measuring container 10. This was set in the measuring device, and the current was applied under the following stripping conditions.

Applied voltage: 5V
Applied current: 20mA
Current ON / OFF cycle: 2 seconds ON, 2 seconds OFF
Current ON / OFFDuty: 50%
Application time: 10 minutes

After the application of the current was completed, the cleaning electrolyte solution 70 was discarded from the measuring container 10.

In Example 1, the above procedure was performed only once, in Example 2, the above procedure was repeated three times, and in Comparative Example, the above procedure was not performed.

After performing the above procedure a specified number of times, washing with 900 μL of distilled water was performed twice.

(3) Plasma spectroscopic analysis The amount of light emitted by plasma spectroscopic analysis of lead using the electrodes of the above Examples and Comparative Examples was measured as follows.

First, ethanol was added in a proportion of 1 part by volume to 20 parts by volume of a lithium hydroxide aqueous solution adjusted to 2 mol / L and mixed to prepare a measuring solution, and 420 μL of this measuring solution was introduced into the measuring container 10.

Then, as a concentration step, a current was applied under the following concentration conditions so that the plasma generating electrode 20 became a cathode and the plasma non-generating electrode 30 became an anode, and lead ions were concentrated in the vicinity of the plasma generating electrode 20. The following applied current is a constant current, and the applied voltage fluctuates according to the resistance of the measuring liquid.

(Concentration conditions)
Applied current: 20mA
Pulse period: 4 seconds Duty (pulse ratio): 50%
Application time: 600 seconds

Immediately after the concentration step, as a detection step, a voltage is applied under the following detection conditions so that the plasma generating electrode 20 becomes an anode and the plasma non-generating electrode 30 becomes a cathode, and each of the generated plasma emission is performed. The emission intensity (count value) at the wavelength was measured. The following applied voltage is a constant voltage, and the applied current varies depending on the resistance of the measuring liquid, but the applied current is larger than the applied current under the concentration conditions.

(Detection condition)
Applied voltage: 500V
Pulse period: 50 μs Duty: 50%
Application time: 2.5 msec

The value obtained by dividing the count value at the specific peak of lead (see FIG. 5) at a wavelength of 368 nm by the background count value was taken as the Pb analysis emission amount.

The above measurement results are shown in Table 1 below.

The “number of measurements” in Table 1 refers to the number of measurements taken by exchanging the measurement liquid with the same electrode.

Since the measurement solution does not contain lead, a lead peak should not be observed by plasma spectroscopic analysis. However, as shown in Table 1 above, in the comparative example, the Pb analysis luminescence amount was observed with an average value of 0.45 in 12 measurements. It is clear that this is due to the lead ions eluted from the plasma non-generating electrode 30 which is a carbon electrode.

Then, in Example 1 in which the washing step was performed once, the average of the six measurements was 0.14, and the amount of Pb emission was reduced to about one-third of that of the comparative example. This indicates that the lead contained in the plasma non-generating electrode 30, which is a carbon electrode, was removed by the cleaning process.

In Example 2 in which the washing step was performed three times, the average of 12 measurements was 0.11, and the amount of Pb analysis luminescence was reduced to about one-fourth of that of the comparative example. However, the difference from Example 1 was small, and it was presumed that the lead that could be removed in advance in the three washing steps was almost completely removed.

Therefore, the number of times the cleaning process is repeated is limited to about 5 times at most because of the time constraint of performing plasma spectroscopic analysis after that and the possibility that the carbon-containing electrode may be damaged by the cleaning process. It is estimated that lead is sufficiently removed and exhausted in three times.

The present invention can be used when a carbon-containing electrode is used in plasma spectroscopic analysis of lead in a sample.

10 Measuring container
11 Translucent part
20 Plasma generating electrode
21 Wet contact part
22 Insulator
30 Plasma non-generating electrode
40 Receiver
50 Voltage application means
60 specimens
61 Liquid level
70 Electrolyte solution for cleaning

Claims (5)

Quantitative analysis of lead in a sample that is supposed to contain lead, which is the metal species to be analyzed, by applying an electric current to a pair of electrodes that are carbon-containing electrodes, one of which is carbon-containing as a material, placed in the measuring container. prior to performing, have row current applied to the pair of electrodes is wetted in cleaning electrolytic solution is the pair of electrodes, for the cleaning of lead contained in the carbon-containing electrode from the carbon-containing electrode A method for cleaning a carbon-containing electrode, which comprises a cleaning step of eluting into an electrolyte solution. The method for cleaning a carbon-containing electrode according to claim 1, wherein the cleaning electrolyte solution is an aqueous solution of hydroxide. The method for cleaning a carbon-containing electrode according to claim 2, wherein the hydroxide is lithium hydroxide. The current applied in the washing step, the claim 1, wherein the performing energization and deenergized and one set to discard the cleaning electrolytic solution after repeating a predetermined time, which one or more sets 5 sets less The method for cleaning a carbon-containing electrode according to any one of claims 3 to 3. The carbon-containing electrode according to claim 4, wherein as one set of the current application, the voltage is 1 mV or more, the current is 10 to 40 mA, and the current is energized for 2 seconds and cut off for 2 seconds for 0.2 to 40 minutes. Cleaning method.

Images