JP2517886B2 - Anion analysis method using complexation reaction - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method using a complex formation reaction in the separation analysis of anions in an aqueous solution by a liquid chromatograph having an electric conductivity detector.

(Prior Art) A method for analyzing anions in an aqueous solution by a liquid chromatograph having an electric conductivity detector is described in Small,
Stevens and Boomer [Reference Anal. Chem. 47 , 180
1 (1975)]. This method uses a solution containing a hydroxide or carbonate of an alkali metal ion as an eluent, separates anions by an anion exchange column, and then guides them to a cation exchange column on which hydrogen ions are adsorbed. It directly exchanges the alkali metal ions and hydrogen ions in the eluent, converts the electrolyte of the eluent into a weak ion type, and analyzes the measured anions with high sensitivity. After that, in order to solve the problem of regeneration due to the limitation of the ion exchange capacity of the cation exchange column and the drawback of adsorbing the specific measurement anion (nitrite ion) on the cation exchange column, a cation exchange membrane was used. This method was developed by Stevens, Davis and Small [Literature Anal. Chem. 53 , 1488 (1981)]. The basic principle of these methods is to directly chemically replace an alkali metal ion hydroxide or carbonate inorganic compound with a cation exchange column or cation exchange membrane to replace an alkali metal ion with a hydrogen ion. It can be expressed as

In the formula (1), M ⁺ is an alkali metal ion such as sodium or potassium, and X ⁻ is a hydroxide ion or a carbonate ion. Therefore, H ⁺ X produced by ion exchange of M ⁺ and H ^{+ -} in the case of hydroxide ions becomes water is converted when the carbonate ions weakly ionic form of carbonate itself. In the method using a cation exchange membrane, a solution of sulfuric acid, which is a strong acid, is passed through the annular structure of the double tube provided outside the cation exchange membrane tube through which the eluent is passed, and alkali metal ions in the eluent are continuously fed. It permeates the cation exchange membrane tube wall and is converted (regenerated) with hydrogen ions.

(Problems to be Solved by the Invention) A problem with such a conventional method is that metal ions that react with hydroxide ions or carbonate ions of the eluent composition to cause precipitation are present in the sample. This is the case. This phenomenon not only affects the chromatogram, but also causes deterioration of the performance of the separation column. Matsushita [Reference
J. Chromatogr., 312 , 327 (1984)] is a complex forming reagent ethylenediaminetetraacetic acid (EDTA), trans-1,2-cyclohexanediamine-N, N, N ′, N′-tetraacetic acid (CyDTA). And glycol ether diamine-N, N, N ', N'-tetraacetic acid (GEDTA) salt as an eluent, in addition to the anions in the sample, alkaline earth metal ions such as calcium and magnesium, nickel and zinc, etc. The transition metal ions are reacted with a complex forming reagent to elute these metal ions as a negatively charged complex. This is an effective method for solving contamination of the separation column with metal ions, which is a problem when measuring anions in a sample by liquid chromatography, but the electrical conductivity (background) of the eluent itself is Since it is high, it is difficult to constantly measure the anions in the sample at the ppb (μg /) level.

As described above, there is no satisfactory method for analyzing anions in an aqueous solution by a liquid chromatograph having an electric conductivity detector, and therefore, high sensitivity detection and prevention of deterioration of a separation column are achieved and stable analysis can be achieved. A method was desired.

(Means for Solving Problems) As a result of earnest research aimed at solving these problems, the present inventors have found that a small amount of anions existing in an aqueous solution are contaminated in the separation column by metal ions in the sample. We have completed a liquid chromatographic analysis method that enables highly sensitive detection with an electric conductivity detector without causing it.

That is, the present invention, when separating and analyzing anions in an aqueous solution by a liquid chromatograph having an electric conductivity detector, an anion in a sample solution is obtained by an eluent containing an alkali metal or alkaline earth metal salt of aminodicarboxylic acid. Is separated by a separation column filled with an anion exchanger, the eluent is led to a complex formation reaction support connected in series with the separation column, and a transition metal ion ionically bonded to the complex formation reaction support. A complex formation reaction characterized by measuring the electric conductivity of an anion in a sample solution by an electric conductivity detector after forming a complex of azole and aminodicarboxylic acid on the complex formation reaction support. An anion analysis method is provided.

(Operation) Hereinafter, the present invention will be described in detail.

FIG. 1 shows a flow diagram of a liquid chromatograph as one embodiment of the analysis method of the present invention. The eluent containing aminodicarboxylate as a developing agent is the eluent reservoir 1
Is sent to the sample liquid injector 3 by the liquid sending pump 2,
Here, after injecting a fixed amount of the sample solution into this eluent flow, it is sent to the separation column 4 filled with an anion exchanger installed in the constant temperature bath 6. The eluent stream is then directed to the complexing reaction support 5, which is preliminarily ion-bonded to the transition metal ion, and upon contact with the eluent stream, the transition metal ion to the aminodicarboxylic acid is transferred to the complexing reaction support 5. Is a place for rapidly forming a transition metal complex.
The eluent flow chemically exchanged as a transition metal complex in the complex formation reaction support 5 passes through the electric conductivity detector 8, and the detection signal is displayed on the recorder 9 as a chromatogram. As a developing agent for separating anions in the present invention, an alkali metal or alkaline earth metal salt of aminodicarboxylic acid is used.

Examples of aminodicarboxylic acids include iminodiacetic acid, methyliminodiacetic acid, N- (γ, γ-dimethylbutyl) -iminodiacetic acid, iminopropionacetic acid, iminodipropionic acid, 2-oxyethyliminodipropionic acid, β-methoxy Ethyliminodiacetic acid, β-aminoethyliminodiacetic acid, N- (acetamido) -iminodiacetic acid, ethylenediamine-N, N'-diacetic acid, ethylenediamine-N, N'-dipropionic acid, phenyliminodiacetic acid, nitroimino Examples include diacetic acid. From the complexation constants with transition metal ions, iminodiacetic acid, methyliminodiacetic acid, N- (γ, γ-
Dimethylbutyl) -iminodiacetic acid, iminopropionic acid, β-methoxyethyliminodiacetic acid, β-aminoethyliminodiacetic acid, ethylenediamine-N, N′-diacetic acid,
Ethylenediamine-N, N'-dipropionic acid is preferably used.

On the other hand, as the transition metal ion, a divalent ion is preferable, and a copper (II) ion is particularly preferable.

The combination of aminodicarboxylic acid and the transition metal ion complex formation is appropriately selected such that the complex formation constant is larger than the hydroxide formation constant of the transition metal ion in relation to the pH of the eluent.

The developing agent concentration in the eluent may be appropriately selected depending on the sample to be analyzed.

The decomposition column 4 used in the present invention may be any one as long as it is packed with an anion exchange resin which is usually used for analysis.

As the complex formation reaction support 5 of the present invention, a cation exchange resin is ionically bonded to a transition metal ion.
The cation exchange resin is preferably a polystyrene-, polyacryl-, polymethacrylate-, or polyvinyl acetate-based gel substrate chemically bonded with a sulfonic acid type cation-exchange group. The larger the ion-exchange capacity, the more transition metal ions are contained. The ability of the complex-forming reaction support as a supply field is increased, and is not particularly limited, but generally 0.3 meq / g
The above is preferable.

As the method for preparing the complex formation reaction support 5, a solution containing, for example, a copper (II) ion as the alkali metal or alkaline earth metal ion of the sulfonic acid type cation exchange gel prepared by a commonly known method is flowed. Therefore, ion exchange may be performed.

If the developing agent cation in the eluent and the copper (II) ion on the complex forming reaction support 5 are ion-exchanged to deactivate the aminodicarboxylic acid-copper complex forming ability, the pump shown in FIG. 7, copper (II) ion solution can be supplied to the complex formation reaction support 5 for regeneration. Such a regeneration operation can be easily performed by providing a switching valve before and after the complex formation reaction support 5.

Although the mechanism of the chemical conversion occurring in the complex formation reaction support 5 is not clear, it can be considered as shown in FIG. 2 as an example. As shown in FIG. 2 (a), copper (II) ions are ionically bonded to sulfonic acid groups on the complex formation reaction support. The eluent contains a salt M ₂ L (M; alkali metal, L; aminodicarboxylic acid) having a considerably high electric conductivity as a developing agent. As shown in FIG. 2 (b), when M ₂ L in the eluent reaches the complex formation reaction support, aminodicarboxylic acid (L) forms an active intermediate complex with copper (II) ion. In general, the complex formation constant for aminodicarboxylic acid is in the order of copper (II) ion >> alkaline earth metal ion >> alkali metal ion. Therefore, as shown in FIG. 2 (c), the copper (II) ion is an aminodicarboxylic acid. In order to form a CuL complex with (L), separate from the sulfonic acid group, and maintain electrostatic equilibrium, M ⁺ ionically bonds with the sulfonic acid group. By repeating such a procedure, the complex of CuL having no charge passes through the detector as shown in Fig. 2 (d), and the electric conductivity of the detector is that of water itself which is the solvent. It is close to the value you have.

Thus, the first feature of the present invention is to use an aminodicarboxylic acid, which is a chelating reagent, as an eluent to prevent contamination of the separation column with metal ions in the sample. In order to obtain highly sensitive detection of ions, the transition metal ion is supplied to the chelating reagent through a medium called a complexing reaction support by using the property of the chelating reagent itself to form a stable zero-valent complex, and thus the conductivity ( The background is to reduce the water itself as a solvent. This is because an inorganic compound such as a hydroxide or carbonate of an alkali metal ion, which has been developed by Small, Stevens, and Boomer 5 and has no coordination ability, is used as an eluent, and an alkali metal is used by a cation exchanger. It is based on a completely different chemical principle from the method in which ions and hydrogen ions are directly ion-exchanged to be converted into water or carbonic acid and led to an electric conductivity detector. That is, the role of the cation exchanger used in both methods is
In the present invention, a transition metal ion such as copper is provided to the aminodicarboxylic acid, with the result that the counterion of the aminodicarboxylic acid is retained on the cation exchanger to maintain electrostatic equilibrium. In the method of Small et al., First, the alkali metal ion of the eluent is captured by the cation exchanger, and as a result, the hydrogen ion is desorbed. In the former case, the field for supplying the metal ion is the cation exchanger,
In the latter case, the mechanism for removing metal ions is a cation exchanger, which is the exact opposite mechanism. Bonner and Smith [Reference J. Phys. Chem., 61 , 326 (1957)] have already experimentally shown selectivity coefficients for various cations in polystyrene sulfonate resin, which is a cation exchanger. Adsorption becomes higher in the order of ⁺ <Na ⁺ <K ⁺ <Cu ²⁺ <Ca ²⁺ . As will be shown later in Examples, an eluent containing iminodiacetic acid as an aminodicarboxylic acid and sodium, potassium, and calcium as its counterions was used as a cation exchanger, and polystyrene sulfone preadsorbed with copper (II) ions was used. When it is passed through a column filled with acid gel, the electric conductivity is uniformly lowered to 10 μS or less and stabilized. Judging from the selection coefficient for the polystyrene sulfonic acid resin, this shows that in the case of the calcium salt of iminodiacetic acid, the copper (II) in the cation exchange column was used.
Since calcium is more absorptive than ions, the small cation exchanger, in which calcium ions are trapped in the gel and copper (II) ions are desorbed as a result, is used as a place for removing metal ions. Although it may be from the same point of view, even sodium and potassium salts, which have lower adsorptivity than copper (II) ions, cause a decrease in electrical conductivity in the same manner as calcium salts. In the first step, iminodiacetic acid complexed with copper (II) ion, and after elimination as a zero-valent complex, in the second step, the counterion of iminodiacetic acid was retained in the gel just to maintain electrostatic equilibrium. This can be easily explained by using the cation exchanger that is said to be a place for supplying metal ions. Although the cation exchange resin column has been described above as the complex formation reaction support of the present invention, the method of using the cation exchange membrane may be used in the present invention.

Even when the cation exchange membrane is used as the complex formation reaction support, the flow diagram of FIG. 1 can be used as it is,
A solution containing a transition metal ion can be continuously supplied by the liquid feed pump 7. As the use as a membrane, a method such as a flat membrane or a hollow membrane is appropriately adopted. When a hollow membrane is used, the eluent containing the developing agent is passed inside the membrane and the solution containing copper (II) ions for regeneration is passed outside to retain the copper (II) ions on the complex formation reaction support. It can be maintained. Although the membrane is manufactured by various synthetic polymers such as sulfonation of polystyrene,
A perfluorocarbon cation exchange membrane having excellent solvent resistance, for example, a copolymer of perfluoro-3,6-dioxa-4-methyl-7-octenesulfonyl fluoride and tetrafluoroethylene (registered trademark Nafion) is cited. To be

(Examples) The present invention is described below with reference to examples, but the present invention is not limited thereto.

Examples 1 to 5, Comparative Example 1 As a device, a liquid feed pump (Toyo Soda Industry Co., Ltd., trade name CCPM), sample injection valve (Toyo Soda Industry Co., Ltd., trade name injection valve unit, sample injection amount 100)
μ), column constant temperature bath (manufactured by Toyo Soda Industry Co., Ltd., trade name
CO-8000), separation column for anion analysis (Toyo Soda Industry Co., Ltd., trade name TSKgel IC-Anion-PW) and electric conductivity detector (Toyo Soda Industry Co., Ltd. trade name CM-8000) And the column constant temperature bath temperature was 40 ° C. Sulfonic acid type cation exchange column (manufactured by Toyo Soda Kogyo Co., Ltd., trade name TSKg
el IC-C, inner diameter 4.6 mm, length 5 cm) was previously fed with 20 mM copper sulfate solution at 1.2 ml / min for 20 minutes to completely bond the copper (II) ion to the complex formation reaction support. After that, the column was washed with deionized water at the same flow rate for 90 minutes and connected immediately after the separation column in the column thermostat. The electrical conductivity before and after complex formation was measured by changing various aminodicarboxylic acid salts as a developing agent in the eluent.

 The above results are shown in Table 1.

The potassium ion on the complexing reaction support saturated with the developing agent of Example 1 was adjusted to 20 mM copper sulfate, 50 mM copper acetate, 20 mM.
FIG. 3 shows the change in electric conductivity when the regeneration treatment was carried out with mM copper acetate. When the copper (II) ion was completely ion-exchanged and washed with deionized water for the time shown in the figure *, the copper (II) ion was replaced within 30 minutes and regeneration was completed within 120 minutes. I was able to finish.

Example 6 A sample solution containing chloride ion 0.5 ppm, nitrite ion 0.5 ppm, bromide ion 1.0 ppm, nitrate ion 1.0 ppm and sulfate ion 2.0 ppm was separated and analyzed with the eluent of Example 3 and the results are shown in FIG. Shown in the chromatogram. Carbonate ions in the sample solution are detected between nitrate ions and sulfate ions. The solution was adjusted by diluting it from a 1000 ppm standard solution immediately before measurement.

Further, an example of a calibration curve obtained by changing the concentrations of various ions is shown in FIG. 5, and all show good linearity.

Example 7 A commercially available Nafion tube (manufactured by DuPont, inner diameter 0.625 mm, outer diameter 0.875 mm, length 4.8 m) was used as the inner diameter of the complexing support.
It was placed in a 2 mm Teflon tube and connected to the diameter of the separation column in the column thermostat. Inside the Nafion tube,
A solution containing 1.2 mM ethylenediamine-N, N'-diacetic acid and 2.4 mM potassium hydroxide was flown, and copper sulfate solutions having various concentrations were flowed at 1.2 ml / min to measure the electric conductivity. Table 2 shows the results. It can be seen that the electrical conductivity is sufficiently reduced from the weight to about 10 times the molar ratio of the developing agent in the eluent.

Similar results were obtained with copper acetate and copper nitrate instead of copper sulfate.

Comparative Example 2 Table 3 shows the results of the same procedure as in Example 7 in which the sulfuric acid concentration was changed using an eluent containing 1 mM iminodiacetic acid and 2 mM potassium hydroxide.

Example 8 The same procedure as in Example 7 was performed using 1.5 mM copper sulfate,
Fluoride ion 0.002ppm, Chloride ion 0.004ppm, Nitrite ion 0.004ppm, Bromide ion 0.010ppm, Nitrate ion 0.01
A sample solution containing 0 ppm, phosphate ion 0.020 ppm and sulfate ion 0.020 ppm was analyzed. The resulting chromatogram is shown in FIG.

Example 9 A chromatogram of the result of analyzing tap water in the same manner as in Example 8 is shown in FIG. Chloride ion, nitrate ion,
Carbonate ion and sulfate ion are detected.

(Effects of the present invention) As described above, by utilizing the complex formation reaction of the method of the present invention, it is possible to prevent contamination by heavy metal ions in the separation column by the complex forming agent in the eluent, and to form the complex. Due to the complex formation on the reaction support, the electric conductivity of the eluent can be set to a very low value of several μS, whereby the anion in the sample solution can be measured with a high sensitivity up to a low concentration.

In addition, as shown in the chromatogram in the examples,
In the method of the present invention, the influence of water dip caused by the water itself having low electric conductivity in the sample, which is a problem when quantifying anions of low concentration with increased sensitivity, is small, and in the method of small 5, the eluent is small. It is difficult to detect the carbonate ion itself in the sample at the same time as other anions because an alkali metal salt of carbonate ion is used as
The method according to the present invention also has an excellent feature that carbonate ions can be detected very easily without interfering with other anions.

[Brief description of drawings]

FIG. 1 is a flow diagram showing an embodiment of the present invention, FIG. 2 is a schematic diagram of an ion exchange reaction mechanism on a complex formation reaction support, and FIG. 3 is a saturated complex formation reaction support made of copper (II).
FIG. 4 is a diagram showing a lapse of regeneration time when regenerating with ions, FIG. 4 is a chromatogram obtained by Example 6 of the present invention, FIG. 5 is an example of a calibration curve obtained by Example 6 of the present invention, and FIG. Is a chromatogram obtained in Example 8 of the present invention, and FIG. 7 is a chromatogram of tap water obtained in Example 9 of the present invention.

Claims (4)

(57) [Claims] 1. An anion in a sample solution is separated by a separation column filled with an anion exchanger with an eluent containing an alkali metal salt or an alkaline earth metal salt of aminodicarboxylic acid and connected in series with the separation column. After introducing the eluent to the complex-forming reaction support, and forming a complex of a transition metal ion ionically bonded to the complex-forming reaction support and aminodicarboxylic acid on the complex-forming reaction support, A method for analyzing anions using a complex formation reaction, characterized in that the electric conductivity of anions in a sample solution is measured by an electric conductivity detector. 2. The analysis method according to claim 1, wherein the transition metal ion ionically bonded to the complex formation reaction support is a copper (II) ion. 3. The analysis method according to claim 1, wherein the complex-forming reaction support is a column packed with a gel in which sulfonic acid type cation exchange groups are chemically bonded. 4. The analysis method according to any one of claims (1) to (3), wherein the complex-forming reaction support is a membrane to which a sulfonic acid type cation exchange group is chemically bonded.