JP2000180429A - Method for separation analysis of anion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform the simultaneous separation analysis of a monovalent anion and a polyvalent anion by using an eluent containing a specific salt compound, easily controlling the elution behavior of an anion to be measured, and reducing background electric conductivity. SOLUTION: An eluent is used in the case when separating and analyzing an anion by the suppresser system ion chromatography, and it contains a salt compound. In the salt compound, the first acid dissociation index pKal (25 deg.C) in a solution at the time the compound having become an acid type chemical seed species is at least 6.5. Concrete speaking, as such salt compound, arsenious acid, boric acid, hypochlorous acid, hydrocyanic acid, hypoiodous acid hydrogen sulfide acid, and silicic acid can be mentioned as the acid compound. In a chromatograph that is obtained by using the eluent, peaks 1 and 2 of, for example, a fluoride ion and a chloride ion appear at both sides of carbonate ion. In this manner, even in a sample containing the carbonate ion, the anions to be measured can be accurately determined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an eluent used for separating and analyzing anions by suppressor-type ion chromatography and a method of separating and analyzing anions by suppressor-type ion chromatography using the eluent. It is about.

[0002]

2. Description of the Related Art Suppressor-type ion chromatography for separating and analyzing ions in an aqueous sample solution is described below.
H. in 1975 Published by Small and his collaborators (Anal. Chem., 47,
1801 (1975), and JP-B-56-2310
No. 0).

This method is a method in which ion exchange chromatography and an electric conductivity detector are combined, and is an ion exchange (suppressor) in which a counter ion opposite to an ion to be measured is electrostatically bound after a separation column. By connecting the column, the electrolyte of the eluent composition is converted into a chemical species having a low degree of dissociation, and high sensitivity detection of the inorganic ion to be measured is achieved.

[0004] Later, a suppressor-type ion chromatography using an ion exchange membrane capable of continuous regeneration instead of the suppressor column was described in T. Suzuki. S. Proposed by Stevens (Anal. Chem., 53, 1488).
(1981) and Japanese Patent Publication No. 62-29024), and this method is now mainstream.

[0005] In the suppressor-type ion chromatography, the eluent is eluted from the anion exchange column for separation and analysis, after the target anion is eluted, before being introduced into the electric conductivity detector. The solution is passed through a hydrogen ion-type cation exchanger used as a suppressor medium for the purpose of lowering the background electrical conductivity. Here, the salt compound in the eluent is ion-exchanged with hydrogen ions and converted into an acid type chemical species having a low degree of dissociation, thereby increasing the peak sensitivity of the anion to be measured in the electric conductivity detector. It is done.

[0006]

Prior to the detection by the electric conductivity detector, for the purpose of lowering the background electric conductivity in the suppressor, conventionally, mainly dilute sodium hydroxide, sodium carbonate or hydrogen carbonate was used. Sodium is used alone or in combination.

For example, sodium hydroxide is used as a suppressor
Passing through the medium should theoretically convert it to water with zero electrical conductivity when introduced into the electrical conductivity detector, thus providing an ideal eluent.

However, in practice, when an eluent containing sodium hydroxide is used, phosphate ions, which are one of the anions to be measured, are completely trivalent negative ions,
Compared to other anions, for example, monovalent chloride ion, nitrate ion, divalent sulfate ion, etc., the selectivity with the gel in the anion exchange column for separation and analysis is greatly different. As a result, It has a problem that it tends to elute with a considerable delay. In addition, since the elution behavior of these anions to be measured can be controlled only by the concentration of sodium hydroxide, if the concentration of sodium hydroxide in the eluate is increased in order to accelerate the elution of phosphate ions, other, especially monovalent, ions can be obtained. This has the problem of adversely affecting the anion separation analysis. Furthermore, if the eluent containing sodium hydroxide is used or left in contact with air, the eluent will gradually absorb carbon dioxide in the air, and as a result, the shade of each measurement object will be affected. There is also a problem of causing a phenomenon in which ion elution is accelerated.

[0009] Any problem that arises when an eluent containing sodium hydroxide is used causes impairment of the reproducibility and quantitativeness of anion separation analysis. At least the latter problem can be solved by not allowing the eluent to come into contact with air.However, for this purpose, operability is sacrificed, such as by strictly controlling the container holding the eluent, and Special equipment and equipment must be used.

In the case of carbonate, C is mainly responsible for elution of the anion to be measured from the anion exchange column for separation and analysis.
O ₃ ^2-or HCO ^3- is sodium hydroxide OH ^- and to have a strong dissolution power compared to, phosphorus since the mixing ratio of sodium carbonate and sodium bicarbonate can be adjusted in pH The valence of the acid ion can be controlled,
As a result, compared to an eluent containing sodium hydroxide, a more suitable eluent can be provided which can elute a plurality of anions to be measured within a relatively short time and in a balanced state of each ion. It should be possible.

However, when the eluent containing a carbonate is passed through a suppressor medium, it does not completely become water after the passage, but is dissociated in a part of hydrogencarbonate ions, though it is a part, However, there is a problem that the electric conductivity is high (the background is high). As a result, the water dip of the negative peak due to the water itself contained in the sample solution becomes remarkably large as compared with the case where the eluent containing sodium hydroxide is used, and a small amount of fluoride eluted in the vicinity. There is a problem in that an influence is exerted on the positive small peak of the ion, for example, and an error occurs in the quantitative result. Furthermore,
In the eluent containing carbonate, as a peak other than the anion to be measured, in addition to the water dip, a peak of the carbonate ion itself in the eluent and a peak of the carbonate ion contained in the sample appear. However, there is also a problem that it interferes with the quantification of the peak of the anion to be measured.

Accordingly, an object of the present invention is to provide a method for separating hydrogen ions between an anion exchange column for separating and analyzing anions and an electric conductivity detector for measuring the electric conductivity of components eluted from the column. Eluent used for the separation and analysis of anions by suppressor-type ion chromatography in which a cation-exchanger is arranged. Compared to the eluent conventionally used, which contains sodium hydroxide as a salt compound. As a result, the elution behavior of the anion to be measured can be easily controlled, and the background electric conductivity can be reduced as compared with an eluent containing sodium carbonate as a salt compound.
In particular, the influence of water dip, which is a problem when quantifying a small amount of fluoride ions, can be reduced, and a peak derived from a salt compound in the eluent and hindering the quantification of the peak of the anion to be measured appears. Another object of the present invention is to provide an eluent that enables simultaneous and short-time separation analysis of monovalent anions and polyvalent anions such as sulfate ions and phosphate ions.

[0013]

Means for Solving the Problems The present invention made to achieve the above object is an eluent used for separating and analyzing anions by suppressor-type ion chromatography, which contains a salt compound. Wherein the salt compound is
An eluent characterized by having a first acid dissociation index pKa1 (25 ° C.) of 6.5 or more in an aqueous solution when it becomes an acid type chemical species. The present invention also relates to a method for separating and analyzing anions by suppressor-type ion chromatography, wherein the first acid dissociation index pKa1 (25 ° C.) in an aqueous solution when it becomes an acid type chemical species is 6.5 or more. The above method, wherein an eluent containing a salt compound is used. Hereinafter, the present invention will be described in more detail.

When sodium carbonate or a mixture of sodium carbonate and sodium hydrogencarbonate is used as a salt compound when anions are separated and analyzed by suppressor-type ion chromatography, this is converted to a hydrogen ion-type cation exchange medium serving as a suppressor medium. It is carbonic acid (H ₂ CO ₃ ) that is produced by passing through the body.

Carbonic acid is partially dissociated in an aqueous solution to form hydrogen ions H ⁺ and hydrogen carbonate ions HCO ₃ ⁻ , and this ionic equilibrium increases the background electrical conductivity compared to water.
For this reason, the water itself in the sample does not undergo ion exchange interaction in the anion exchange column for separation and analysis, and appears as a water dip as a negative peak when introduced into the electric conductivity detector. Quantification is obstructed by, for example, overlaying a positive minute peak of a small amount of fluoride ion eluted in the vicinity thereof.

Therefore, in the present invention, in order to reduce the size of the water dip, the water is passed through a hydrogen ion type cation exchanger, which is a suppressor medium, so that the electric conductivity of the aqueous carbonate solution is reduced. Provided is an eluent containing a salt compound that produces an aqueous acid solution that is low, that is, has a lower degree of dissociation than carbonic acid.

The salt compound contained in the eluent of the present invention has a first acid dissociation index pKa1 (25 ° C.) of pKal of an aqueous carbonate solution.
It is an acid compound having a pKal (25 ° C.) of 6.5 or more, which is higher than (6.35). In such a salt compound, the degree of dissociation with hydrogen ions is smaller than that of carbonic acid, and therefore, the background electric conductivity can be reduced as compared with that of the aqueous carbonate solution.

More specifically, as the salt compound contained in the eluent of the present invention, arsenous acid (H ₃ AsO ₃ , pKal (2
9.1), boric acid (H ₃ BO ₃ , pKal (25
C) 9.24), hypobromous acid (HBrO, pKal
(25 ° C) 8.62), hypochlorous acid (HClO, pKa)
1 (25 ° C.) 7.53), hydrocyanic acid (HCN, p
Kal (25 ° C.) 9.22), hypoiodic acid (HIO,
pKal (25 ° C.) 10.64), hydrosulfide acid (H
_{2 S, pKal (25 ℃)} 7.02), silicic acid (H ₂ Si
O ₂ (OH) ₂ , pKal (25 ° C. 9.86) can be exemplified. Above all, sodium tetraborate (Na ₂ B ₄ ) which is a boric acid salt compound (sodium salt)
A boric acid salt compound represented by O ₇ ) is a particularly preferred salt compound in view of the toxicity and stability of the compound itself.

The above-mentioned salt compound, particularly preferably a salt compound of boric acid such as sodium tetraborate, is detected by an electric conductivity detector when an impurity is present (chromatogram).
Therefore, it is preferable to use one having as high a purity as possible. When a commercially available salt compound is purchased and used, the use of a high-purity special-grade standard can be exemplified. The salt compound is present in the eluent at a final concentration of 0.1
What is necessary is just to use it so that it may be set to about 10 mM.

The eluent of the present invention may contain, in addition to the salt compound described above, an acidic or basic compound for adjusting the pH of the eluent to a predetermined range. pH range is 5
The pH can be appropriately determined in the range of from 1 to 12, and particularly preferably when a boric acid salt compound such as sodium tetraborate is used, the pH is preferably adjusted to about 11. Depending on the pH of the eluent, the peak appearance position of the measurement target anion appearing in the chromatogram fluctuates, and in some cases, it affects the quantification of the peak due to, for example, the appearance of a carbonate ion appearing as a result of sample injection. This is because there are times. In the eluent containing sodium tetraborate, the peak appearance position of the anion to be measured can be controlled by adjusting the pH of the eluent to about pH 11, so that the detection is not affected by the peak of the carbonate ion. .

When the eluent contains only a salt compound, its pH is naturally determined by the salt compound used. Therefore, when adjusting the pH of the eluent for the purpose of eliminating the influence of the fluctuation of the peak appearance position of the measurement target anion as described above, an acidic compound or a basic compound should be added. Can be exemplified. The acidic compound or the basic compound may be appropriately selected and used in consideration of the salt compound to be used, and is not particularly limited. For example, when sodium tetraborate is particularly preferably used as a salt compound, boric acid having a final concentration of 10 to 200 mM as an acidic compound and sodium hydroxide having a final concentration of 1 to 10 mM as a basic compound can be exemplified. .

As described above, for example, boric acid or sodium hydroxide is added to an eluent containing sodium tetraborate, and
By adjusting H, it is possible to control the elution of the anion species to be measured and to simultaneously detect and quantify many kinds of anions to be measured.

The eluent of the present invention is used in a method for separating and analyzing anions by suppressor type ion chromatography. Suppressor-type ion chromatography dissociates the salt compound ions contained in the eluate before introducing the eluate containing the anion to be measured eluted from the ion exchange column for anion separation analysis into the electric conductivity detector. The present invention is characterized in that a suppressor device for weakly ionizing ions by exchanging ions with low degrees is used.

The eluent of the present invention, as suppressor devices, for example, how a column type using a cation exchange column capable of releasing H ^+, ions eluate suppressor medium supplied as a suppressor device for example H ⁺ It can be used in any of the membrane type methods of contacting through an exchange membrane.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION In the following, in order to explain the present invention in more detail, examples will be described as embodiments, but the present invention is not limited to the described examples.

Example 1 A commercially available ion exchange column for anion separation / analysis (manufactured by Tosoh Corporation, trade name: TSKgel IC-A)
(nion-PWXL, inner diameter 4.6 mm, length 35 mm)
Was analyzed for anions using an eluent containing 3 mM sodium tetraborate as eluent. The eluent contains
The final concentration was 12 to adjust the pH to pH 5.4.
Boric acid was added to 0 mM.

The analysis was performed under the conditions of a column temperature of 40 ° C. and an eluent flow rate of 1.0 ml / min. As a suppressor device, a column filled with H ⁺ releasing cation exchange resin after the separation column (Alltech) Product name: Solid-phase chemical suppressor module for ion chromatography, Model 335, 4.6 mm inside diameter, 100 m length
m) was arranged and carried out in a column type manner.

The sample was prepared as standard anions (1) fluoride ion (1 mg / l) and (2)
Chloride ion (2 mg / l), (3) nitrite ion (4
mg / l), (4) bromide ion (4 mg / l),
(5) nitrate ion (4 mg / l), (6) phosphate ion (10 mg / l) and (7) sulfate ion (10 mg / l)
1) is a mixed aqueous solution (20 μl) containing 1).

The sample was injected into an ion exchange column for separation and analysis, the eluted eluate was passed through a suppressor device, and the eluate was detected by an electric conductivity detector to obtain a chromatogram. The obtained chromatogram is shown in FIG.

In this embodiment, the pH of the eluate introduced into the electric conductivity detector is 5.4, the background electric conductivity is 9 μS / cm, and seven kinds of anion peaks appear. Took about 12 minutes. In FIG. 1, asterisks (*) are attached to the carbonate ion peak elution positions. In this example, the respective fluoride ion and chloride ion peaks appear on both sides of the carbonate ion, and even in the sample containing the carbonate ion. It can be seen that these anions to be measured can be accurately quantified.

In FIG. 1, of the two negative peaks eluted before the fluoride ion, the first peak is a water dip derived from water, and the second is boric acid added to the eluent. This is a peak derived from

For comparison, a chromatogram was obtained in the same manner as in Example 1, except that an eluent containing 4 mM sodium hydroxide was used. The results are shown in FIG. The pH of the eluent is pH 11.6. In this case, the background electric conductivity of the eluate inserted into the electric conductivity detector was close to that of water, but it took about 24 minutes for seven kinds of anion peaks to appear.

For comparison, a chromatogram was obtained in the same manner as in Example 1 except that an eluent containing 0.75 mM sodium carbonate and 0.71 mM sodium hydrogen carbonate was used. FIG. 4 shows the results. The pH of the eluent is pH 10.21. In this case, the background electric conductivity of the eluate inserted into the electric conductivity detector is 12
μS / cm, and it took about 14 minutes for seven kinds of anion peaks to appear. In FIG. 4, the * mark is added to the elution position of the carbonate ion peak. In this comparative example, a carbonate ion peak appears immediately before the fluoride ion peak, which impedes the determination of nitrite ion. Moreover,
It can also be seen that a negative water dip appears in front of the fluoride ion peak and hinders the quantification of trace fluoride ions.

Example 2 A chromatogram was obtained in the same manner as in Example 1, except that an eluent containing 0.6 mM sodium tetraborate and 3.3 mM sodium hydroxide was used. The results are shown in FIG. The pH of the eluent is pH 11.2.

In this embodiment, the pH of the eluate inserted into the electric conductivity detector is 11.2, the background electric conductivity is 7 μS / cm, and seven kinds of anion peaks appear. Took about 13 minutes. In this example, the elution positions of phosphate ions and sulfate ions were reversed by changing the pH of the eluent. In FIG. 2, the elution position of the carbonate ion is marked with an asterisk (*). In this example, the peaks of the nitrate ion and the sulfate ion appear on both sides of the carbonate ion. It can be seen that the target anion can be measured accurately.

[0036]

As described above, according to the present invention, for example, a suppressor-type ion chromatography for separating and analyzing anions in a mixture of sodium tetraborate and an eluent containing boric acid or sodium hydroxide. By adopting the eluent as the negative eluent, it becomes possible to reduce the background electric conductivity due to the suppressor effect as compared with the eluent containing carbonate which is usually used.
Further, for example, when an eluent containing sodium hydroxide is used, the problem is that trivalent negative phosphate ions are eluted much slower than other monovalent chloride ions or divalent sulfate ions. Also, there is a problem that the water dip, which is a large negative peak, affects the positive minute peak of trace fluoride ions, and the anion to be measured is affected by the peak of carbonate ions derived from the eluent or the sample. The problem can be solved at once.

For example, in an eluent containing sodium tetraborate and boric acid or sodium hydroxide for adjusting the pH, sodium carbonate, which is a commonly used salt compound, and p
Compared to the eluent containing sodium bicarbonate or the like for adjusting H, the background electric conductivity after passing through the hydrogen ion type cation exchanger as the suppressor medium is about 25.
-40% lower, so that the drop in the negative peak of the water dip due to the water itself in the sample can be remarkably reduced.

By using the eluent of the present invention, in addition to the above effects, it is possible to separate and analyze polyvalent anions such as phosphoric acid in a short time.

[Brief description of the drawings]

FIG. 1 is a chromatogram showing the results of Example 1.
In the figure, peak 1 is a fluoride ion, peak 2 is a chloride ion, peak 3 is a nitrite ion, peak 4 is a bromide ion, peak 5 is a nitrate ion, peak 6 is a phosphate ion, and peak 7 is a phosphate ion. Is a peak of sulfate ion.

FIG. 2 is a chromatogram showing the results of Example 2.
The peaks 1 to 7 in the figure are the same as in FIG.

FIG. 3 is a chromatogram obtained by a comparative operation performed in Example 1. The peaks 1 to 7 in the figure are the same as in FIG.

FIG. 4 is a chromatogram obtained by a comparative operation performed in Example 1. The peaks 1 to 7 in the figure are the same as in FIG.

Claims (3)

[Claims] 1. An eluent used for separating and analyzing anions by suppressor-type ion chromatography, wherein the eluent contains a salt compound, wherein the salt compound is used when it becomes an acid type chemical species. First acid dissociation index pKa1 in aqueous solution
(25 ° C.) 6.5 or more. 2. The eluent according to claim 1, wherein said salt compound is sodium tetraborate. 3. A method for separating and analyzing anions by suppressor-type ion chromatography, wherein the first acid dissociation index pKa1 (2) in an aqueous solution when it becomes an acid type chemical species.
(5 ° C.) using an eluent containing a salt compound having a value of 6.5 or more.