JP3408903B2 - Analysis method by ion chromatography - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ion chromatography analysis method in which a sample is caused to flow through a column together with an eluent, and target ions in the sample separated and eluted by the column are detected by an electric conductivity detector. .

[0002]

2. Description of the Related Art As one of the detection methods of ion chromatography (hereinafter abbreviated as IC), there is an electric conductivity detection method. A non-suppressor type IC by the electric conductivity detection method uses an organic acid ion having a low conductivity as an eluent, but the suppressor type IC mainly uses a carbonate buffer solution. In the suppressor type, a suppressor column was initially used in order to reduce the background of the eluate, but there was a problem in that continuous operation could not be performed for a long time because the ion exchange resin had to be regenerated. At present, this problem has been solved by the advent of ion-exchange membrane suppressors, but new problems have arisen in the durability of exchange liquids for suppressors and ion-exchange membranes.

A suppressor method has been developed in which a carbonate buffer solution as an eluent is mixed with a fine suspension of H ⁺ -type cation exchange resin to reduce the background of the eluent (Ana.
l. Chem., 62, 612 (1990)). An IC using a suspension prepared by refining a commonly used H ⁺ -type cation exchange resin has also been reported (Analytical Chemistry, 43, 75).
(1994)). Currently, ion exchange resin suspensions are commercially available. In this method, there is a problem in durability and stability of the pump used for feeding the suspension.

[0004]

All of the above suppressor methods are of the ion exchange type, but the object of the present invention is to reduce the background of the eluate without using the ion exchange type suppressor method. It is a thing.

[0005]

In the ion chromatography of the present invention, a quaternary ammonium hydroxide solution is used as the eluent, and the eluent in the flow path from the column outlet to the detector is ion-associating with an organic acid. A suppressor solution containing a reagent and a nonionic surfactant is added and mixed.

The present invention utilizes the neutralization of the eluent components and the reduction of electrical conductivity due to the extraction of micelles of the ionic associations.
In order to reduce the background electric conductivity of the eluate, a quaternary ammonium ion hydroxide (Q ⁺ · O
The H ⁻ ) solution is used as an eluent, and after passing through the column, it is mixed with an organic acid solution containing a nonionic surfactant as a suppressor solution. As a result, the OH ⁻ ion is neutralized, the quaternary ammonium ion and the organic acid ion form an ion association body, and form a mixed micelle with the nonionic surfactant. This reaction is called micelle extraction. Thus, the electrical conductivity of the eluate is significantly reduced by the neutralization and the micelle extraction. Further, by neutralizing the eluent with carbon dioxide and making a part of the eluted ions into HCO ₃ ⁻ and CO ₃ ²⁻ , the elution of the ions is accelerated.

[0007]

The quaternary ammonium hydroxide as eluant PREFERRED EMBODIMENTS eluent decyl trimethyl ammonium hydroxide ^{_{(C 10 H 21 -TMA + OH}} -), dodecyl trimethyl ammonium hydroxide (C ₁₂ H ₂₅ - TMA ⁺ OH ^-), or hydroxide tetradecyltrimethylammonium (C ₁₄ H
₂₉ -TMA ⁺ OH ^-) and alkyltrimethylammonium hydroxides such as tetrabutylammonium hydroxide ((n-
_{C 4 H 9) 4 NO 3} - N + OH -) and the like can be used.

The organic acid used in the suppressor solution is
p-Pentylbenzoic acid (H.PBC), myristic acid (H.MA), dodecylsulfonic acid (H.LS), dodecylbenzenesulfonic acid (H.DBS), or the like can be used. As the nonionic surfactant used in the suppressor solution, for example, the electric conductivity of itself is low,
Polyoxyethylene easy to prepare as an aqueous solution (10)
Octyl phenyl ether (TX-100) and the like can be used.

It was examined what kind of combination of ion-associating reagents is preferable. Various quaternary ammonium hydroxide (Q ⁺ · OH ⁻ ) and organic acid (H ⁺ · A ⁻ ) solutions having the same concentration were prepared, and the same volume was mixed to examine the electric conductivity value.
1.5 x 1 each of quaternary ammonium hydroxide and organic acid
The measurement result of the electric conductivity of the equimolar mixed solution in which 0 ^-3 M was mixed is shown. Toa Denpa CM for measuring the electrical conductivity of the solution
-30ET was used.

[Table 1] The numerical value in the parentheses represents the electric conductivity of each solution before mixing, and ethanol (4% (V /
V)) and the nonionic surfactant TX-100 (2%
(M / m)) was added. TX-100 of mixed solution
Is 1%, the electric conductivity of a 1% (m / m) aqueous solution of a commercially available diluted TX-100 solution is 10.1 μS.
It was cm- ¹ .

(Study of Ion-Associating Reagent) From the results shown in Table 1, the organic acid which is an anionic ion-associating reagent was examined. As a result, dodecylbenzenesulfonic acid (HD
(BS) is most likely to undergo ion association. Further, from the comparison of dodecyl sulfonic acid (H · LS) and myristic acid _{(H · MA), -OSO 3} - the it is -COO ^- seen to be susceptible to ionic association than.

Among the cation and anion pairs shown in Table 1, dodecyltrimethylammonium hydroxide (C ₁₂ H ₂₅ -T
MA ⁺ OH ^-) and tetradecyl trimethyl ammonium hydroxide ^{_{(C 14 H 29 -TMA + OH}} -) is the easiest to ion association and dodecylbenzenesulfonic acid (H · DBS), the electrical conductivity of the mixed solution TX-100 The value is close to the electric conductivity of the aqueous solution containing only. This means that the neutralization of H ⁺ and OH ⁻ and the ionic association are almost completely performed. As the anionic ion-associating reagent, dodecylbenzenesulfonic acid (H · DBS), which has high hydrophobicity and high reactivity, is preferable.

[0012] The (eluent selection) suppressor was dodecylbenzenesulfonic acid and (H · DBS) and a solution containing TX-100, dodecylbenzenesulfonate ions eluent (DBS ^-) and to form an ion associate , TX-1
Micellar extraction of dodecyltrimethylammonium hydroxide (C ₁₂ H ₂₅ -TMA ⁺ ) and tetradecyltrimethylammonium hydroxide (C ₁₂
Separate detection of anions was carried out using a solution of ₁₄ H ₂₉ -TMA ⁺ ) hydroxide. Tetradecyltrimethylammonium hydroxide ^{_{(C 14 H 29 -TMA + OH}} -) if was used as eluent, to increase the quaternary ammonium ions to be adsorbed to the filler, also increases ion exchange capacity, retention of sample ions Time will increase. Also, the more hydrophobic the eluent, the later the system peak appears. For this reason,
Tetradecyl trimethyl ammonium hydroxide (C ₁₄ H ₂₉
-TMA ⁺ OH - ^If) of the eluent peak system peak and Cl- ions overlap even at low concentrations. Therefore, it is preferable to use dodecyltrimethylammonium hydroxide ion (C ₁₂ H ₂₅ -TMA ⁺ ) as the eluent.

Further, as a result of examination on rapid determination of SO ₄ ²⁻ of divalent ions, it was possible to shorten the retention time of SO ₄ ²⁻ by using an eluent of carbonate in which carbon dioxide was blown. .

Next, the principle of the micelle extraction suppressor method for ion associations according to the present invention will be described in detail. This method reduces the concentration of the conductive substance by utilizing the reaction of forming an ion association product in an aqueous solution represented by the following formula (1) -mixing micelle formation reaction with a nonionic surfactant. Q ⁺ + A ⁻ = (Q ⁺ · A ⁻ ) _m (1) Here, Q ⁺ , A and (Q ⁺ · A ⁻ ) _m are a hydrophobic cation, a hydrophobic anion and between these ions, respectively. The mixed micelle of the formed ion association body with a nonionic surfactant is shown. Further, the equilibrium constant K _ass of the equation (1) is expressed by the following equation. ^{_{K ass = [(Q + ·}} A -) m] / [Q +] [A -] (2)

Now, consider the case where an eluent of Q ⁺ .OH ⁻ is used. In the eluent Q ⁺ · OH ^-, H suppressor solution
Assuming that the concentrations of A (sulphonic acid, carboxylic acid, etc. of organic acid) are the same, and that they are mixed at a molar ratio of (1: 1) after passing through the column, the neutralization reaction of the following formula (3) and the formula (3) The ion association micelle extraction reaction of 1) occurs. ^{Q + · OH - + HA =} Q + + A - + H 2 O (3)

After passing through the detector, the following relationships are established. (A) In the absence of sample ion S- (which is a conjugate base of strong acid), the principle of mass balance and electrical neutrality
Equations (4) and (5) are established. _{^{[Q +] o + [(}} Q + · A -) m] o = [A -] o + [HA] o + [(Q + · A -) m] o (4) [Q +] o + [ H ⁺ ] _o = [A ⁻ ] _o + [OH ⁻ ] _o (5) Here, [] _o indicates the time when the sample ion S ⁻ does not exist. Since Q ⁺ .OH ⁻ is a strong base and HA is a sulfonic acid or a carboxylic acid, it can be considered that these react quantitatively (99.9% or more), and [HA] _o ,
_{^{[H +] o, [OH}} -] o can be ignored with respect to the other terms. Therefore, equations (4) and (5) are both equation (6). _{^{[Q +] o = [A}} -] o (6)

(B) When the sample ion S − is present, the following relationship is established. _{^{[Q +] s + [(}} Q + · A -) m] s = [A -] s + [HA] s + [(Q + · A -) m] s (7) [Q +] s + [ ^{_{H +] s = [a -}} ] s + [S -] s + [OH -] s (8) where, _s is the sample ions S [] ^- indicates the concentration at which are present. Since some of which remain to be neutralized acid HA in this ^{case, [H +] >> [OH} -] , and the right side of equation (8) - for the other terms ^[OH] You can ignore it,
Equations (7) and (8) become equations (9) and (10), respectively. _{^{[Q +] s = [A}} -] s + [HA] s (9) [Q +] s + [H +] s = [A -] s + [S -] s (10) In addition, the usual inorganic It can be considered that the ionic association (Q ⁺ · S ⁻ ) of the anionic sample ions is not subjected to micelle extraction.

The electric conductivity under the condition (a) is (C
D) _o, when the electric conductivity of the (CD) _s under the conditions of _{(b), (CD) o} = λ Q + [Q +] o + λ A- [A -] o + λ H + [H +] o ^{_{+ λ OH- [OH -] o}} (11) (CD) s = λ Q + [Q +] s + λ A- [A -] s + λ H + [H +] s + λ S- [S -] s + λ OH- [ OH ⁻ ] _s (12) Here, λ represents the equivalent electric conductivity. Of these, [H ⁺ ] _o ,
Since [OH ⁻ ] _o and [OH ⁻ ] _s are usually very small, about 10 ⁻⁶ to 10 ⁻⁸ M, their electrical conductivity can be ignored. From the equations (11) and (12), the electric conductivity corresponding to the peak when the sample ions are present is shown by the following equation. Δ (CD) = (CD) s - (CD) o = λ Q + ([Q +] s - [Q +] o) + λ A- ([A -] s - [A -] o) + λ H + [H ⁺ ] _s + λ _S- [S ⁻ ] _s (13) When the ion association micelle extraction constant of the equation (2) is sufficiently large, the following equation is satisfied from the equations (6) and (7). _{^{[Q +] o = [A}} -] o ≒ 0 [A -] may be considered that _s = 0. Therefore, the equation (13) becomes the following equation. Δ (CD) = λ _{Q +} [Q ⁺ ] _s + λ _{H +} [H ⁺ ] _s + λ _S− [S ⁻ ] _s (14) From the equations (14) and (9), the following equation is obtained. Δ (CD) = λ _{Q +} [HA] _s + λ _{H +} [H ⁺ ] _s + λ _S− [S ⁻ ] _s (15) Alternatively, the following equation (16) can be obtained from the equation (10).

Further, in the case of the strong acid HA has a -SO ₃ H is, Δ (CD) = (λ Q + -λ H +) [HA] s + (λ H + + λ S-) [S -] s ( 16) Further, when HA is a strong acid having —SO ₃ H, [HA] _s = 0 [Q ⁺ ] _s = [A ⁻ ] _s ≈0, and the equation (10) becomes the following equation. [H ⁺ ] = [S ⁻ ] (17) Therefore, the following relation is obtained from the equation (14). _{Δ (CD) = λ H +} [H +] s + λ S- [S -] s = (λ H + + λ S-) [S -] s (18)

Since (λ _{Q +} -λ _{H +} ) [HA] <0 in Formula 16, it can be seen that ΔCD is considerably smaller when HA is a weak acid than when it is a strong acid of Formula (18). In the case of a strong acid, ΔCD (peak height) is proportional to [S ⁻ ], and the proportionality constant is the sum of equivalent electric conductivity of H ⁺ and S ⁻ (λ _{H +} + λ
_S- ), whereas it is not necessarily proportional in the case of weak acids. Further, it can be seen from the following equation that the background electric conductivity is also greatly reduced by the neutralization and micelle extraction / suppressor methods. ^{_{CD = λ Q + [Q +}} ] + λ OH- [OH -] Δ (CD) = λ Q + [Q +] + λ OH- [OH -] - (λ Q + [Q +] o + λ A- [A -] o ^{_{) ≒ λ Q + [Q +}} ] + λ OH- [OH -] where, CD is the electrical conductivity of the case where no micelles extraction.

It is also understood that when the carbonate is used, the reaction of the following formula occurs and the background electric conductivity is lowered. (HA is a strong _{^{acid) Q + (HCO 3 -)}} + HA = (Q + · A -) m + CO 2 + H 2 O (Q +) 2 (CO 3 2-) + 2HA = 2 (Q + _{^{· A -) m + CO 2}} + H 2 O

[0022]

EXAMPLE FIG. 1 shows the outline of ion chromatography to which the present invention is applied. The eluent 1 and the suppressor liquid 2 are sent by the liquid feed pump 3 at 1.0 ml / min. The eluent 1 is supplied to the column 5, and a sample injection valve (50 μml or 100 μml) 4 is arranged upstream of the column 5 in the eluent flow path to inject the sample. Column 5 is a column for anion analysis,
For example, Tosoh TSKgel IC-Anion PW (4.6 mm
× 50 mm) can be used. 6 is a constant temperature water tank, which keeps the column 5 at a constant temperature of 35 ° C. The eluate of the column 5 and the suppressor liquid 2 are mixed in the flow path and enter the mixing coil (inner diameter 0.5 mm, length 50 cm) 7, and after being mixed uniformly, they are guided to the electric conductivity detector 8 for detection. Is performed. 9 is a recorder for recording the detection output, 10
Is a waste liquid container for receiving the waste liquid after detection.

Dodecyltrimethylammonium hydroxide was used as the eluent for Eluent 1. It was prepared by passing a solution of quaternary ammonium bromide or chloride through an anion exchange resin (Amberlite IRA-400) in the OH ⁻ form. The concentration of dodecyltrimethylammonium hydroxide in the eluent was 1.5 × 10 ⁻³ M, and carbon dioxide was blown into the eluent to adjust the pH to 10.8.

Dodecylbenzene sulfonic acid (H.DBS) was used as the organic acid contained in the suppressor solution. This was prepared by passing a solution of sodium salt through a cation exchange resin (Amberlite IR-120B) which was H type. Suppressor solution is H ・ DBS (1.5 × 1
0 ^-3 M) and TX-100 (4% (W / W)) was intended to include.

Using this eluent and suppressor solution, a sample containing 2 × 10 ⁻⁴ M each of F ⁻ , Cl ⁻ , NO ₂ ⁻ , Br ⁻ , NO ₃ ⁻ , I ⁻ and SO ₄ ²⁻ was separated. The chromatogram of the result is shown in FIG. 1 is a system peak, 2 is F ⁻ , 3 is Cl ⁻ , 4 is NO ₂ ⁻ , 5 is Br ⁻ , 6
Is NO ₃ ⁻ , 7 is SO ₄ ²⁻ , 8 is I ⁻ , and it is understood that they are well separated from each other.

[0026]

INDUSTRIAL APPLICABILITY In the present invention, a quaternary ammonium hydroxide solution is used as an eluent, and an ion-associating reagent composed of an organic acid and a nonionic surfactant are used as the eluent in the flow path from the column outlet to the detector. Since the suppressor solution containing is added and mixed, the background of the eluate can be reduced without using the ion exchange type suppressor system, and various anions can be separated and quantified.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing ion chromatography to which the present invention is applied.

FIG. 2 is a waveform diagram showing a chromatogram obtained in one example.

[Explanation of symbols]

1 Eluent 2 suppressor liquid 3 Liquid feed pump 4 sample injection valve 5 columns 6 constant temperature water tank 7 mixing coil 8 Electric conductivity detector

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-63-300960 (JP, A) JP-A-3-172758 (JP, A) JP-A-63-44166 (JP, A) JP-A-61- 2072 (JP, A) Analytical Chemistry, Japan, 1989, 38, 147-170 Analytical Chemistry, Japan, December 5, 1995, 44, 1041-1048 Fresenius Z Anal Chem, 1987, 327, 451-455 ( 58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 30/84 G01N 27/06 G01N 30/26 G01N 30/64

Claims (2)

(57) [Claims] 1. An analytical method by ion chromatography in which a sample is caused to flow through a column together with an eluent, and a target ion in the sample separated and eluted by the column is detected by an electric conductivity detector. A feature is that an ammonium solution is used, and a suppressor solution containing an ion-associating reagent composed of an organic acid and a nonionic surfactant is added to and mixed with the eluent of the flow path from the column outlet to the detector. Analysis method. 2. The analysis method according to claim 1, wherein the eluent is neutralized with carbon dioxide.