JPH0752176B2 - Electrochemical detector - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrochemical detector for high performance liquid chromatography.

(Prior Art) An electrochemical detector (ECD) is used as one of detectors for a substance to be measured in high performance liquid chromatography. The ECD is a detector that performs quantification by measuring the current generated by changes (oxidation, reduction, etc.) in the measured substance due to electrochemical reactions. This ECD is composed of a cell having a working electrode, a counter electrode and a reference electrode, and a detector. As the working electrode of ECD, glassy carbon obtained by molding graphite carbon at a pitch and sintering it at around 2000 ° C is widely used. Glassy carbon has high hardness, excellent chemical stability, and high sensitivity when used as an electrode. However, when the detection sensitivity is increased to measure a minute amount of the substance to be measured (for example, when the measurement is performed at a high potential of 1000 mV or higher), the noise level increases, and thus the sensitivity is substantially reduced. Although the cause thereof is not clear, it is considered that the surface portion of the glassy carbon electrode is eroded and partially collapsed due to the application of voltage. Further, since the glassy carbon is obtained by sintering at a high temperature of around 2000 ° C as described above, variations (density, uniformity, etc.) occur among lots due to subtle differences in the conditions.

On the other hand, a solid potential (polymer-carbon electrode) formed by adding a synthetic polymer compound as a binder to graphite carbon has been proposed. Examples of the polymer compound (polymer) used include Kel-F (manufactured by Daiflon), Teflon (registered trademark), polypropylene, and styrene-divinylbenzene copolymer. These polymer-carbon electrodes have the advantage that they have a lower background current (BGC) value and therefore a lower background noise level than the above-mentioned glassy carbon. However, among the above polymers, for example, polypropylene and styrene-divinylbenzene copolymer are dissolved in an organic solvent or swollen by the organic solvent. Therefore, when the eluent contains an organic solvent such as acetonitrile or methanol, the electrode surface swells over time, making measurement difficult. Fluorine resins such as Teflon (registered trademark) have excellent resistance to organic solvents, but have poor wettability with carbon and high melt viscosity, making it difficult to uniformly disperse fine carbon particles. As a result, the electrode tends to have unstable electrode characteristics.

(Problems to be Solved by the Invention) The present invention solves the above-mentioned conventional drawbacks, and an object of the present invention is to perform stable and highly accurate measurement even when measurement is performed in a high potential state. An ECD having a working electrode capable of Another object of the present invention is to
An object of the present invention is to provide an ECD having a polymer-carbon working electrode having the above-mentioned excellent properties and high organic solvent resistance.

(Means and Action for Solving Problems) An electrochemical detector of the present invention is an electrochemical detector for high performance liquid chromatography having a cell having a working electrode, a counter electrode and a reference electrode; The working electrode is composed of a composition mainly composed of polyphenylene sulfide resin and graphite carbon, whereby the above object is achieved.

The polyphenylene sulfide contained in the composition constituting the working electrode of the ECD of the present invention is considered to be a polymer having a basic unit represented by the formula C ₆ H ₄ —S and having a complicated crosslinked structure. . The organic solvent resistance of this polymer is considered to be slightly inferior to that of fluororesin, but it does not dissolve at all in organic solvents for chromatography such as acetonitrile and alcohol solvents. Thus, it has excellent resistance to organic solvents, as well as excellent acid and alkali resistance. Since this polymer has good adhesion to graphite carbon, a uniform working electrode can be obtained. Further, since it is a thermoplastic resin, an electrode having a complicated shape can be easily prepared by, for example, injection molding.

The particle size of the graphite carbon contained in the composition is not particularly limited, but is usually 0.5 to 100 μm. This graphite carbon is contained in the composition in a proportion of 30 to 80% by weight. If the amount is too small, the conductivity of the working electrode is low and the absolute sensitivity is low. When it is excessive, the BGC value increases and the relative sensitivity decreases.

The working electrode used in the present invention is obtained by mixing the above-mentioned polymer and graphite carbon, and optionally other additives, and heating and molding. As the counter electrode and the reference electrode of the ECD of the present invention, any of the counter electrode and the reference electrode used in a usual ECD can be used. As the material for the counter electrode, stainless steel (SUS316), which has good conductivity and is resistant to corrosion, is used. A silver / silver chloride electrode or the like is used as the reference electrode. For the detector that measures the current due to the electrochemical reaction of the substance to be measured, a normal ECD detector is used, such as amplifying a minute current and measuring it as a voltage.

ECD cell 1 with working electrode, counter electrode and reference electrode set
The structure of 0 is shown, for example, in the sectional view of FIG.

This ECD cell 10 has cell channels 11 and 12 and a reference electrode 1
5 holds the cell block 1, a block holding plate 3 that rotatably connects the cell block 1 to a cell holder (not shown), and a working electrode 21 attached to the block holding plate 3.
And a working electrode portion 2 for holding

The cell block 1 has a rectangular parallelepiped shape, and has a protrusion 1a at the center of one side surface. The block holding plate 3 has a through hole that fits into the protruding portion 1a, and one side surface of the cell block 1 on which the protruding portion 1a is formed is brought into contact with one side surface of the block holding plate 3 to form the protruding portion. 1a is fitted in the through hole of the block holding plate 3.

The block holding plate 3 has connecting portions 31 and 32 projecting in a direction opposite to the side where the cell block 1 is arranged, on the upper and lower sides of the side surface opposite to the contact surface of the cell block 1, and on the lower side thereof. The connecting portion 32 is pivotally supported by a cell holder (not shown). The upper connecting portion 31 is attachable to and detachable from the cell holder.

At the center of the cell block 1, a pair of upper and lower cell channels 11 and 12 penetrating the cell block 1 in the horizontal direction are arranged. An opening at one end of each of the cell flow paths 11 and 12 is formed on the end surface of the protrusion 1 a of the cell block 1. Cell flow path
The inner wall of 12 is formed of a conductive metal, and this constitutes the liquid contact part 13. A counter electrode (not shown) is provided at the liquid contact portion 13 so as to come into contact with the fluid flowing through the liquid contact portion 13. A reference electrode insertion hole 14 is provided in the upper portion of the cell block 1. The reference electrode insertion hole 14 is open upward from the upper surface of the cell block 1, and the lower end portion thereof reaches the cell flow passage 12 above the central portion of the cell block 1. A reference electrode is inserted in the reference electrode insertion hole 14.

The working electrode portion 2 is arranged between the connecting portions 31 and 32 of the block holding plate 3. The working electrode portion 2 is fixed to a side surface of the block holding plate 3 opposite to the cell block 1 contact surface by a pair of guide pins 25. The working electrode portion 2 has a working electrode 21 facing the end face of the protrusion 1 a of the cell block 1. The working electrode 21 is supported in a through hole provided in the central portion of a disk-shaped supporting member 22. The support member 22 is supported by the guide pin 25, and the working electrode 21
Are opposed to the open end faces of the cell channels 11 and 12 located in the end face of the protrusion 1a of the cell block 1. The working electrode
A gasket 23 having a through hole in its central portion is interposed between 21 and the end surface of the protrusion 1a of the cell block 1 so that a liquid-tight cell space 16 is formed. And the gasket
The open end faces of the cell channels 11 and 12 are located in the cell space 16 at the center of 23, and the working electrode 21 is
It opposes the open end faces of the cell flow paths 11 and 12 via. The working electrode 21 is pressed by the electrode pressing member 24 supported by the guide pin 25 against the end surface of the protrusion 1a of the cell block 1 via the gasket 23. The electrode pressing member 24 is
It is composed of an electrode pressing plate 24a which is in contact with the working electrode 21 and is relatively soft and highly elastic, an electrode pressing plate 24b having no elasticity and an electrode pressing plate 24c of high hardness.

A block cover plate 4 is attached to the side surface of the cell block 1 opposite to the side surface on which the protrusion 1a is formed. The block cover plate 4 has a through hole 4a in the center thereof, and the open end faces of the cell channels 11 and 12 located in the side surface of the cell block 1 are located in the through hole 4a.

When the ECD having the cell 10 is set in high performance liquid chromatography, the effluent containing the substance to be measured from the high performance liquid chromatography passes through the cell flow passage 11, the cell space 16, and the cell flow passage 12 and the cell 10 flows. It is discharged outside. A voltage is applied between the working electrode 21 and the liquid contact part 13, and the effluent containing the substance to be measured undergoes an electrochemical reaction while passing through the cell 10. The current generated at this time is amplified and measured by the detection unit (not shown).

Since the working electrode of the ECD of the present invention uses polyphenylene sulfide as a binder resin as described above, it may dissolve or swell even when an eluent containing an organic solvent such as methyl alcohol or acetonitrile is used. Measurement sensitivity does not decrease. Since it is also excellent in acid resistance and alkali resistance, it is possible to use any solvent having a pH range of 3 to 9 which is usually used in liquid chromatography.

The sensitivity of this working electrode is almost the same as that of conventional glassy carbon. Although the surface state of Glacier Carbon changes when the high potential is measured, the sensitivity decreases, but no change in the surface state is observed on this working electrode even when a voltage of 1 V is continuously applied for one month. Furthermore, in the conventional glassy carbon electrode, it takes several hours for the baseline to stabilize during high potential measurement, whereas the working electrode stabilizes in about 20 minutes, so measurement can be performed in a short time.

(Example) Hereinafter, the present invention will be described with reference to Examples.

Example 1 Polyphenylene sulfide resin Ryton GR01 (manufactured by Philippe Oil Co., Ltd.) and a predetermined amount of graphite carbon 32
Add 5 mesh passing product (manufactured by Tokai Carbon Co., Ltd.),
Lightly mixed in a mortar. This is MINI MAX MOLDER (Costo
m Scientific Instruments; Cedar Knlls NJUSA)
Was molded at a resin temperature of 320 ° C to a thickness of 3 mm and a diameter of 12 mm. The surface of the obtained molded body was polished by an automatic polishing machine, Emmet IV type (manufactured by Buehler Ltd (USA)). The resistance value of the test piece thus obtained was measured. The relationship between the graphite carbon content in the electrode and the resistance value is shown in FIG. 2 (a). It can be seen from FIG. 2 (a) that the resistance value exceeds 10 ⁶ Ω when the graphite carbon content falls below 30% by weight. It can be seen that the working electrode needs to have a certain degree of conductivity, and that the graphite carbon content of at least 30% by weight is necessary for this test piece to function as the working electrode.

Next, this was attached to the cell shown in FIG. 1 as a working electrode, and this cell was set to an ECD-12 type detector (manufactured by Sekisui Chemical Co., Ltd.). This detector was connected to LC6A high performance liquid chromatography (Shimadzu). A sample solution containing estriol (E ₃ ) at a rate of 1 μg / ml was subjected to the above high performance liquid chromatography for measurement. The measurement conditions are as follows.

Eluent: 50 mM phosphate buffer (pH 3.5) 50% by volume / acetonitrile 35% by volume / methanol 15% by volume Flow rate: 1.0 ml / min Applied voltage: 1000 mV Test substance: estriol (E ₃ ) Concentration: 1 μg / ml Injection Amount: 5μ Separation column: Medipora ODS-113N Sekisui Chemical Co., Ltd. Fig. 2 shows the relationship between the content of graphite carbon contained in the working electrode and the peak height of estriol (E ₃ ) at the time of measurement (indicating sensitivity). It shows in (b). From FIG. 2 (b), it can be seen that the sensitivity drops sharply when the graphite carbon content falls below 50% by weight, but almost constant sensitivity is obtained above 50% by weight. Fig. 2 (b)
From this, it can be seen that the graphite carbon content must be approximately 30% by weight or more.

Furthermore, the content of graphite carbon contained in the working electrode and the background current (BGC) during measurement
The relationship with is shown in FIG. It can be seen from Fig. 2 that BGC increases rapidly when the graphite carbon content exceeds 60% by weight. It can be used as an electrode with a graphite carbon content of up to about 90% by weight.
It is preferably 0% by weight or less.

It is desirable that the sensitivity of the ECD detector is high, but even if the sensitivity is high, the substantial sensitivity decreases if BGC is high. From FIGS. 2 (a) to 2 (c), low resistance value, good sensitivity, small BGC and suitable use as working electrode of ECD are graphite carbon of 30 to 80% by weight. It is clear that the range.

Example 2 A working electrode was prepared according to the method of Example 1 with a graphite carbon content of 50% by weight. This was immersed in acetonitrile and methanol for 7 days at 24 ° C. Visual observation of the surface of this working electrode showed no deterioration of the electrode surface. Chromatography using this working electrode under the same conditions as in Example 1 revealed that E
₃ appeared with a peak height equivalent to that of Example 1, and it was confirmed that the sensitivity did not decrease.

Comparative Example 1 As a resin having relatively good solvent resistance, 50 parts by weight of novolac type phenol resin RM1006P (manufactured by Asahi Organic Co., Ltd.) was added with 50 parts by weight of graphite carbon of the same quality as in Example 1 and mixed in a mortar. This is hot press molded at 150 ° C to a thickness of 3
A compact with a diameter of 12 mm and a diameter of 12 mm was obtained. The molded body was polished in the same manner as in Example 1 to obtain a test piece, which was evaluated by liquid chromatography. The conductivity, sensitivity as a working electrode and BGC value of this test piece were almost the same as those of the test piece of Example 1 at the time of preparation. However, the sensitivity gradually decreased after several hours from the start of the measurement, and the E ₃ peak did not appear in about 24 hours. When the electrode was taken out and visually observed, it was found that the liquid contact part swelled and almost no liquid flowed.

Comparative Example 2 Epoxy resin Epicoat 828 and curing agent Epicure Z
50 parts by weight of the same carbon as in Example 1 was mixed with a 1: 1 (weight ratio) mixture (all made by Yuka Shell Co., Ltd.), and a curing reaction was carried out at 130 ° C. for 4 hours in a closed stainless steel container. After that, it was taken out to obtain a rod-shaped molded body having a length of 50 mm and a diameter of 12 mm. The obtained molded body was sliced to a thickness of 3 mm, polished in the same manner as in Example 1, and evaluated by liquid chromatography. Since the baseline was not stable at the time of measurement and no measurement was possible, it was found that the surface was swollen when the electrode was taken out and observed.

(Effects of the Invention) According to the present invention, there is thus provided an electrochemical detector having a working electrode having a specific composition and capable of measuring a substance to be measured with high sensitivity. Since the working electrode has excellent organic solvent resistance and acid / alkali resistance, it can be widely used as a detector for high performance liquid chromatography using various eluents.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of the ECD cell of the present invention; FIG. 2 (a) shows the relationship between the graphite carbon content in the working electrode of the ECD of the present invention and the resistance value of the electrode. Fig. 2 (b) is a graph showing the relationship between the content of graphite carbon in the working electrode and the peak of the substance to be measured in the sample; and Fig. 2 (c) is the graphitic carbon in the working electrode. It is a graph which shows the relationship between the content rate of and the background current. 1 …… Cell block, 10 …… ECD cell, 15 …… Reference electrode, 21
...... Working electrode.

Front page continuation (72) Inventor Hideo Matsumura 5-2, Suzaku, Nara City, Nara Prefecture (72) Hirohito Nishino 536-53, Minamikasacho, Kusatsu City, Shiga Prefecture

Claims (2)

[Claims] 1. An electrochemical detector for high performance liquid chromatography having a cell having a working electrode, a counter electrode and a reference electrode; and a detector, wherein the working electrode is mainly composed of polyphenylene sulfide resin and graphite carbon. An electrochemical detector composed of. 2. The electrochemical detector according to claim 1, wherein the composition contains the graphite carbon in a proportion of 30 to 80% by weight.