Electrical conductivity detector

Abstract

To reduce the error of a measurement signal caused by the influence of charge transfer resistance when the electric conductivity of a sample liquid is high.SOLUTION: The apparatus includes a sample cell (2) having a flow channel (10) through which a sample liquid flows and a pair of electrodes (12 - 12) disposed across the flow channel (10), a voltage applying unit (4) configured to apply an alternating measurement voltage between the pair of electrodes (12 - 12) of the sample cell (2), and a control unit (8) configured to reduce an error included in a measurement signal by changing a frequency of the measurement voltage applied by the voltage applying unit (4) to the pair of electrodes (12 - 12) during measurement of the electrical conductivity of the sample liquid in accordance with a magnitude of the measurement signal output from a measuring unit (6).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrical conductivity detector used as a detector for an ion chromatograph. [Background technology]

[0002] In an ion chromatograph, the ionic components in a sample are generally separated using a separation column, and then the sample liquid eluted from the separation column is passed through a suppressor to remove unnecessary ions other than the ionic components to be analyzed. The electrical conductivity of the sample liquid that has passed through the suppressor is measured to quantify the ionic components to be analyzed.

[0003] An electrical conductivity detector for measuring the electrical conductivity of a sample liquid has a sample cell with a pair of electrodes disposed on either side of a flow path through which the sample liquid flows, and measures the electrical conductivity of the sample liquid flowing through the flow path in the sample cell by applying a measurement voltage between the pair of electrodes and measuring the current flowing between those electrodes.

[0004] The measurement voltage applied between the pair of electrodes is an alternating current (e.g., a sine wave) with a constant amplitude and frequency. When the alternating current measurement voltage is applied between the pair of electrodes, a current corresponding to the electrical conductivity of the sample liquid flowing through the flow path between the electrodes flows between the electrodes. The current flowing between the pair of electrodes is input to an amplifier, and a voltage signal (called a detection signal) corresponding to the current flowing between the pair of electrodes is read at regular intervals by an A / D converter. The detection signal read by the A / D converter is multiplied by a reference signal having the same frequency and phase as the measurement voltage in a multiplier. The output signal from the multiplier is passed through a low-pass filter to remove frequency components, resulting in a measurement signal (DC voltage) corresponding to the electrical conductivity of the sample liquid (see Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2017 / 208561 Summary of the Invention [Problem to be solved by the invention]

[0006] The magnitude of the current flowing between the electrodes in the sample cell is affected not only by the solution resistance Rsol (the reciprocal of electrical conductivity) of the sample solution flowing through the channel between the electrodes, but also by the charge transfer resistance Rct and the electric double layer capacitance Cdl. The charge transfer resistance Rct indicates the difficulty of the charge transfer reaction occurring, and is small when the charge transfer reaction is easy to occur and large when the charge transfer reaction is difficult to occur. To accurately measure the electrical conductivity of the sample solution, it is desirable that the effects of the charge transfer resistance Rct and the electric double layer capacitance Cdl be small. However, the charge transfer resistance Rct increases as the electrical conductivity of the sample solution increases, and therefore the higher the electrical conductivity of the sample solution, the greater the error due to the charge transfer resistance Rct contained in the measurement signal.

[0007] The present invention has been made in view of the above problems, and has as its object to reduce errors in measurement signals caused by the influence of charge transfer resistance when the electrical conductivity of a sample solution is high. [Means for solving the problem]

[0008] Figure 3 shows the equivalent circuit of the sample cell, which is composed of the solution resistance Rsol, the charge transfer resistance Rct, and the electric double layer capacitance Cdl. The charge transfer resistance Rct and the electric double layer capacitance Cdl are in a parallel relationship with each other, and the solution resistance Rsol is in a series relationship with the parallel circuit of the charge transfer resistance Rct and the electric double layer capacitance Cdl. The impedance Z of the entire equivalent circuit is Z=Z'-jZ” Z' and Z" can be expressed as TIFF2026003801000002.tif43165. "f" is the frequency of the measurement voltage applied between the pair of electrodes in the sample cell. From the above equation, the real part of the impedance Z of the entire equivalent circuit approaches the solution resistance Rsol as the frequency f increases, and approaches the sum of the solution resistance Rsol and the charge transfer resistance Rct as the frequency f decreases. Therefore, it can be said that the higher the frequency f, the higher the accuracy of the electrical conductivity measurement.

[0009] As mentioned above, the detection signal output from the amplifier is periodically sampled by the A / D converter. However, if the frequency f of the voltage applied between the electrodes is high, the number of samples taken per cycle of the detection signal decreases, which increases the effects of sine wave distortion and the phase angle of the detection signal integrated per sampling. This increases the error contained in the phase difference between the detection signal read by the A / D converter and the reference signal. The effect of this error increases as the electrical conductivity of the sample solution decreases, and the accuracy and linearity of the measurement values ​​deteriorate, especially when the electrical conductivity of the sample solution is low. Therefore, simply increasing the frequency of the measurement voltage applied between the electrodes of the sample cell is not sufficient.

[0010] Therefore, the present invention solves the above problem by changing the frequency of a voltage applied between the electrodes of a sample cell in accordance with the electrical conductivity of sample water flowing through a flow path in the sample cell. That is, the electrical conductivity detector of the present invention includes: a sample cell having a flow path through which a sample liquid flows and a pair of electrodes disposed on either side of the flow path; a voltage application unit configured to apply an AC measurement voltage between the pair of electrodes of the sample cell; a measurement unit that outputs a measurement signal having a magnitude corresponding to the electrical conductivity of the sample liquid flowing through the flow path of the sample cell based on the magnitude of the current flowing between the pair of electrodes; and a control unit that controls the voltage application unit, the control unit being configured to change the frequency of the measurement voltage applied to the pair of electrodes by the voltage application unit during measurement of the electrical conductivity of the sample liquid in accordance with the magnitude of the measurement signal output from the measurement unit, thereby reducing errors contained in the measurement signal. [Effects of the Invention]

[0011] The electrical conductivity detector according to the present invention is configured to reduce errors contained in the measurement signal by changing the frequency of the measurement voltage applied to a pair of electrodes to measure the electrical conductivity of the sample liquid flowing through the flow path of the sample cell during electrical conductivity measurement in accordance with the magnitude of the measurement signal output from the measurement signal, thereby reducing errors in the measurement signal due to the influence of charge transfer resistance when the electrical conductivity of the sample liquid is high. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of an electrical conductivity detector. [Figure 2] 4 is a flowchart showing an example of control of a measurement voltage according to the embodiment; [Figure 3] FIG. 2 is a diagram for explaining an equivalent circuit of a sample cell of the electrical conductivity detector. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the electrical conductivity detector according to the present invention will be described below with reference to the drawings.

[0014] As shown in FIG. 1, the electrical conductivity detector of this embodiment comprises a sample cell 2, a voltage application unit 4, a measurement unit 6, and a control unit 8.

[0015] The sample cell 2 includes a flow path 10 through which the sample liquid flows, and a pair of electrodes 12-12 arranged opposite each other across the flow path 10. The voltage application unit 4 is configured to apply an AC measurement voltage Vosin(ωt) between the pair of electrodes 12-12 of the sample cell 2.

[0016] The measurement unit 6 is a single-phase lock-in amplifier and includes an amplifier 14, an A / D converter 16, a multiplier 18, and a low-pass filter (hereinafter, LPF) 20. The amplifier 14 outputs a detection signal Asin(ωt+φ) having an amplitude A corresponding to the magnitude of the current flowing between the pair of electrodes 12-12 of the sample cell 2. The A / D converter 16 reads the detection signal Asin(ωt+φ) from the amplifier 14 at regular time intervals and outputs it to the multiplier 18 and the LPF 20. The detection signal Asin(ωt+φ) output from the A / D converter 16 is multiplied by a reference signal sin(ωt) having the same frequency (ωt) as the detection signal Asin(ωt+φ) in the multiplier 18, and then frequency components are removed in the low-pass filter 20, resulting in a measurement signal (A / 2)cosφ having an amplitude A corresponding to the magnitude of the current flowing between the pair of electrodes 12-12 and a phase difference φ. The phase difference φ between the detection signal Asin(ωt+φ) and the reference signal sin(ωt) is adjusted in advance.

[0017] The control unit 8 is realized by an electronic circuit including a CPU (central processing unit) and information storage memory, and controls the measurement voltage applied by the voltage application unit 4 between the pair of electrodes 12-12 of the sample cell 2 and the reference signal to be multiplied by the detection signal by the multiplier 18. The control unit 8 monitors the magnitude of the measurement signal obtained by the measurement unit 6 during measurement of the electrical conductivity of the sample water, and is configured to adjust the measurement voltage and the frequency f of the reference signal according to the magnitude of the measurement signal. The control unit 8 also includes a reference value memory unit 22 that stores a reference value of the measurement signal obtained by the measurement unit 6, and changes the measurement voltage and the frequency of the reference signal depending on whether the measurement signal exceeds the reference value stored in the reference value memory unit 22. The reference value memory unit 22 is realized by a partial storage area of ​​the information storage memory. The reference value can be, for example, 1 + α (mS / cm) when switching from a low electrical conductivity to a high electrical conductivity, or 1 - α (mS / cm) when switching from a high electrical conductivity to a low electrical conductivity, where α is 10 to 100 (μS / cm). However, the present invention is not limited to these.

[0018] The control of the frequency of the measurement voltage will be described with reference to the flowchart of FIG. 2 as well as FIG.

[0019] When the measurement of electrical conductivity is started, the control unit 8 controls the voltage application unit 4 to apply an AC measurement voltage having a predetermined amplitude and a first frequency (e.g., 1 kHz) between the pair of electrodes 12-12 of the sample cell 2 (step 101). During the measurement of electrical conductivity, the control unit 8 constantly determines whether the measurement signal obtained by the measurement unit 6 is less than a reference value (step 102). If the measurement signal is less than the reference value (step 102: Yes), the control unit 8 sets the frequency of the measurement voltage to the first frequency (step 103). If the measurement signal exceeds the reference value (step 102: No), the control unit 8 sets the frequency of the measurement voltage to a second frequency (e.g., 12.5 kHz) higher than the first frequency (step 104). In response to the change in the frequency of the measurement voltage, the control unit 8 also changes the frequency of the reference signal by which the detection signal is multiplied in the multiplier 18. The measurement unit 6 reads the detection signal at regular time intervals, and since the number of samples per detection signal cycle decreases as the frequency of the measurement voltage increases, a lower frequency of the measurement voltage enables more accurate phase adjustment. Therefore, when the electrical conductivity is low, at which the charge transfer resistance Rct becomes dominant, the frequency of the measurement voltage is reduced, and only when the electrical conductivity is high, at which the solution resistance Rsol and the electric double layer capacitance Cdl become dominant, the frequency is increased.

[0020] In the above embodiment, one value is set in advance as a reference value, and the frequency of the measurement voltage and the reference signal is changed depending on whether the measurement signal exceeds that reference value. However, the present invention is not limited to this, and multiple reference values ​​may be set, and the frequency of the measurement voltage and the reference signal may be changed in multiple stages depending on the magnitude of the measurement voltage.

[0021] The embodiment described above is merely one example of an embodiment of the electrical conductivity detector according to the present invention. The embodiment of the electrical conductivity detector according to the present invention is as follows.

[0022] In one embodiment of the electrical conductivity detector according to the present invention, a sample cell having a flow path through which a sample liquid flows and a pair of electrodes disposed on either side of the flow path; a voltage application unit configured to apply an AC measurement voltage between the pair of electrodes of the sample cell; a measurement unit that outputs a measurement signal having a magnitude corresponding to the electrical conductivity of the sample liquid flowing through the flow path of the sample cell based on the magnitude of the current flowing between the pair of electrodes; and a control unit that controls the voltage application unit, the control unit being configured to reduce errors contained in the measurement signal by changing the frequency of the measurement voltage applied to the pair of electrodes by the voltage application unit to measure the electrical conductivity of the sample liquid flowing through the flow path of the sample cell, depending on the magnitude of the measurement signal output from the measurement unit during measurement of the electrical conductivity of the sample liquid.

[0023] In a specific aspect of the above embodiment, a reference value memory unit is provided that stores a reference value of the measurement signal, and the control unit is configured to set the frequency of the measurement voltage to a first frequency when the measurement signal is less than the reference value.

[0024] In the above specific aspect, the control unit may be configured to set the frequency of the measurement voltage to a second frequency that is greater than the first frequency when the measurement signal is equal to or greater than the reference value. [Explanation of symbols]

[0025] 2. Sample cell 4. Voltage application section 6 Measuring part 8 Control Unit 10 Flow path 12 electrodes 14 Amplifier 16 A / D converters 18 Multiplier 20 Low-pass filter 22 Reference value storage section

Claims

1. a sample cell having a flow path through which a sample liquid flows and a pair of electrodes disposed on either side of the flow path; a voltage application unit configured to apply an AC measurement voltage between the pair of electrodes of the sample cell; a measurement unit that outputs a measurement signal having a magnitude corresponding to the electrical conductivity of the sample liquid flowing through the flow path of the sample cell based on the magnitude of the current flowing between the pair of electrodes; a control unit that controls the voltage application unit, the control unit being configured to reduce errors included in the measurement signal by changing the frequency of the measurement voltage applied to the pair of electrodes by the voltage application unit to measure the electrical conductivity of the sample liquid flowing through the flow path of the sample cell, depending on the magnitude of the measurement signal output from the measurement unit during measurement of the electrical conductivity of the sample liquid.

2. a reference value storage unit that stores a reference value of the measurement signal; The electrical conductivity detector according to claim 1 , wherein the control unit is configured to set the frequency of the measurement voltage to a first frequency when the measurement signal is less than the reference value.

3. 3. The electrical conductivity detector according to claim 2, wherein the control unit is configured to set the frequency of the measurement voltage to a second frequency that is greater than the first frequency when the measurement signal is equal to or greater than the reference value.

Images