JPH10267896A - Electron capture type detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the need for switching operation of a range for concentration. SOLUTION: The output voltage of a differential amplifier 13 is inputted to a V/F converter 15 through a non-linear amplifier 14 of square characteristics. Then the taken-out output voltage of the differential amplifier 13 is corrected for non-linear characteristics using a calculation processing part 19, resulting in a direction signal. Thus, when the concentration of a target material introduced into a detection cell 10 is low, the fluctuation in output voltage of the deferential amplifier 13 against concentration changes is large, while higher the concentration of it smaller the fluctuation. Thus, a measurement of high sensitivity is performed given at low concentration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron capture detector (hereinafter, referred to as "ECD (Electron Capture Detector)") used as a detector of a gas chromatograph or the like.

[0002]

2. Description of the Related Art Various types of detectors have been put into practical use in gas chromatography apparatuses.
Is useful for measuring electrophilic compounds such as halogen compounds and nitro compounds. Therefore, organic mercury, pesticides, PCB
Is used for the measurement of residual amounts of steroids, amino acids and the like, or the measurement of trace amounts of steroids and amino acids converted to electrophilic derivatives.

FIG. 3 is a configuration diagram of a conventional ECD. Into the detection cell 10, a carrier gas and a sample gas that has passed through a column of a chromatograph are introduced. Detection cell 10
The electrode 11 installed in the inside is connected to one coil L1 of the transformer 12, and the difference amplifier 1 is connected through the coil L1.
3 has been entered. The output voltage of the difference amplifier 13 is supplied to a voltage / frequency (V / F) converter 15 via an inverting amplifier 20.
, And the V / F converter 15 outputs a pulse signal having a frequency corresponding to the input voltage value. The range switching section 21 is 1 /
It is composed of a frequency divider 22 that performs frequency division by 10 and a switch 23, and a pulse signal selectively output by the switch 23 is given to the pulse shaper 16. Pulse shaper 16
Converts the pulse width and the pulse height appropriately while maintaining the frequency of the pulse signal, and sends it to the other coil L2 of the transformer 12 via the driver unit 17.

The operation of the ECD is as follows. Detected in the cell 10 and radioactive isotopes, such as ⁶³ Ni is sealed, the molecules of the carrier gas by the radiation is ionized. When a pulse voltage is applied to the electrode 11, electrons generated by ionization of the carrier gas are taken in and a pulse current flows. When molecules of the electron-capturing substance enter the atmosphere, the molecules easily absorb surrounding free electrons and become negative ions. For this reason, the density of free electrons decreases, and the moving speed of negative ions is slower than that of free electrons, so that the time to reach the positive electrode becomes longer. Therefore, the current flowing by one pulse voltage application is reduced.

The difference amplifier 13 integrates the difference current between the pulse current and the target current IR and converts it into a voltage. As a result, a voltage corresponding to the difference between the average value of the pulse current (the pulse current per unit time) and the target current IR appears at the output of the difference amplifier 13. Since the V / F converter 15 outputs a pulse signal having a frequency corresponding to this voltage, for example,
When the molecules of the electron-capturing substance enter the detection cell 10 and the pulse current decreases, the output voltage of the difference amplifier 13 increases, and the frequency of the pulse signal output from the V / F converter 15 increases. That is, the number of pulses generated per unit time increases to compensate for the decrease in the number of electrons captured by one pulse signal. When such a pulse signal is supplied to the transformer 12, the average value of the pulse current increases and the output voltage of the difference amplifier 13 decreases.

As described above, if the total number of electrons per unit time, that is, the current, is kept constant by increasing the number of pulses in a unit time by an amount corresponding to the decrease in the number of electrons.
The change Δf in the number of pulses is proportional to the concentration of the electron-capturing substance. That is, the following equation (1) holds. Δf = f−f0 = K · a (1) f: pulse frequency f0: pulse frequency in the case of carrier gas only K: constant depending on electron capture speed, etc. a: concentration of electron-capturing substance

Since the detection voltage V obtained from the output of the difference amplifier 13 depends on the fluctuation of the average value of the pulse current, the detection voltage V is recorded as time elapses, so that the target electron trapping substance can be obtained. A chromatogram for the concentration of is obtained.

[0008]

In the ECD having the above structure, the range can be switched in accordance with the concentration of the target substance by switching the switch 23 of the range switching section 21. That is, when the target substance has a low concentration, the switch 23 is tilted upward and the output of the frequency divider 22 is supplied to the pulse shaper 16 to reduce the number of pulses to 1/10. On the other hand, if the target substance has a high concentration, switch 2
3 is tilted downward, and the pulse signal output of the V / F converter 15 is given to the pulse shaper 16 as it is.

If the relationship between the input voltage in the V / F converter 15 and the frequency of the pulse signal output is linear, the relationship between the detection voltage V and the concentration of the electron-capturing substance is as follows from equation (1). I get it. In the case of the 1 × range where the switch 23 is tilted downward, C · VC−V0 = K · a (2) In the case of the 10 × range where the switch 23 is tilted upward, C · VC−V0 = 10 · K · a (3) where V0 is the detection voltage when only the carrier gas is used, C
Is a constant depending on V / F conversion. The relationship between the detected voltage V and the concentration Ka based on the equations (2) and (3) is a straight line having a different slope depending on the range as shown in FIG.
That is, the change in the detection voltage with respect to the density change is relatively large in the 10-fold range, and the change in the detection voltage with respect to the density change is relatively small in the 1-fold range. Therefore, high-precision measurement can be performed by switching to the 10-fold range when the density is low, and output saturation can be avoided by switching to the 1-fold range when the density is high.

However, in the above-mentioned conventional ECD, the operator has to switch the range in accordance with the concentration of the target substance, so that the operation is troublesome, and when the concentration is not completely known, the range is switched and the measurement is performed twice. Some things have to be done.

An object of the present invention is to provide an ECD having a wide input dynamic range that does not require a range switching operation.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention is directed to a method in which a carrier gas is introduced into a detection cell to be ionized, and a sample gas is sent into the atmosphere to be disposed in the detection cell. An electron capture detector for detecting a current flowing through the electrode, wherein a) a pulse voltage is applied to the electrode, and a current measuring means for detecting a current flowing through the electrode when the voltage is applied; and b) a current measured by the current measuring means. Difference detection means for outputting a voltage corresponding to the difference between the obtained current and the predetermined current; c) amplification means for amplifying the output voltage of the difference detection means with a characteristic that the amplification factor increases when the input is large; d) Pulse generating means for generating a pulse signal having a frequency corresponding to the output voltage of the amplifying means and supplying the pulse signal to the current measuring means; e) correcting means for taking out the output voltage of the difference detecting means and correcting the characteristics of the amplifying means When It is characterized in that it comprises.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In an electron capture detector according to the present invention, a feedback loop is formed by current measuring means, difference detecting means, amplifying means and pulse generating means. In this feedback loop, when the amount of the electron-capturing substance introduced into the detection cell increases and the current measured by the current measuring means decreases, the difference between the current and the predetermined current increases, so that the output voltage of the difference detecting means increases. Become. The pulse generating means increases the frequency of the pulse signal as the input voltage increases, so that the number of pulses per unit time increases. This increases the chances of the electrodes taking in electrons in the detection cell, thereby compensating for a decrease in the current flowing in response to one pulse voltage, so that the current per unit time approaches the predetermined current.

The amplification means may be, for example, an amplifier having a square characteristic. When the amount of the electron trapping substance introduced into the detection cell is small (that is, the concentration is low) and the difference from the current flowing when the carrier gas alone is small, the output voltage of the difference detection means is relatively small. Is also relatively small. Conversely, when the amount of the electron-capturing substance introduced into the detection cell is large (that is, the concentration is high) and the difference from the current flowing when the carrier gas alone is large, the output voltage of the difference detection means is relatively large. Therefore, the amplification factor of the amplification means becomes relatively large. Therefore, when the feedback loop operates to maintain the equilibrium state, when the concentration of the electron-capturing substance is low, the output voltage change of the difference detection means with respect to the concentration change becomes large, and conversely, when the concentration of the electron-capturing substance is high The output voltage change with respect to the density change becomes small. Since the change in the output voltage of the difference detecting means with respect to the change in the concentration of the electron-capturing substance depends on the change characteristic of the amplification factor of the amplifying means, the correcting means performs a process for correcting this characteristic, and obtains the concentration and the detection signal. Is made to be, for example, a straight line.

[0015]

According to the electron capture detector of the present invention, the detection sensitivity increases when the concentration of the sample is low, and the detection sensitivity decreases when the concentration of the sample is high. Therefore, a highly accurate measurement value can be obtained even when the concentration of the sample is low, and the output does not saturate even when the concentration of the sample is high. Thereby, the optimum measurement according to the concentration of the sample can be performed without the troublesome operation of switching the range by the operator himself as in the related art.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the electron capture detector according to the present invention will be described with reference to FIGS. FIG. 1 is a configuration diagram of an ECD according to the present embodiment. In this ECD, a nonlinear amplifier 14 having a square characteristic is provided between a difference amplifier 13 and a V / F converter 15 in place of the range switching unit provided in the conventional ECD of FIG. Also,
The polarity of the voltage extracted from the output of the difference amplifier 13 is inverted by the inverting amplifier 18 and then input to the arithmetic processing unit 19. The nonlinear amplifier 14 squares the input voltage (or
The output is further multiplied by an appropriate constant). Therefore, the amplification factor increases as the input voltage increases, and the amplification factor increases sharply.

In the ECD of this embodiment, the transformer 1
2. The feedback loop including the difference amplifier 13, the non-linear amplifier 14, the V / F converter 15, the pulse shaper 16, and the driver unit 17, as described above, is a pulse flowing by the pulse voltage applied to the electrode 11. The operation is performed so that the average value of the current becomes the target current IR. Therefore, the relationship between the detection voltage V and the concentration of the electron-capturing substance is obtained from the above equation (1) as follows. ^{C · V 2 -C · V0 2} = K · a ... (4)

From the above equation (4), V ² = (K · a / C) + V 0 ² (5) Therefore, the relationship between the detection voltage V and the concentration Ka is shown in FIG.
As shown in FIG. That is, when the concentration of the electron-capturing substance is low, the rate of change of the detection voltage V with respect to the concentration change (that is, the slope of the curve in FIG. 2) is large, and as the concentration increases, the rate of change of the detection voltage V with respect to the concentration change becomes gentle. Therefore, a characteristic can be realized in which the lower the density, the higher the detection sensitivity, but the output is not saturated even at a high density.

The detected voltage V is input to the arithmetic processing unit 19, and the arithmetic processing unit 19 performs a process of calculating the density. For example, after the detected voltage is converted into a digital value by an A / D converter, a square operation is performed to obtain an output signal. As a result, the output signal has a characteristic proportional to the density Ka. Also, if a calibration curve indicating the relationship between the concentration and the detection voltage as shown in FIG. 2 is created using a standard sample whose concentration is known in advance and stored in the arithmetic processing unit 19, the square calculation can be performed without performing By correcting the characteristics of the nonlinear amplifier 14, an output signal proportional to the density can be obtained.

In the above embodiment, the non-linear amplifier 14 has the square characteristic. However, the characteristic is not limited to this, and it is sufficient that the characteristic is such that the amplification factor increases when the input voltage is large. For example, the non-linear amplifier 14 can be based on an exponential characteristic or an amplitude extension characteristic (linear characteristic).

The above embodiment is merely an example, and it is apparent that changes and modifications can be made within the spirit of the present invention.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment of an electron capture detector according to the present invention.

FIG. 2 is a diagram showing a relationship between the concentration of an electronic capture substance and a detection voltage in the electron capture type detector of the present embodiment.

FIG. 3 is a configuration diagram of a conventional electron capture detector.

FIG. 4 is a diagram showing a relationship between the concentration of an electronic capture substance and a detection voltage in a conventional electron capture detector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Detection cell 11 ... Electrode 12 ... Transformer 13 ... Difference amplifier 14 ... Nonlinear amplifier 15 ... Voltage / frequency (V / F) converter 16 ... Pulse shaper 17 ... Driver part

Claims (1)

[Claims] 1. An electron capture detector for introducing a carrier gas into a detection cell, ionizing the carrier gas, sending a sample gas into the atmosphere, and detecting a current flowing through an electrode provided in the detection cell. Current measuring means for applying a pulse voltage to the electrode and detecting a current flowing through the electrode when the voltage is applied; b) difference detection for outputting a voltage corresponding to a difference between the current measured by the current measuring means and a predetermined current C) amplifying means for amplifying the output voltage of the difference detecting means with a characteristic that the amplification factor increases when the input is large; d) generating a pulse signal having a frequency corresponding to the output voltage of the amplifying means, An electron capture type detector comprising: a pulse generation unit to be supplied to a measurement unit; and e) a correction unit that extracts an output voltage of the difference detection unit and corrects a characteristic of the amplification unit.