JP2001056315A - Electron capture detector - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To avoid a change in sensitivity over time due to contamination or the like inside a detection cell. In a steady state before a sample is introduced into a detection cell, a control unit scans a signal to be sent to a D / A converter, and a pulse frequency obtained through the A / D converter is a key. A signal value that matches the fundamental frequency input and set from the input unit 22 is found. The signal of this value is D
/ A converter 19 to determine the set current IS,
The pulse frequency is controlled so that the anode current ID becomes the set current value IS. Then, the signal of the analysis READY is sent to the analyzer or the automatic sample injection device to prompt the sample injection. As a result, even if the anode current ID becomes difficult to flow due to contamination or the like,
As a result, IS is reduced and the sensitivity is kept constant.

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 an 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.

The operation principle of the ECD is as follows. A radioactive isotope such as ⁶³ Ni is sealed in the detection cell,
An inert gas (such as N ₂ ) is introduced into the detection cell, and the inert gas molecules are ionized by radiation (β rays) to emit electrons (free electrons). When a voltage is applied to an electrode (anode) arranged in the detection cell, the free electrons are taken in and a certain amount of current flows through the electrode.

The voltage applied to the electrode is pulsed, and a difference between a pulse current flowing through the electrode and a predetermined set current IS by the pulse voltage is input to an integrator. 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 set current IS appears in the output of the integrator. The output of this integrator is expressed as voltage / frequency (V
/ F) A pulse signal having a frequency corresponding to the difference between the two currents is output to the converter, and a pulse voltage to the electrode is generated based on the pulse signal. Thereby, the frequency f of the pulse voltage applied to the electrode in the steady state has a value corresponding to the set current IS.

[0005] When molecules of an electron-capturing substance are introduced into the detection cell, the molecules absorb free electrons emitted from the inert gas, and the density of the free electrons in the detection cell decreases.
Since the negative ions of the electron-capturing substance that have absorbed the free electrons have a much lower moving speed than the free electrons, the current flowing through the electrode decreases due to the decrease in the number of free electrons. Then, the output voltage of the integrator increases, and the frequency f of the pulse signal output from the V / F converter increases. That is, the number of pulses generated per unit time increases to compensate for the decrease in the number of electrons taken in by one pulse voltage.

The concentration a of the electron trapping substance and the pulse frequency f
Is known to have the following relationship (eg, RJ Maggs, et al., "The Electron Capture Det.
ector-A New Mode of Operation ", ANALYTICAL CHEMIS
TRY, VOL.43, NO.14, DECEMBER 1971, p.1967). k1 · a + KD = (b · q / IS) · f (1) k1, KD: constant, b: electron generation rate, q: elementary charge, IS: average cell current (that is, set current) When a = 0, That is, assuming that the frequency f without introducing an electron-capturing substance into the detection cell is f0, f0 = (KD.IS) /b.q (2), and the change .DELTA.f in the pulse frequency obtained as an output
Is as follows: Δf = f−f0 = (IS · k1 / b · q) · a (3), which is proportional to the concentration a. Therefore, the concentration a of the sample is
From the frequency change Δf, it can be calculated as follows: a = Δf / (IS · k1 / b · q) (4) In general, (IS · k1 / b ·
The value of q) is obtained in advance by an experiment.

The detection voltage V extracted from the output of the integrator
Is based on the change in the pulse frequency f. By recording this detection voltage V over time,
A chromatogram relating to the concentration of the target electron-capturing substance is obtained.

[0008]

According to the above equation (3), Δf is inversely proportional to the electron generation rate b. In general, when the detection cell is new, the electron generation rate b is large. If the electrodes and the inside of the detection cell become contaminated with the sample while the measurement is repeated, the electron generation rate b decreases. Therefore, as the electrode and the inside of the detection cell become dirty with the sample, Δf increases.
That is, a phenomenon that the sensitivity is increased occurs. (However, this is an apparent sensitivity, and in reality, if the inside of the detection cell becomes dirty, noise increases, so that the S / N ratio does not improve.) As a result, the sensitivity changes with time. This causes a problem in the reproducibility of measurement and the like.

The present invention has been made in order to solve such a problem, and an object of the present invention is to provide an ECD capable of avoiding a change in sensitivity with time due to contamination or the like inside a detection cell. It is in.

[0010]

According to the above equation (3),
Since Δf is proportional to the average cell current IS, it can be seen that the sensitivity can be kept constant by decreasing IS in accordance with the decrease in the electron generation rate b. That is, in the steady state before introducing the electron-capturing substance into the detection cell, the sensitivity can be maintained substantially constant by determining IS such that f0 shown in the equation (2) is constant.

Specifically, the electron capture detector (ECD) according to the present invention comprises: a) ionizing a carrier gas introduced into a detection cell;
Electron emission means for emitting electrons, b) current value setting means for setting a reference current value, and c) applying a pulse voltage to an electrode disposed in the detection cell, and causing a current flowing through the electrode by the electrons. Pulse voltage applying means for controlling the frequency of the pulse voltage so that the reference current value is obtained; d) determining a reference current value such that the frequency of the pulse voltage becomes a predetermined frequency before introducing the sample into the detection cell. And control means for causing the current value setting means to set the reference current value.

In this configuration, a frequency input means for inputting and setting the frequency to be a reference before the sample is introduced, ie, the predetermined frequency, may be provided.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In an ECD according to the present invention, in a steady state before a sample is introduced into a detection cell, a control means changes a current value by a current value setting means within a predetermined range. The frequency of the pulse voltage generated by the application means is monitored. Then, a current value at which the frequency becomes a predetermined frequency is found, and the value is set as a reference current value by the current value setting means. The pulse voltage applying means controls the frequency of the pulse voltage so that the current flowing through the electrode has the reference current value. In this state, when the sample is introduced and the electrode current decreases, the frequency of the pulse voltage is increased to compensate for the decrease. The change in the frequency is extracted and used as a detection output.

Since the frequency of the pulse voltage in the steady state where no sample is introduced is kept constant, if the inside of the detection cell or the electrodes becomes dirty and the above-mentioned electrode current becomes difficult to flow, the electrode current will decrease. Accordingly, the reference current value also decreases. Thereby, the detection sensitivity can always be kept constant.

[0015]

According to the ECD of the present invention, even when the state of the detection cell changes due to contamination by the sample or the like, the concentration of the sample can always be detected with a constant sensitivity.
Therefore, high precision and reproducibility can be achieved.

[0016]

An ECD according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a configuration diagram of a main part of the ECD. The detection cell 12 connected to the column 11 of the gas chromatograph is a sealed type provided with a carrier gas inlet and an outlet, and has a radioactive isotope ⁶³ N therein.
A β-ray source (not shown) carrying i and the like and an anode 13 are provided. Note that the main body of the detection cell 12 is grounded. The anode 13 is connected to a coil L2 on the secondary side of the transformer 14, and is connected to the difference amplifier 15 via the coil L2. The output voltage of the difference amplifier 15 is provided to a voltage / frequency (V / F) converter 16, and the V / F converter 16 outputs a pulse signal having a frequency according to the input voltage value. The pulse voltage generating circuit 17 includes a pulse shaper, a current driving transistor, and the like. The pulse voltage generating circuit 17 receives a pulse signal from the V / F converter 16 and outputs a pulse voltage having the frequency to the primary side of the transformer 14. Give to coil L1.

The output voltage of the difference amplifier 15 is also A / D
The signal is converted into a digital signal by the converter 18 and sent to the control unit 20. The control unit 20 is configured by a microcomputer including a CPU, for example, and a D /
The A converter 19, the memory 21, the key input unit 22, and the data processing device 23 are connected.

The operation of the ECD is as follows. When an inert gas (N ₂ ) is caused to flow through the detection cell 12, the inert gas molecules are ionized by the β-ray source, and emit electrons having low kinetic energy (free electrons). According to the above configuration, since a positive pulse voltage is applied to the anode 13 from the secondary coil L2 of the transformer 14, free electrons are taken in and a current flows to the anode 13 only during the voltage application period. Therefore, when the frequency f of the pulse voltage is increased, the average current (anode current) ID of the anode 13 within a predetermined period longer than the pulse cycle is increased, and when the frequency f is decreased, the anode current ID is increased.
Decreases.

The operator inputs and sets the value of the basic frequency f0 'of the pulse voltage from the key input section 22. Since this fundamental frequency f0 'is related to the detection sensitivity, it may be set appropriately according to the desired detection sensitivity. Before analysis, that is, in a steady state before a sample is introduced from the column 11 into the detection cell 12, the control unit 20 changes the set current IS by scanning a digital signal to be sent to the D / A converter 19 within a predetermined range. Let it. On the other hand, since the value of the anode current ID is defined by the pulse frequency f as described above, the difference i (= ID-IS) between the anode current ID and the set current IS is calculated by the difference amplifier 1
Given from 5. The difference amplifier 15 outputs a voltage corresponding to the difference current i and supplies the voltage to the V / F converter 16 and the A / D converter 18.

As described above, when the set current IS changes, the difference i also changes, so that the digital value sent from the A / D converter 18 to the control unit 20 also changes. Since this digital value can be associated with the frequency of the pulse signal output from the V / F converter 16, the control unit 2
0 determines the frequency from the input digital value, and the value DS of the digital signal to be sent to the D / A converter 19 when the frequency matches the previously set fundamental frequency f0 '.
To get. This value DS is a value that gives the set current IS that can set the frequency of the pulse signal to the basic frequency f0 'in the above configuration. This control for determining DS is performed for a predetermined period of time or for a time set by the operator, and after sending DS to the D / A converter 19, the control is terminated. The value DS may be stored in the memory 21.

If the inside of the detection cell 12 becomes dirty due to the adhesion of a sample or the like, the electron generation rate decreases and the anode current ID
Is difficult to flow. However, when the above-described control is performed, the set current IS is also reduced, and the difference current i is maintained substantially constant.

When the above control is completed and the value DS is sent to the D / A converter 19, the apparatus sends the analysis READY signal to the control unit 2.
0 or display of data processing device 23 (not shown)
To inform the analyst that the sample is ready for injection. Alternatively, an analysis READY signal is sent to an automatic sample injection device to prompt sample injection. D / A converter 19
Converts DS into an analog signal and causes a set current IS to flow therethrough. As described above, the difference i between the anode current ID and the set current IS is supplied from the difference amplifier 15 and
5 outputs a voltage corresponding to the difference current i and gives it to the V / F converter 16. The V / F converter 16 outputs a pulse signal having a frequency corresponding to the input voltage value, and the pulse voltage generating circuit 17 supplies a voltage having the frequency to the primary coil L1 of the transformer 14. The V / F converter 16 is set to increase the pulse frequency f when the difference current i is larger than a predetermined value (for example, zero). Therefore, the difference current i becomes a predetermined value by this feedback loop control. The frequency f is controlled as described above.

The sample introduced from the column 11 captures free electrons in the detection cell 12 and reduces the anode current ID. Accordingly, the value of the pulse frequency for setting the difference current i to a predetermined value increases to f1. The control unit 20 is an A / D
This frequency f1 is detected by the output signal of the converter 18,
Using the fundamental frequency f0 ', the concentration a of the sample is calculated based on the above equations (3) and (4). As described above, since the fundamental frequency f0 'is always constant (unless changed externally), the detection sensitivity is kept constant.

It should be noted that the digital value DS corresponding to the set current IS does not greatly change unless the state of contamination of the detection cell 12 and the anode 13 changes. Therefore, the operation of determining the digital value DS as described above and sending it to the D / A converter 19 does not necessarily have to be performed before each analysis. For example, each time the power of the apparatus is turned on, or It may be performed at least when it is assumed that a state change such as dirt will occur, such as every time a certain period of use. Of course, when the value of the input and set fundamental frequency f0 'is changed, a new digital value DS must be obtained.

Since the detection sensitivity increases as the basic frequency f0 'increases, the operator may appropriately set the basic frequency f0' according to the desired detection sensitivity. However, it is not always necessary to set the basic frequency f0 'from the outside.
0 'may be stored, and the value may always be used.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a main part of an electron capture detector (ECD) according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Column 12 ... Detection cell 13 ... Anode 14 ... Transformer 15 ... Difference amplifier 16 ... Voltage / frequency (V / F) converter 17 ... Pulse voltage generation circuit 18 ... A / D converter 19 ... D / A converter 20 ... Control unit 21 ... Memory 22 ... Key input unit 23 ... Data processing device

Claims (1)

[Claims] 1. An electron emission means for ionizing a carrier gas introduced into a detection cell and emitting electrons, b) a current value setting means for setting a reference current value, and c) a current value setting means for setting a reference current value. A pulse voltage applying means for applying a pulse voltage to the provided electrode and controlling the frequency of the pulse voltage so that the current flowing through the electrode by the electrons becomes the reference current value; andd) introducing a sample into the detection cell. Control means for determining a reference current value so that the frequency of the pulse voltage becomes a predetermined frequency, and causing the current value setting means to set the reference current value. vessel.