JP4569606B2 - Electron capture detector - Google Patents

Description

The present invention relates to an electron capture detector (hereinafter referred to as ECD (= Electron Capture Detector)) used as a detector for a gas chromatograph apparatus or the like.

Various detectors for gas chromatograph apparatuses have been put into practical use. Among them, ECD is useful for measuring electrophilic compounds such as halogen compounds and nitro compounds. For this reason, it is used for the residual measurement of organic mercury, agricultural chemicals, PCB, etc., or the trace amount measurement by converting steroids, amino acids, etc. into electrophilic derivatives.

The operating principle of ECD is as follows. A radioactive isotope such as ⁶³ Ni is sealed in the detection cell, a carrier gas is introduced into the detection cell, and carrier gas molecules are ionized by radiation to emit electrons (free electrons). When a voltage is applied to the electrode (anode) disposed in the detection cell, in a steady state, a certain amount of current flows through the electrode due to the free electrons.

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

When a molecule of an electron-capturing substance is placed in the detection cell, the molecule absorbs free electrons emitted from the carrier gas and decreases the density of free electrons. Since the negative ions of the electron-trapping substance that has absorbed free electrons have a moving speed much lower than that of the free electrons, the current flowing through the electrode decreases due to the decrease in 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 in order to compensate for the decrease in the number of electrons captured by one pulse voltage.

It is known that the concentration a of the electron-capturing substance and the pulse frequency f have the following relationship (for example, RJMaggs, et al., “The Electron Capture Detector-A New Mode of Operation”, ANALYTICAL CHEMISTRY, VOL.43, NO.14, DECEMBER 1971, p.1967).
K.f = (k1.a + KD) (K, k1, KD: constant) (1)
Therefore, the change Δf in pulse frequency from the state where a = 0, that is, the state where no electron-capturing substance is introduced into the detection cell is
Δf = f−f0 = (k1 · a + KD) / K−KD / K
= (K1 / K) · a (2)
And proportional to the concentration a. That is, the concentration a of the sample is determined from the change Δf of the frequency f,
a = Δf / (k1 / K) (3)
Can be calculated. The value of (k1 / K) is obtained in advance by experiments.

Since the detection voltage V extracted from the output of the integrator is in accordance with the change of the pulse frequency f, the detection voltage V is recorded over time, so that a chromatogram relating to the concentration of the target electron-capturing substance is obtained. Is obtained.

If the electrode and the inside of the detection cell become contaminated with the sample while the measurement is repeated, the current becomes difficult to flow and the frequency f increases. For this reason, even if it measures with the same sample, the problem that a measurement result changes with time arises. On the other hand, since ECD uses a radioisotope as described above, special techniques are required for cleaning the inside of the detection cell, and there is a restriction that it cannot be easily cleaned.

The present invention has been made to solve such a problem, and the object of the present invention is to appropriately correct a change over time due to contamination inside the detection cell and always output a correct measurement result. It is to provide an ECD that can be used.

The electron capture detector according to the present invention, which has been made to solve the above problems, includes a) an electron emission means for ionizing a carrier gas introduced into a detection cell and emitting electrons, and b) a predetermined current. Current value setting means for setting a value; and c) pulse control for applying a pulse voltage to the electrode provided in the detection cell and controlling the frequency of the pulse voltage so that the current caused by the electrons becomes the predetermined current value. Means, d) frequency storage means for storing the frequency value f00 in the initial state of the detection cell, and the frequency value f0 before introducing the sample into the detection cell at the time of sample analysis, and e) the detection cell Current value storage means for storing the current value Id in the initial state and the current value Is before introducing the sample into the detection cell at the time of sample analysis, and f) after injecting the analysis sample into the detection cell The frequency f of the pulse voltage and the both frequencies And a concentration calculating means for calculating the concentration of the analysis sample using the numerical values f00 and f0 and the two current values Id and Is.

The ECD according to the present invention can always detect the correct concentration even when the state of the detection cell changes due to contamination by a sample or the like. Further, since the degree of contamination of the detection cell can be known from the change in the frequency value stored in the frequency storage means, the necessity of cleaning can be appropriately determined and a warning can be given.

In a state in which the detection cells are clean immediately after manufacturing the ECD or immediately after cleaning (these are called initial states), first, a pulse frequency f00 corresponding to a predetermined current value is measured, and this value (initial frequency) is obtained. The frequency is stored in the frequency storage means.

When analyzing the sample, the pulse frequency f0 before injecting the analysis sample is measured, and this value (frequency before measurement) is also stored in the frequency storage means. These values stored in the frequency storage means can be used in several ways.

The first method of use can be used to automatically correct the detector characteristics that change with time due to contamination inside the detection cell and to always calculate a correct detection value. That is, when the sample concentration a is calculated by the above equation (3) using the pulse frequency f after the analysis sample is injected, the values f00 and f0 stored in the frequency storage means are used instead of the value f. The value f1 corrected by using is used. In particular,
f1 = f · (f00 / f0) (4)
And using the calculated value f1, the density a is set to a = Δf / k1 = (f1−f0) / k1 (5)
And calculate. Or directly using the values of f00, f0, f,
a = {f · (f00 / f0) −f0} / k1 (6)
May be calculated.

The reason why such correction (or calculation) can be performed is as follows. When the inside of the ECD becomes dirty, the constant K in the equation (1) changes to K1.
K1 · f = (k1 · a + KD) (K1, k1, KD: constant) (1b)
When a = 0, the frequency is f00 = KD / K according to the equation (1) when the detection cell is clean (initial state).
However, when the detection cell is dirty, f0 = KD / K1 according to the equation (1b).
It becomes. Therefore,
f00 / f0 = K1 / K (7)
It becomes. When the detection cell is dirty, Δf ′ = f′−f0 = (k1 / K1) · a according to the equation (2).
= (F0 / f00) ・ (k1 / K) ・ a
= (F0 / f00) · Δf (8)
It becomes. Therefore, as shown in the equations (5) and (6), by using f1 = f · (f0 / f00) instead of f, the sample concentration a can always be calculated by the same equation.

The second usage method is a method in which the detection value is not corrected according to the sensitivity change 2 as described above, but the sensitivity is made constant. That is, the set current value is changed so that the pre-measurement frequency f0 always coincides with the initial frequency f00. Note that the method for correcting the detection value and the method for changing the set current value can be switched as appropriate.

The third method of use is to compare the pre-measurement frequency f0 with the initial frequency f00 each time to determine the degree of contamination of the detector. For example, when the difference or ratio between the two values is equal to or greater than a predetermined value, a warning can be issued and the cell can be replaced.

An embodiment of an electron capture detector (ECD) according to the present invention will be described with reference to FIG. The detection cell 12 connected to the gas chromatograph column 11 is a sealed type provided with a carrier gas introduction port and a discharge port, and a β-ray source and an anode 13 carrying a radioisotope ⁶³ Ni or the like inside. Is provided. The detection cell 12 body is grounded. The anode 13 is connected to the secondary coil of the transformer 14 and is connected to the differential amplifier 15 via the coil. The output voltage of the differential amplifier 15 is supplied to a voltage / frequency (V / F) converter 16, and the V / F converter 16 outputs a pulse signal having a frequency corresponding to the input voltage value. The pulse voltage generation circuit 17 receives the pulse signal from the V / F converter 16 and applies a voltage of that frequency to the primary coil of the transformer 14.

The output voltage of the differential amplifier 15 is also converted into a digital signal by the A / D converter 18, and this digital signal is sent to the current setting unit 19 and the calculation control unit 20. A memory 21, a key input unit 22, and a data processing device 23 are connected to the arithmetic control unit 20.

The operation of this ECD is as follows. When a carrier gas is allowed to flow through the detection cell 12, the carrier gas is ionized by the β-ray source and emits electrons (free electrons) having low kinetic energy. With the above configuration, since a positive pulse voltage is applied to the anode 13 from the secondary coil of the transformer 14, a current flows through the anode 13 by free electrons only during the period in which the voltage is applied. 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 period increases, and when the frequency f is decreased, the anode current ID decreases.

When the operator inputs the value of the set current IS from the key input unit 22, the arithmetic control unit 20 sends a corresponding set current signal to the current setting unit 19, and the current setting unit 19 outputs a set current IS corresponding to the set current signal. . However, since the value of the anode current ID is defined by the pulse frequency f as described above, the difference i (= ID−IS) between it and the set current IS is given from the difference amplifier 15. The difference amplifier 15 outputs a voltage corresponding to the difference current i and supplies it to the V / F converter 16. As described above, the V / F converter 16 outputs a pulse signal having a frequency corresponding to the input voltage value, and the pulse voltage generation circuit 17 applies the voltage having the frequency to the primary coil 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), so that the difference current i becomes a predetermined value by this loop control. The frequency f is controlled. The pulse frequency f when the carrier gas flowing in the detection cell 12 is in a steady state and the difference current i is controlled to a predetermined value is called a zero frequency.

In the ECD of the present embodiment, after the detection cell 12 is washed, the operator operates the key input unit 22 to store the zero frequency f00 in the memory 21. This is the initial frequency. In Japan, the user cannot clean the ECD detection cell 12 at this time, and it is a manufacturer's engineer or serviceman who performs such an operation.

When the user analyzes a sample using the ECD, the zero frequency f 0 is stored in the memory 21 by operating the key input unit 22 before introducing the sample into the detection cell 12. This is the frequency before measurement. The measurement of the pre-measurement frequency is always performed before the sample analysis even after the detection cell 12 has been washed and the detection cell 12 has already been used several times. During this time, the inside of the detection cell 12 is gradually contaminated by the sample, and the anode current ID is less likely to flow, so the pre-measurement frequency f0 generally increases gradually.

Thereafter, the sample delivered from the column 11 is introduced into the detection cell 12. The introduced sample captures free electrons in the detection cell 12 and decreases the anode current ID. Accordingly, the value of the pulse frequency for setting the difference current i to a predetermined value increases and becomes f1.

The arithmetic control unit uses the value f1, the initial frequency f00 and the pre-measurement frequency f0 stored in the memory to calculate the concentration a of the sample by the above formula (4) and (5) or (6). .

Note that the initial frequency f00 and the pre-measurement frequency f0 may not be manually stored in the memory 21 by the operator's key operation as described above, but may be automatically stored at a predetermined timing. For example, in many cases, the detection cell 12 is heated at the time of analysis, but the zero frequency f00 and f0 when the temperature of the detection cell 12 reaches a predetermined value can be stored in the memory 21. . In addition, when the zero frequency is automatically and sequentially stored in the memory after the detection cell 12 is washed, the zero frequency stored at the oldest time is set as the initial frequency f00, and the zero frequency stored at the newest time is measured. The previous frequency f0 can be used.

Next, a calculation method different from the above will be described. From equation (8), Δf ′ = (f0 / f00) · Δf
Where f0 / f00 = c,
Δf ′ = c · Δf (9)
And After the detection cell 12 is washed, the detection cell 12 is gradually soiled experimentally in advance, and f0 is obtained at each soiling stage, c is calculated by f0 / f00, and stored in the memory 21. At the time of sample analysis, a pre-measurement frequency f0 is obtained, c corresponding thereto is read from the memory 21, and the concentration a is calculated by the equations (9) and (5).

In the above embodiment, the set current value IS at the time of analysis must be equal to the set current value IS at the time of storing f0 after washing the detection cell. Since the set current value IS generally differs depending on the analysis sample, when analyzing various samples, the initial frequency f00 and the pre-measurement frequency f0 must be stored in the memory 21 for each set current value IS. . The following shows how to simplify it.

The constant K in equation (1) is
K = b.q. {1-exp [-(k1.a + KD) / f]} / ID
(B: constant, q: electron charge, ID: set current value) (the above-mentioned document). here,
K = k / ID
Then, the zero frequency f0 (including the initial frequency f00 and the pre-measurement frequency f0) is
f0 / ID = KD / k
It becomes. By storing the value of f0 / ID at a certain set current value ID in the memory 21, the zero frequency f0S at an arbitrary set current value IS is
f0S = (f0 / ID) · IS
Is calculated. Therefore, using this and equations (8) and (9),
Δf ′ = (IS / ID) · c · Δf
Δf ′ is calculated by the above, and based on this, the concentration a of the sample can be obtained.

As described above, the ECD of this embodiment can always detect the correct concentration even if the detection cell 12 is dirty. Further, the current zero frequency value f0 is stored in the memory 21, and this value corresponds to the degree of contamination of the detection cell 12. Therefore, it is possible to output a warning or a message that prompts cleaning of the detection cell 12 to the data processing device 23 or the like when this value exceeds a predetermined threshold. Further, since the zero frequency immediately after cleaning is stored in the memory 21, this value is compared with the current zero frequency, and a threshold is set in advance for the ratio or difference (for example, f00). /F0=0.5) Similarly, a warning or message of contamination of the detection cell 12 can be output.

Note that the function of the arithmetic control unit 20 may be further strengthened, and the function of the V / F converter 16 may be executed by the arithmetic control unit 20. For example, as shown in FIG. 2, the output of the differential amplifier 15 is input to the arithmetic control unit 20 via the A / D converter 18, and the arithmetic control unit 20 generates a pulse signal based on the value to generate a pulse voltage generation circuit. 17 to give. As a result, any operation can be performed by the key input unit 22 or the like of the arithmetic control unit 20, so that the operability is greatly improved. Further, when the second usage method is used, control is performed such that the calculation control unit 20 calculates a corresponding setting current based on the detected pre-measurement frequency f 0 and sends a current setting signal to the current setting unit 19. be able to.

1 is a schematic circuit diagram of an electron capture detector (ECD) that is an embodiment of the present invention. FIG. 6 is a schematic configuration circuit diagram of an ECD that is another embodiment of the present invention.

Explanation of symbols

11 ... Chromatograph column 12 ... Detection cell 13 ... Anode 14 ... Transformer 15 ... Differential amplifier 16 ... Voltage / frequency (V / F) converter 17 ... Pulse voltage generation Circuit 18 ... A / D converter 19 ... Current setting unit 20 ... Operation control unit 21 ... Memory 22 ... Key input unit 23 ... Data processing device

Claims (1)

a) electron emission means for ionizing the carrier gas introduced into the detection cell and emitting electrons; b) current value setting means for setting a predetermined current value; and c) an electrode provided in the detection cell. A pulse control means for applying a pulse voltage and controlling the frequency of the pulse voltage so that the current due to the electrons becomes the predetermined current value; and d) the frequency value f00 in the initial state of the detection cell, and the sample analysis Frequency storage means for storing the frequency value f0 before introducing the sample into the detection cell, e) current value storage means for storing any set current value set in the current value setting means, f) Using the frequency f of the pulse voltage after the analysis sample is injected into the detection cell, the values f00 and f0 of both the frequencies, and any set current value stored in the current value storage means, the concentration of the analysis sample is determined. Calculated concentration An electron capture detector comprising: a calculation means.

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