JP2008232783A - Electron capture detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable analysis of a sample of a low concentration by improving detection sensitivity. <P>SOLUTION: The amount of a radiation source 16 that is a radioactive isotope element enclosed in a detection cell 11 is set in the range of not less than 20MBq and less than 100MBq that is lower than before, for instance, at 90MBq. Thus, the amount of free electrons generated following positive ionization of inert gas in the detection cell 11 is reduced, so that the change in the amount of electrons captured when an object substance (electrophilic molecule) to be detected of low concentration is introduced is more easily indicated in a detection signal and thus the detection sensitivity is improved. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electron capture detector (ECD = Electron Capture Detector) used as a detector such as a gas chromatograph.

  Various detectors for gas chromatographs have been put into practical use. Among them, the electron capture detector is a detector particularly suitable for measuring electrophilic compounds such as halogen compounds and nitro compounds. For this reason, it is mainly 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 operation principle of the electron capture detector is as follows (see, for example, Patent Document 1). A radiation source that is a radioactive isotope such as ⁶³ Ni is sealed inside the detection cell, and an inert gas such as N ₂ is introduced into the detection cell. Then, inert gas molecules are ionized by the action of radiation (β rays) emitted from the radiation source, the molecules are positively ionized, and free electrons are emitted from the molecules. When a pulsed voltage is applied to the electrode (positive electrode) installed in the detection cell in this state, free electrons are taken in and a current flows through the electrode. If an electrophilic (electron capturing) molecule having a high ability to absorb electrons is introduced there, the electrophilic molecule absorbs free electrons and becomes negative ions. Density decreases.

  Molecules that have become negative ions move toward the positive electrode in the same way as free electrons, but since negative ions have a remarkably larger mass than free electrons, the movement speed is slow and it takes time to reach the positive electrode. Further, since it has a high probability of binding to positive ionized inert gas molecules before reaching the positive electrode, unlike free electrons, it hardly contributes as a current flowing through the electrode. Therefore, when the density of free electrons in the detection cell decreases, the number of electrons taken into the electrode with respect to one voltage pulse also decreases, and the current flowing through the electrode decreases. When the number of voltage pulses is controlled so that the total number of electrons per unit time, that is, the current value is kept constant, the number of voltage pulses increases as the concentration of electrophilic molecules increases and the degree of decrease in free electrons increases. Therefore, the concentration of the introduced electrophilic molecule can be obtained by changing the number of voltage pulses. Specifically, a detection signal corresponding to the concentration of the electrophilic substance can be obtained by converting the change in the number of voltage pulses per unit time, that is, the change in frequency into a voltage (FV conversion). .

  Although the electron capture detector is originally a relatively high-sensitivity detector, in recent years it has become more sensitive than ever due to increasing interest in environmental pollutants such as pesticide residues and organic solvents, and tightening regulations. It is getting demanded. In response to such demands, attempts have been made to improve sensitivity by improving the efficiency of ionization of an inert gas by improving the structure of the detection cell (see, for example, Patent Document 2). However, these conventional attempts have limitations, and it is difficult to meet the demand for higher sensitivity.

JP-A-11-153579 JP 2000-65799 A

  The present invention has been made to solve the above-mentioned problems, and the main object of the present invention is to provide an electron capture type detection capable of improving detection sensitivity and detecting low concentrations and trace amounts of substances as compared with the prior art. Is to provide a vessel.

  In the electron capture detector, since the free electrons that cannot be captured by the electrophilic molecules introduced into the detection cell become a current that flows to the electrode, the amount of remaining free electrons is small. If no voltage pulse is applied to the electrode, no current flows. As a result, the frequency of the voltage pulse is increased and the result of the FV conversion is increased, and the influence of noise and the like is relatively reduced. In order to reduce the amount of remaining free electrons in the detection cell even when the concentration (density) of the electrophilic molecule is low, the amount of free electrons in a state where no electrophilic molecule is introduced may be reduced. Since the free electrons are generated when the inert gas molecules are ionized by the radiation emitted from the radiation source, the amount of free electrons depends on the intensity of the radiation.

  Conventionally, the amount of radioactive isotope elements used in electron capture detectors is 300 MBq or more, but the inventors of the present application pay attention to the radiation intensity of these radiation sources and reduce the amount of radiation sources. This has led to the idea of reducing the amount of free electrons in the detection cell and increasing detection sensitivity.

  The present invention, which has been made to solve the above problems, has a radiation source that is a radioisotope and an electrode for electron capture inside, and a gas inlet for introducing a sample gas containing a component to be detected at the bottom. A detection cell is provided, a voltage is applied between both electrodes with the detection cell serving as a negative electrode and the electron capture electrode serving as a positive electrode, and is introduced into the detection cell from the gas inlet by radiation emitted from the radiation source. In an electron capture detector for ionizing gas molecules and detecting a current flowing through an electron capture electrode due to electrons generated thereby, the amount of radioactive isotope of the radiation source is in a range of 20 MBq to less than 100 MBq It is characterized by being set to.

  That is, in the electron capture detector according to the present invention, the amount of the radiation source is reduced to 1/3 or less of the conventional one. Therefore, the amount of free electrons generated by ionization by radiation in the detection cell is considerably smaller than before, and even if a low concentration of electrophilic molecules is introduced and a small amount of free electrons are captured, the effect is relatively Appear greatly in Thereby, detection sensitivity can be raised more than before.

However, if the amount of the radiation source is small, the difference between the amount of free electrons generated and the amount of electrons captured by the electrophilic molecule is small, and in some cases the amount of free electrons is insufficient. The linearity (dynamic range) of the response with respect to the concentration of is reduced. Specifically, according to the present inventor's experiments, whereas the concentration ranges the radiation source volume can guarantee the linearity in the case of 370MBq is about 10 ^4, a straight line when the source volume is 90MBq concentration range that can guarantee the sex is about 10 ³ and 1 digit poor. This linearity is further decreased when reducing the radiation source volume than, it is less than 20MBq is estimated that concentration range that can guarantee the linearity is less than about 10 ^2, not practical as a detector. Therefore, the lower limit of the radiation source amount is set to 20 MBq.

  As described above, according to the electron capture detector according to the present invention, it is possible to detect a low-concentration substance or a substance contained in a small sample introduction amount by improving the detection sensitivity as compared with the related art.

  Furthermore, there is a completely different advantage by suppressing the radiation source amount to less than 100 MBq. In other words, since radiation may have a negative impact on human health, radiation-based devices are subject to legal regulations in Japan, and the use of electron capture detectors was also specified for use. It is necessary to report (or obtain a use permission) to a predetermined institution above. Therefore, there is an inconvenience that the apparatus cannot be moved freely. On the other hand, recently, this regulation has been partially relaxed, and devices having a radiation source amount of less than 100 MBq have been excluded from the regulation target. Therefore, the electron capture detector according to the present invention does not need to be subject to the regulation. As a result, it is possible to move the detector freely, realizing a portable detector that was impossible in the past, transporting the detector to the site where the substance to be measured is located, and measuring on the spot Can also be performed.

  An electron capture detector according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of a main part of the electron capture detector of this embodiment.

A detection chamber 12 is formed inside the detection cell 11, and a gas introduction port 13 to which an outlet end of a column 14 of the gas chromatograph and a makeup gas flow path 15 are connected is provided below the detection chamber 12. On the other hand, an exhaust gas flow path 18 for discharging the gas in the chamber 12 to the outside is connected to the upper portion of the detection chamber 12. The detection chamber 12 is provided with a radiation source 16 carrying a radioisotope ⁶³ Ni around it, and a rod-like collector electrode (electron capturing electrode) 17 is disposed at the center. The collector electrode 17 and the detection cell 11 as a conductor are electrically insulated, and a pulsed positive voltage is applied between the collector electrode 17 and the detection cell 11 by an electric circuit (not shown). It has become. The amount of the radiation source 16 is set to 90 MBq, which is about ¼ of the amount of radiation source 370 MBq in a commercially available electron capture detector.

The operation of the electron capture detector of the embodiment is as follows. When an inert gas such as N ₂ is flowed as makeup gas from the gas inlet 13 through the makeup gas channel 15 to the detection chamber 12, the inert gas molecules are ionized by radiation (β rays) emitted from the radiation source 16. Then, electrons (free electrons) with low kinetic energy are emitted and become positive ions. As described above, since a positive pulse voltage is applied to the collector electrode 17, free electrons are taken in and a current flows through the collector electrode 17 only during the period in which this voltage is applied. Accordingly, when the frequency of the pulse voltage is increased, the average current flowing through the collector electrode 17 during a predetermined unit time longer than the pulse cycle increases, and when the frequency of the pulse voltage is decreased, the average current decreases.

  As described above, since the amount of the radiation source 16 is considerably smaller than the conventional one, the number of inert gas molecules to be ionized is small and the number (density) of free electrons leaving the inert gas molecules is also small. Therefore, when the frequency of the pulse voltage applied to the electrode 17 is controlled so that a certain constant current flows to the collector electrode 17, the frequency becomes higher.

  In this state, when an electrophilic molecule as a detection target substance is introduced from the outlet end of the column 14 into the detection chamber 12 through the gas inlet port 13, the molecule absorbs free electrons and becomes negative ions. Free electrons decrease. As described above, since the number of free electrons is further reduced due to electron capture from the state where the number of free electrons is relatively small, a constant current is secured unless the frequency of the pulse voltage applied to the collector electrode 17 is further increased. I can't. Since the number of free electrons is originally relatively small, at least the influence of the number of free electrons trapped by the electrophilic molecule is significantly reflected in the increase of the pulse voltage frequency. This makes it possible to detect electrophilic molecules at a lower concentration than in the past.

  The difference in sensitivity between the electron capture detector according to this embodiment and the conventional detector will be described based on the actual measurement result. FIG. 2 shows an actual measurement of the relationship between the sample injection amount and the response value for the electron capture detector of the present embodiment having a radiation source amount of about 90 MBq and the conventional electron capture detector having a radiation source amount of 370 MBq. It is a graph which shows the result. The sample is γ-BHC.

As can be seen from this result, the electron capture detector of the present example is more than 10 times more sensitive than the conventional one, and it is possible to detect even a low-concentration sample of 0.1 pg or less that could not be detected conventionally. It has become. Although linearity (dynamic range) is reduced reducing the radiation source volume, whereas conventional (radiation source volume 370 MBq) is linear in the concentration range of about ¹⁰⁴ is guaranteed, this embodiment (Radiation Even with a source amount of 90 MBq), linearity is guaranteed in a concentration range of about 10 ³ , which is practically sufficient.

Also, from these experimental results, it can be inferred that the linearity is further reduced by an order of magnitude when the radiation source amount is less than 20 MBq, which is about ¼ or less of the present embodiment. That is, the concentration range that can guarantee the linearity will be becomes narrower than about 10 ^2, not practical as a narrower detector to this extent. Therefore, even if the detection sensitivity is improved, the lower limit of the radiation source amount should be set to about 20 MBq.

  The above-described embodiment is an example of the present invention, and it is a matter of course that modifications, corrections, and additions may be appropriately made within the scope of the present invention, and included in the scope of the claims of the present application.

The block diagram of the principal part of the electron capture type | mold detector which is one Example of this invention. The graph which shows the result of having actually measured the relationship between the sample injection amount in the electron capture type detector of a present Example, and the conventional electron capture type detector, and a response value.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Detection cell 12 ... Detection chamber 13 ... Gas inlet 14 ... Column 15 ... Makeup gas flow path 16 ... Radiation source 17 ... Collector electrode 18 ... Exhaust gas flow path

Claims (1)

A detection cell having a radiation source that is a radioisotope and an electrode for electron capture inside, and a gas introduction port for introducing a sample gas containing a component to be detected at the bottom; Due to the electrons generated by applying a voltage between both electrodes with the capture electrode as the positive electrode, ionizing the gas molecules introduced into the detection cell from the gas inlet by the radiation emitted from the radiation source, and In the electron capture detector that detects the current flowing through the electron capture electrode,
An electron capture detector, wherein the amount of the radioisotope element of the radiation source is set in a range of 20 MBq or more and less than 100 MBq.

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