JP2012117926A - Method for analyzing isotopic concentration - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for analyzing isotopic concentration capable of obtaining accurate measurement data by suppressing the influence caused by residual water in an ion source in mass spectrometry adapting electron ionization as an ionization method.SOLUTION: In mass spectrometry by introducing a sample to an ion source and ionizing it by electron ionization, the sample and at least one kind of inert gas such as helium, nitrogen, argon, neon, krypton and xenon are simultaneously introduced to the ion source when determining concentration of a stable isotope of a specific element in the sample.

Description

  The present invention relates to an isotope concentration analysis method, and more particularly to an isotope concentration analysis method for determining the abundance ratio of a stable isotope of a specific element in a sample.

  Mass spectrometry and infrared spectroscopy are widely used as methods for obtaining the isotope labeling rate of a specific element in a compound. Specifically, in mass spectrometry, a pretreated sample or an untreated sample is analyzed with a mass spectrometer such as GCMS (Gas Chromatography Mass Spectrometry), LCMS (Liquid Chromatography Mass Spectrometry), or IRMS (Isotope Ratio Mass Spectrometry). The peak ratio between the molecular ion peak and mass number + α (α is the difference in mass number between two nuclides that are isotopes) or the peak ratio between a specific fragment ion and mass number + α is The isotope purity (isotope labeling rate) is obtained based on being equal to the abundance ratio of the nuclide as a body. By performing mass spectrometry in combination with GC or LC, the isotopic purity of a specific compound in the mixture can also be determined (see, for example, Patent Document 1).

JP 2005-30816 A

However, when the isotope concentration is obtained by mass spectrometry employing an electron ionization method as the ionization method, accurate data may not be obtained due to the influence of substances remaining in the ion source, for example, residual moisture. That is, in the electron ionization method, an electron beam is emitted from a sample molecule gasified by heating or the like to emit electrons from the molecule of molecular weight M, and M ⁺ ions (molecular ions) and bonds in the ions are formed. In order to generate a broken ion (fragment ion), there is no particular problem if there are only ions generated by electron ionization, and the obtained data, for example, the intensity ratio between the peak of the molecular ion and the peak of its isotope ion From the above, it is possible to obtain the isotope abundance ratio of a specific element in the molecule.

  However, when the sample is ionized by the electron ionization method, particularly when the residual moisture is large in the ion source, this moisture becomes a proton (proton) supplier to the sample, and there are few ions with protons added to the molecules. When the peaks of isotope ions and proton-added ions overlap, an error is generated in the analysis result and the isotope ratio increases.

  Therefore, the present invention has an object to provide an isotope concentration analysis method capable of obtaining accurate measurement data while suppressing the influence of residual moisture in an ion source in mass spectrometry employing an electron ionization method as an ionization method. It is said.

  In order to achieve the above object, the isotope concentration analysis method of the present invention is a stable isotope of a specific element in a sample by introducing the sample into an ion source and ionizing the sample by electron ionization to perform mass spectrometry. In the isotope concentration analysis method for obtaining the concentration of an inert gas, an inert gas is simultaneously introduced into the ion source together with the sample.

  Furthermore, in the method for analyzing an isotope concentration of the present invention, the inert gas is at least one of helium, nitrogen, argon, neon, krypton, and xenon, and the sample has a hydroxyl group, a ketone group, or a carboxyl group. Or a molecule containing a nitrogen atom having the same bond structure as amines or imines, and the sample is glycine, indole, benzaldehyde, benzyl alcohol, benzoic acid, It is characterized by being any one of phenol, pyruvic acid, N, N-dimethylformamide, dimethyl carbonate, diethyl carbonate, pyridine and melamine.

  According to the present invention, by introducing an inert gas into the ion source simultaneously with the sample, it is possible to suppress the generation of proton-added molecular ions due to the reaction between residual moisture in the ion source and sample molecules. In particular, samples containing a nitrogen atom having the same bond structure as that of a hydroxyl group, ketone group, carboxyl group, or amines or imines in the molecule are oxygen atoms or unpaired electrons of the nitrogen atom in these molecules. Since it functions as an acceptor, generation of proton-added molecular ions can be suppressed more reliably.

It is the schematic which shows an example of a mass spectrometer. It is explanatory drawing which shows an example of an ion source.

  First, in order to mass-analyze ions obtained by ionizing a sample, for example, as shown in FIGS. 1 and 2, an ion source 11 adopting an electron ionization method as an ionization method, and velocity convergence of ions generated in the ion source 11 are used. A mass spectrometer including a mass separation unit 12 having a toroidal electric field 12a for performing the above and a sectoral magnetic field 12b for performing direction convergence and a detector 13 for detecting the amount of ions is used.

  The ion source 11 includes an ion source chamber 11b provided with an ion source block 11a, a sample introduction unit 14 for introducing a sample into the ion source block 11a, and an inert gas that similarly introduces an inert gas into the ion source block 11a. And an introduction part 15. The sample introduction unit 14 is formed so that a predetermined amount of sample injected into the sample reservoir 17 from the sample injection port 17a with the microsyringe 16 is introduced by adjusting the flow rate with the needle valve 18, and the inert gas introduction unit 15 is introduced. Is configured such that an inert gas in an inert gas source 19 such as a gas cylinder is pressure-adjusted by a pressure regulating valve 20 and a flow rate is regulated by a needle valve 21 to be introduced.

  When using such a mass spectrometer to determine the concentration of a stable isotope of a specific element in a sample, a predetermined amount of sample is introduced from the sample introduction unit 14 into the ion source 11, and A predetermined amount of inert gas is introduced from the inert gas introduction unit 15. As the inert gas, a gas that is inert when the sample is ionized and does not adversely affect the analysis result is used. For example, a gas that does not have the same peak as the peak appearance position used for calculation of the isotope concentration of the sample, or various impurity components, particularly a gas having a water content of 1 ppm or less, such as helium, nitrogen, argon, neon, krypton Xenon can be used, and it is possible to use generally used high purity gas.

  Furthermore, in view of the effect of suppressing the generation of proton-added molecular ions, it is preferable to use an inert gas having a large molecular weight. For example, when comparing the aforementioned helium, nitrogen, argon, neon, krypton, and xenon, helium, neon, nitrogen , Argon, krypton, and xenon, the suppression effect increases in order, and xenon is maximized, but it is only necessary to select the sample type, mass spectrometer configuration, and gas cost. It can also be used as a mixture.

  The inert gas is preferably performed at a constant pressure and a constant flow rate, and the introduction pressure may be set according to the pressure of the ion source 11, and the secondary side of the pressure regulating valve 20 is set to about atmospheric pressure. Good. The amount introduced varies depending on the type of sample and the configuration of the mass spectrometer, particularly the configuration of the ion source and the amount of residual water in the ion source, but usually a range of 0.1 to 5 ml per minute is appropriate. If the introduction amount of the inert gas is less than 0.1 ml / min, it becomes difficult to obtain the effect of introducing the inert gas into the ion source 11 sufficiently, and the introduction amount of the inert gas is more than 5 ml / min. If it becomes too much, the sample concentration in the ion source 11 is relatively decreased, making it difficult to perform accurate measurement, or maintaining the inside of the ion source 11 at an appropriate pressure. The introduction temperature of the inert gas may be room temperature and does not need to be heated or cooled.

  In addition, as shown in FIG. 2, the inert gas is introduced into the ion source 11 by providing an inert gas introduction part 15 separately from the sample introduction part 14, and the inert gas is separated from the sample. Is preferably introduced directly into the ion source 11, but depending on the type and properties of the sample, an inert gas is introduced along the path from the sample reservoir 17 or the sample reservoir 17 to the ion source 11, and the sample and the inert gas are introduced. Can be introduced into the ion source 11 in a mixed state. Portions where the inert gas flows, such as pipes and valves through which the inert gas flows, are preferably made of a material such as stainless steel or glass that is less likely to contain impurities in the inert gas.

  As the sample mass analysis procedure and setting conditions, conventional general procedures and conditions can be adopted except for the introduction of the inert gas. In the apparatus configuration, the mass separation means in the mass separation unit 12 may be any of a magnetic field type, a quadrupole electric field type, and a time-of-flight type. Or the double convergence type of the reverse arrangement | sequence arranged in order of the magnetic field-electric field may be sufficient. The detection means in the detector 13 may be a general detector such as a secondary electron multiplier or a Faraday cup as a mass spectrometry detector.

  Furthermore, the sample can be introduced in accordance with the state of the sample. In the case of a liquid sample having a relatively high vapor pressure, as shown in FIG. Thus, a sample direct introduction method in which a minute amount is leaked to the ion source can be employed, and the sample introduction amount into the sample reservoir at this time is usually 1 to 3 μL. In the case of a liquid sample or a solid sample having a low vapor pressure, a capillary tube filled with the sample is set at the tip of a direct probe or the like, and the ion source chamber 11b is preliminarily evacuated, and then the capillary tube is inserted into the ion source. The introduction can be carried out by the direct introduction method. In this case, the introduction amount of the liquid sample is 1 to 2 μL, and the introduction amount of the solid sample is suitably 1 to 2 mg.

  As other conditions, the temperature of the ion source 11 is about 200 ° C., the ionization voltage is about 30 to 70 eV, the ion source filament current is about 4 A, the mass scanning range is a range including a peak used for calculating the isotope concentration, and the mass The scanning speed is suitably from several seconds to several tens of seconds per SCAN.

  In this way, in mass spectrometry that has been generally performed in the past, by introducing an inert gas into the ion source 11 at the same time as the sample, the proton-added molecular ions caused by the reaction between the residual moisture in the ion source and the sample molecules Generation can be suppressed, and accurate measurement data can be obtained while suppressing the influence of residual moisture in the ion source. Moreover, it is only necessary to introduce an inert gas into the ion source 11, and it is possible to carry out by simply adding the inert gas introduction part 15 to a normal mass spectrometer, so that an increase in equipment cost is also suppressed slightly. be able to. Furthermore, since a commercially available high-purity gas can be used as the inert gas, an increase in cost required for analysis can be suppressed slightly. Therefore, the concentration and abundance ratio of stable isotopes in the sample can be accurately measured at a low cost.

In particular, a sample containing a hydroxyl group (—OH), a ketone group (—CO—) or a carboxyl group (—COOH) in the molecule, or amines (R ¹ —N (—R ² ) —R ³ (R ¹ , R ² and R ³ represent hydrogen or a hydrocarbon group, excluding ammonia)) or imines (R ⁴ —C (═NR ⁵ ) —R ⁶ (R ⁴ , R ⁵ and R ⁶ are hydrogen or hydrocarbon) A sample containing a nitrogen atom having the same bond structure as in ())), for example, glycine, indole, benzaldehyde, benzyl alcohol, benzoic acid, phenol, pyruvic acid, N, N-dimethylformamide, dimethyl carbonate, diethyl carbonate, In the case of pyridine and melamine, oxygen atoms and nitrogen atoms contained in the hydroxyl groups and the like in these samples become proton acceptors and are affected by residual moisture when ionized by the ion source 11. Although it is easily affected, since the generation of proton-added molecular ions from the sample can be suppressed by the inert gas introduced into the ion source 11, the sample has a large amount of oxygen atoms and nitrogen atoms serving as proton acceptors. However, accurate analysis can be reliably performed as compared with the conventional case.

  Experiments were performed using a mass spectrometer having the configuration shown in FIG. Unlabeled pyruvic acid was used for the sample, and a secondary electron multiplier was used for the detector 13. Nitrogen (pure gas) was used as the inert gas, the pressure was adjusted to atmospheric pressure with the pressure adjustment valve 20, and the flow rate was adjusted with the needle valve 21. As the inert gas introduction pipe, a stainless steel pipe and a glass capillary pipe whose inner surface was subjected to an inert treatment were used. The sample was introduced into the ion source by the direct sample introduction method.

  The amount of sample introduced into the sample reservoir is 2 μL, the ion source temperature is 200 ° C., the ionization voltage is 30 eV, the ion source filament current is 4 A, the mass scanning range is M / Z = 40 to 300, and the mass scanning speed is 1 SCAN / 2 sec. The mass spectrum was acquired by changing the introduction amount of nitrogen, which is an inert gas, to 0 (for comparison), 0.5 ml / min, 1 ml / min, and 10 ml / min. When the nitrogen flow rate was set to 10 ml / min, in the mass spectrometer used in this experiment, the protection circuit worked due to the vacuum degree in the ion source, and the mass spectrometer stopped and data could not be acquired.

Hydrogen and oxygen isotope abundance ratios are natural abundance ratios ( ¹ H: 99.985 atom%, ² H: 0.015 atom%, ¹⁶ O: 0.99759 atom%, ¹⁷ O: 0.037 atom%, ¹⁸ O: 0.0. In the obtained mass spectrum, the ¹³ C concentration of carbon was calculated using the peak intensity ratio values of M / Z = 88 and M / Z = 89. Table 1 shows the relationship between the nitrogen flow rate [ml / min] and the ¹³ C concentration [atom%].

From the results shown in Table 1, by introducing nitrogen into the ion source at the same time as the sample, the calculated ¹³ C concentration approaches the expected value of 1.108 atom%, and the adverse effect of residual moisture of the ion source on the analytical value is reduced. I understand that

The mass was the same as in Example 1 except that the sample was unlabeled N, N-dimethylformamide, the inert gas was argon, and the amount of argon introduced was changed to 0 ml / min, 1 ml / min, and 2 ml / min. A spectrum was acquired and ¹³ C concentration was calculated using peak intensity ratios of M / Z = 73 and M / Z = 74. Table 2 shows the relationship between the argon flow rate [ml / min] and the ¹³ C concentration [atom%]. This result also shows that the ¹³ C concentration approaches the expected value by introducing argon gas into the ion source simultaneously with the sample.

A ¹³ C concentration was calculated by acquiring a mass spectrum in the same manner as in Example 2 except that the amount of gas introduced was 1 ml / min and the type of inert gas was changed to neon, nitrogen, argon, krypton, and xenon. Table 3 shows the relationship between the type of inert gas introduced and the ¹³ C concentration [atom%]. From this result, it can be seen that the ¹³ C concentration approaches the expected value as the molecular weight of the introduced inert gas (Ne <N ₂ <Ar <Kr <Xe) increases.

Mass spectrum in the same manner as in Example 1 except that the sample was unlabeled melamine, the nitrogen introduction amount was 1 ml / min, and the nitrogen introduction pressure was changed to 0 MPa, 0.1 MPa, and 0.2 MPa (all gauge pressures). The ¹³ C concentration was calculated from the peak intensity ratio of M / Z = 126 and M / Z = 127. Table 4 shows the relationship between the nitrogen pressure [MPaG] and the ¹³ C concentration [atom%]. From this result, it can be seen that the nitrogen introduction pressure has little influence on the analysis result.

A mass spectrum was obtained in the same manner as in Example 1 except that the sample was unlabeled pyridine, the nitrogen introduction amount was 1 ml / min, and the nitrogen introduction temperature was changed to room temperature (26 ° C.), 75 ° C. and 150 ° C. The ¹³ C concentration was calculated from the peak intensity ratio of M / Z = 79 and M / Z = 80. The relationship between the nitrogen temperature [° C.] and the ¹³ C concentration [atom%] is shown in Table 5. From this result, it can be seen that the nitrogen introduction temperature hardly affects the analysis result.

  DESCRIPTION OF SYMBOLS 11 ... Ion source, 11a ... Ion source block, 11b ... Ion source chamber, 12 ... Mass separation part, 12a ... Toroidal electric field, 12b ... Fan magnetic field, 13 ... Detector, 14 ... Sample introduction part, 15 ... Inert gas introduction , 16 ... micro syringe, 17 ... sample reservoir, 17a ... sample inlet, 18 ... needle valve, 19 ... inert gas source, 20 ... pressure regulating valve, 21 ... needle valve

Claims (4)

In an isotope concentration analysis method for obtaining a stable isotope concentration of a specific element in a sample by introducing a sample into an ion source and performing mass spectrometry on ions ionized by an electron ionization method, the ion source includes the ion source An isotope concentration analysis method that simultaneously introduces an inert gas together with a sample. The isotope concentration analysis method according to claim 1, wherein the inert gas is at least one of helium, nitrogen, argon, neon, krypton, and xenon. The isotope according to claim 1 or 2, wherein the sample is a molecule containing a hydroxyl group, a ketone group or a carboxyl group, or a molecule containing a nitrogen atom having the same bond structure as amines or imines. Body concentration analysis method. 4. The sample according to claim 1, wherein the sample is any one of glycine, indole, benzaldehyde, benzyl alcohol, benzoic acid, phenol, pyruvic acid, N, N-dimethylformamide, dimethyl carbonate, diethyl carbonate, pyridine and melamine. The method for analyzing an isotope concentration according to any one of the preceding claims.

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