JP2002228636A - Method of analyzing trace amount of impurity in gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of analyzing a trace amount of impurities in a gas whereby the trace amount of impurities can be analyzed with a high sensitivity and a high accuracy in an apparatus in which a gas chromatograph and an atmospheric pressure ionization mass spectrometer are combined. SOLUTION: In analyzing the trace amount of impurities by separating a main component in a sample gas carried by a carrier gas and the trace amount of impurities from each other by the gas chromatograph 11 and introducing an outflow gas from the gas chromatograph into the atmospheric pressure ionization mass spectrometer 12, an addition gas to which a main component gas of the sample gas is mixed by a concentration of several ppm to several % is added to the outflow gas from the gas chromatograph, and then the outflow gas is introduced to the atmospheric pressure ionization mass spectrometer. Accordingly impurities conventionally hard to measure with high sensitivity are detected with high sensitivity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for analyzing trace impurities in a gas. The present invention relates to an analysis method effective for measuring sub-ppb level trace impurities with high sensitivity.

[0002]

2. Description of the Related Art Conventionally, when impurity analysis in a high-purity gas is measured by a combined analyzer using a gas chromatograph and an atmospheric pressure ionization mass spectrometer, the effluent gas of a gas chromatograph using a normally used packed column ( Usually 30
(50 cc / min) alone does not reach the gas amount required for the atmospheric pressure ionization mass spectrometer (100 to 500 cc / min). The measurement is performed after adding a fixed amount.

[0003] For example, Japanese Patent Application Laid-Open No. Hei 6-34616 discloses that "when the amount of carrier gas required by an atmospheric pressure ionization mass spectrometer cannot be supplied, the atmospheric pressure ionization is performed after the outlet of a column of a gas chromatograph. Injecting a gas of the same type as the carrier gas as a diluent gas between the mass spectrometer and the mass spectrometer.
No. 207 discloses that “Ultra-high-purity inert gas having a high ionization energy is used as a carrier gas, so that a gas component introduced into an atmospheric pressure ionization mass spectrometer per unit time is simply separated from the carrier gas by a column. By making two components of one trace impurity ... ".

[0004] By performing analysis in this manner, impurities in a gas having a low ionization energy, such as methane, hydrogen, and carbon dioxide in oxygen, are converted into a carrier gas composed of an ultra-high-purity inert gas having a high ionization energy. It can be analyzed as impurities in the material. The diluent gas or carrier gas used as an additive gas in these examples is high-purity helium, argon, or a mixed gas thereof, and the like.
This is a gas intended to dilute the main component gas, and a gas type different from the main component gas is used as the additional gas.

[0005]

However, for example, when helium or argon is used as a carrier gas and the sample gas is hydrogen, helium or argon ions ionized by an ion source have a high ion-molecule reaction rate with hydrogen. In addition, hydrogen ions eluted from the separation agent and gas chromatograph, and the main component residual hydrogen that remains in the ion source of the analyzer in trace amounts also cause an ion-molecule reaction in the ion source. And a proton transfer reaction with impurities such as nitrogen, methane and carbon monoxide.

At this time, the amount of hydrogen eluted from the separation column or the like or the amount of hydrogen remaining in the ion source is extremely small, and its concentration changes every moment. The amount of ions generated in the proton transfer reaction changes, resulting in an analysis result as if the impurity concentration itself had changed. Thus, there is a problem that the reproducibility of the analysis value is deteriorated due to the influence of the residual hydrogen, and it is difficult to perform the measurement with high sensitivity.

When measuring a component having a high adsorptivity or a reactivity to a separating agent or a tube wall of a gas chromatograph, blank measurement due to consumption of the measured component or the like is large, so that high-sensitivity measurement may be difficult. Therefore, a method of adding a component which is easily adsorbed or reacted to the carrier gas at a ppm level for the purpose of measuring a trace component which is easily adsorbed or reacted in the main component gas, or a main component itself of the sample gas is easily adsorbed or reacted In the case of a sample, a method has been proposed in which a sample gas is introduced several times in advance as a pretreatment gas for a column into a sample introduction section of a gas chromatograph before the start of analysis in order to enable the trace measurement. These methods are all applied when the measurement gas is easily adsorbed or reacted in the analytical system, particularly in the column, and aims to saturate the reactive or adsorbable site with the gas. By these methods, the problem in the column of the gas chromatograph can be solved.

However, according to the study of the present inventors, depending on the type of gas, the ability to adsorb and desorb to and from the inner wall at the downstream of the column rather than inside the column, that is, at the ion source of the mass spectrometer, etc. It has been found that since the measurement components remain for a long time due to the low value, the blank is high and the influence of the fluctuation becomes large. Therefore, in order to suppress the influence, the solution at the former stage of the column as in the above two methods is not suitable, but rather, it is necessary to take the solution at the latter stage.

Accordingly, the present invention provides a compound analyzer using a gas chromatograph and an atmospheric pressure ionization mass spectrometer, which controls the influence of the main component gas to remove trace amounts of ppb to sub-ppb level trace impurities in various high-purity gases. An object of the present invention is to provide a method for analyzing trace impurities in a gas, which can be analyzed with high reproducibility and high sensitivity.

[0010]

In order to achieve the above object, a method for analyzing trace impurities in a gas according to the present invention comprises separating a main component and trace impurities in a sample gas carried by a carrier gas by gas chromatography. And analyzing the trace impurities by introducing the effluent gas from the gas chromatograph into an atmospheric pressure ionization mass spectrometer,
It is characterized in that an additive gas containing a main component gas of a sample gas is added to an outflow gas from the gas chromatograph and then introduced into the atmospheric pressure ionization mass spectrometer.

Further, the additive gas contains hydrogen gas, and the main component gas contains oxygen,
It is characterized by being a hydride such as hydrogen, argon, carbon dioxide, carbon monoxide, silane, a halogen-based gas such as hydrogen chloride and a fluorocarbon-based gas, or a mixed gas thereof.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a system diagram showing one embodiment of a compound analyzer capable of implementing the method of the present invention. This analyzer is provided downstream of a gas chromatograph (GC) 11 for separating a main component gas and a trace impurity in a sample gas.
An atmospheric pressure ionization mass spectrometer (APIMS) 12 is disposed as a detection unit of the gas chromatograph 11, and a first purified gas supply path 13 for supplying a first purified gas used as a carrier gas of the gas chromatograph 11 is provided.
And a second purified gas supply path 14 for supplying a second purified gas for mixing with the additive gas of the atmospheric pressure ionization mass spectrometer 12, and accurately mixing the first purified gas and the second purified gas And a gas mixing unit 15 for supplying to the atmospheric pressure ionization mass spectrometer 11 such that the concentrations of various impurity components separated by the gas chromatograph 11 are measured by the atmospheric pressure ionization mass spectrometer 12. Is formed. In addition, the gas chromatograph 11 and the atmospheric pressure ionization mass spectrometer 12 are not particularly limited, and commercially available ones can be used in a usual manner.

The gas chromatograph 11 shown in the present embodiment.
For example, a separation column 22 filled with an appropriate separating agent such as a molecular sieve type or a unibead type, two measuring tubes 23a and 23b for measuring a sample gas, and a sample gas introduction A path 24, a carrier gas introduction path 25, and an exhaust path 27 provided with a flow meter 26 are connected to each other.
By operating 1, the sample gas measured by the measuring tubes 23a and 23b is introduced into the separation column 22 to separate the components, and each component is sequentially discharged to the separation gas outlet path 28.

The atmospheric pressure ionization mass spectrometer 12 has an ion source section 31 and a mass separation section / detection section 32.
The ion source 31 is connected to a gas introduction path 33 connected to the separation gas outflow path 28 and a gas outlet path 34 for discharging excess gas. Further, a pressure gauge 35, a back pressure regulator 36, and a flow meter 37 are provided in the gas outlet path 34 as needed to maintain the ion source 31 at a constant pressure.

The purified gas supply passages 13 and 14 are adjusted to a predetermined pressure by pressure regulating valves 41 and 42,
Each of the purified gases purified at 44 is supplied as a carrier gas of the gas chromatograph 11 and an additive gas of the atmospheric pressure ionization mass spectrometer 12, respectively, and is a carrier gas supply path branched downstream of the purifier 43. 45 is connected to the carrier gas introduction path 25 via a flow controller 46 such as a mass flow controller or an orifice for maintaining the flow rate of the carrier gas supplied to the gas chromatograph 11 at a predetermined amount.

The gas mixing section 15 includes flow controllers 51, 51 provided in the purified gas supply paths 13, 14, respectively.
The composition (mixing ratio) and flow rate of the additive gas supplied from the additive gas supply path 55 to the atmospheric pressure ionization mass spectrometer 12 are set by the valve 52 and the valves 53 and 54. In order to optimize the analysis sensitivity according to the kind of trace impurities, the mixing ratio and flow rate of both purified gases can be set arbitrarily.

As the kind of the purified gas used as the carrier gas or the additive gas, any gas can be used as long as it does not adversely affect the separation by the gas chromatograph 11 or the analysis by the atmospheric pressure ionization mass spectrometer 12. However, it is desirable to conduct various studies and select an optimal gas.

Here, the concept of setting the composition and flow rate of the additive gas will be described. First, the inventors of the present invention, in an analyzer in which a gas chromatograph 11 and an atmospheric pressure ionization mass spectrometer 12 are combined, a carrier gas in the gas chromatograph 11 and a necessity for measurement by the atmospheric pressure ionization mass spectrometer 12 As an additional gas to be added, for example, in the case of analyzing methane (trace impurities) in hydrogen (sample gas), purified helium or purified argon or a mixed gas thereof as described in the gazette exemplified as the above-mentioned prior art is used. Hydrogen, the main component gas of the sample gas,
The methane was analyzed by mixing so that the concentration of hydrogen in the gas to be analyzed introduced into the ion source 31 of the atmospheric pressure ionization mass spectrometer 12 was 1 ppm to 10%.

The reason for setting the hydrogen concentration in the range of 1 ppm to 10% is that a sufficient reaction must take place with respect to methane, which is an impurity to be analyzed, so that it is considered that addition at a ppm level or more is necessary. If the hydrogen concentration exceeds the% level, it is considered that the meaning of separating impurities in the gas chromatograph 11 becomes less significant.

When a gas obtained by mixing purified hydrogen gas with purified argon gas to a concentration of 10 ppm was used as an additive gas, the carrier gas of the purified helium or purified argon alone and the additive gas (diluent gas) were used.
Unlike the combination, the main component gas having a higher concentration than the hydrogen gas concentration of the main component eluted in a trace amount was added as an additional gas, so that the proton transfer reaction always sufficiently occurred in the ion source. As a result, it was confirmed that the baseline of the chromatogram was stable, the blank noise was reduced, and the sensitivity was improved several times to one digit or more.

If the discharge in the ion source is likely to be unstable, such as when analyzing nitrogen in argon, a small amount of argon as a main component gas is added to stabilize the argon concentration in the ion source. By making
It is possible to stabilize the discharge in the ion source, not only reduce the baseline noise, but also to reduce the occurrence of ghost peaks and the like, which facilitates the determination of impurity peaks.

As described above, by using a mixed gas containing a main component gas as an additional gas to be added to supplement the amount of gas introduced into the ion source section 31, an ion molecule reaction with the main component gas is sufficiently promoted. And gas chromatograph 1
It is possible to suppress the influence of the concentration change of the trace residual main component flowing out of the first separating agent, and it is also possible to greatly improve the stability of the discharge in the ion source unit 31. As a result, it is possible to increase the sensitivity of the impurity analysis, which has been difficult to measure with a single gas.

That is, by adding an additive gas in which argon or helium and a main component gas are mixed at an accurate ratio to the impurities accompanying the carrier gas flowing out of the gas chromatograph 11 in a fixed amount and accurately, High-precision quantification is possible in an atmospheric pressure ionization mass spectrometer.

Since the main component gas mixed with the additive gas has a low mixing ratio, even if the main component gas to be added contains impurities of the same kind as that of the object to be analyzed, it is measured by the atmospheric pressure ionization mass spectrometer 12. In some cases, it does not affect the impurity analysis. In such a case, the sample gas can be used as it is as the main component gas mixed with the additive gas.

Next, a procedure for analyzing trace impurities in a sample gas using the analyzer shown in FIG. 1 will be described. First, 3
The flow controllers 46, 51, and 52 at the locations are set to a predetermined flow rate, and the first purified gas supplied from the first purified gas supply path 13 is used as a carrier gas and an additive gas in the carrier gas supply path 45. The pressure and flow rate are adjusted to a predetermined value, and the main component gas (second purified gas) supplied from the second purified gas supply path 14 is adjusted to a predetermined pressure and a flow rate to be used as an additive gas. Further, the sample gas is introduced from the sample gas introduction path 24 and
The sample gas is caused to flow through any one of a and b.

Next, by switching the eight-way gas switching cock 21 so that the carrier gas flows into the measuring pipe through which the sample gas flows, a predetermined amount of the sample gas measured by the measuring pipe is accompanied by the carrier gas. The sample gas is introduced into the separation column 22, travels through the separation column while being separated by the separating agent, and each component in the sample gas flows out of the separation gas flow path 28 in a predetermined order.

In the gas flowing out of the separation gas outflow passage 28, an additional gas obtained by mixing the first purified gas and the main component gas at a predetermined ratio in the gas mixing section 15 is added to the additional gas supply passage 55.
And a predetermined amount is added to the ion source 31 via the gas introduction path 33. Part of the ionized gas introduced into the ion source unit 31 is introduced into the mass separation unit through the slit, separated by mass, and the ion current of each component is detected by the detection unit.

At this time, when the sample gas contains hydrogen as a main component and trace impurities in the hydrogen, for example, trace nitrogen, trace methane, and trace carbon monoxide are analyzed, the carrier gas and the additive gas may be used. Helium is used as the first purified gas serving as a base component, hydrogen is used as the main component gas as the second purified gas, the concentration of hydrogen in the additive gas is adjusted, and the effluent from the separation gas outflow passage 28 is adjusted. The addition amount of the additional gas from the additional gas supply path 55 is adjusted in accordance with the flow rate of the gas so that the concentration of hydrogen in the gas introduced from the gas introduction path 33 to the atmospheric pressure ionization mass spectrometer 12 is 1 ppm to 10%. Range. Thereby, the ion-molecule reaction of hydrogen in the atmospheric pressure ionization mass spectrometer 12 and the target impurity nitrogen,
The proton transfer reaction with methane and carbon monoxide can be sufficiently performed, and the impurity concentration can be analyzed in a stable state.

FIG. 2 is a system diagram showing another embodiment of the combined analyzer capable of implementing the method of the present invention. In the following description, the same components as those of the analyzer shown in FIG. 1 are denoted by the same reference numerals, and detailed description is omitted.

In this analyzer, a third purified gas supply path 6 for supplying a third purified gas to the gas mixing section 15 through a pressure regulating valve 61, a purifier 62 and a flow regulator 63.
4 are provided. Thus, by forming so that three types of gas can be supplied at a predetermined pressure and a predetermined flow rate,
In the atmospheric pressure ionization mass spectrometer 12, analysis by a so-called hydrogenation method can be performed. That is, when the ionization potential of the main component gas and the ionization potential of the impurities are close, as in the case of analyzing nitrogen in argon, the analysis sensitivity in the atmospheric pressure ionization mass spectrometer 12 deteriorates, and the ppb level Although measurement becomes difficult, in such a case, by adding hydrogen at a% level to the gas and utilizing the proton transfer reaction, ppb
Analysis at the level becomes possible.

Therefore, helium is supplied from the first purified gas supply path 13, hydrogen is supplied from the second purified gas supply path 14,
A main component gas, for example, argon is supplied from the third purified gas supply path 64 at a predetermined pressure and a predetermined flow rate, respectively, and introduced into the atmospheric pressure ionization mass spectrometer 12 for analysis.
By mixing a sufficient concentration of hydrogen sufficient for performing the hydrogenation method and argon as a main component gas for stabilizing the discharge state in the ion source section 31 at a predetermined concentration, the influence of the main component gas is obtained. , While controlling the amount of nitrogen, it is possible to analyze trace nitrogen in argon with high sensitivity. The concentration of argon at this time is appropriately in the range of 1 ppm to 1% for the same reason as described above. In the apparatus of the present embodiment, the second purified gas supply path 14
If the supply of hydrogen from is stopped, analysis can be performed in the same manner as the method described with reference to FIG.

In the method of the present invention, the main component gas of the sample gas and the impurity component to be analyzed are not particularly limited, but it is difficult to directly analyze with an atmospheric pressure ionization mass spectrometer, It becomes necessary to separate components by chromatography, and the measurement cannot be performed by the atmospheric pressure ionization mass spectrometer alone, or the sensitivity becomes insufficient. That is, in the combination of the main component gas and the impurity to be analyzed, the former has a lower ionization energy or a higher proton affinity than the latter.

Further, depending on the resolution of the mass separation unit of the atmospheric pressure ionization mass spectrometer, a combination of a main component gas and an impurity, or a combination of impurities, for example, nitrogen and one ion, which become ions having the same mass number, depends on the resolution. Combinations of carbon oxide, carbon dioxide and nitrous oxide are also sample gases targeted by the present invention.

Further, the present invention relates to a gas having a property that a main component gas is adsorbed on a separating agent of a gas chromatograph or a pipe surface from the gas chromatograph to an atmospheric pressure ionization mass spectrometer and gradually desorbed. This is also effective in the case of a gas having a property that does not quickly move out of the system from the ion source part of the mass spectrometer. Examples of the main component gas corresponding to such conditions include oxygen, hydrogen, argon, carbon dioxide, carbon monoxide, hydrides such as silane, arsine, and phosphine, halogen-based gases such as hydrogen chloride, perfluoroethane, and the like. Can be exemplified.

[0035]

EXAMPLES Example 1 Impurities (nitrogen, methane, carbon monoxide) contained in hydrogen gas were analyzed using an apparatus having the configuration shown in FIG. Helium was used as the carrier gas for the gas chromatograph, and a stainless steel column (diameter 3 mm,
A material having a length of 2 m) filled with molecular sieves 13XS was used. The sampling amount of the sample gas was 3 cc, and the flow rate of the carrier gas was 42 cc / min. First, the additive gas was helium alone at 600 cc / min, and methane in hydrogen was measured. The resulting chromatogram is shown in FIG.

As is apparent from FIG. 3, only a small peak was obtained at a mass number of 15 for measuring methane, and residual hydrogen in the gas system caused a proton transfer reaction with ionized protons. Appears at 17. The chromatogram also shows a mass number of 15 and a mass number of 17
In both cases, the baseline is not stable for a long time because the amount of residual hydrogen in the gas system changes.

Next, nitrogen, methane, and carbon monoxide in the hydrogen were analyzed with 0.5 cc of hydrogen as a main component gas added to the above-mentioned additional gas. The resulting chromatogram is shown in FIG.

As is clear from FIG. 4, the proton transfer reaction proceeds by adding hydrogen, and each impurity is clearly obtained by the mass number to which the proton is added, and the baseline is very stable. You can see that.

The calibration curves at this time are shown in FIGS. 5 is a calibration curve for carbon monoxide, FIG. 6 is a calibration curve for methane, and FIG.
Is a calibration curve of nitrogen, in each case, good linearity was obtained, and it was found that the analysis accuracy was sufficient. Thus, when the additive gas containing the main component gas is used, the detection limit of carbon monoxide in hydrogen is 0.6 ppb, the detection limit of methane is 0.3 ppb, and the detection limit of nitrogen is 0.0 ppb.
It could be calculated to be 5 ppb.

Example 2 Using a device having the structure shown in FIG. 2, nitrogen impurities contained in argon gas were measured by a hydrogenation method. Helium was used as a carrier gas, and a stainless steel column (diameter 3 mm, length 2 m) filled with molecular sieves 13XS was used as a separation column. The sampling rate of the sample gas was 3 cc, and the flow rate of the carrier gas was 70
cc. First, the added gas was helium 600 cc / m
Nitrogen in argon was measured as a mixed gas of in and hydrogen at 0.5 cc / min.

FIG. 8 shows the resulting chromatogram. As is clear from FIG. 8, the noise of the baseline increases, and a large fluctuation of the baseline such as a ghost peak is observed particularly at the time of the rising of the peak. At this time, the detection limit of nitrogen was 0.6 ppb.

The same analysis was performed by adding 14 cc of argon, which is a main component gas, to the additional gas. The resulting chromatogram is shown in FIG. As is clear from FIG. 9, the discharge in the ion source portion was stabilized by adding argon, and the baseline was extremely stabilized. The calibration curve at this time is shown in FIG.
As shown in FIG. 10, a calibration curve having good linearity was obtained, and the detection limit of nitrogen in argon was 0.2 ppb due to a decrease in baseline noise. Note that the detection limit was S / N = 2 in all cases including Example 1.
Was calculated.

[0043]

As described above, according to the method for analyzing trace impurities in a gas of the present invention, the influence of the main component of the sample gas can be controlled, and the ppb to ppb level of various high-purity gases can be controlled. Sub-ppb level trace impurities can be analyzed with high reproducibility and high sensitivity.

[Brief description of the drawings]

FIG. 1 is a system diagram showing one embodiment of a compound analyzer capable of implementing the method of the present invention.

FIG. 2 is a system diagram showing another example of a combined analyzer capable of performing the method of the present invention.

FIG. 3 is a diagram showing a chromatogram of methane when analysis is performed without adding a main component gas to an additive gas in Example 1.

FIG. 4 is a diagram showing a chromatogram of methane, carbon monoxide, and nitrogen when a main component gas is added to an additive gas and analyzed in Example 1.

FIG. 5 is a diagram showing a calibration curve of carbon monoxide in Example 1.

FIG. 6 is a diagram showing a calibration curve of methane in Example 1.

FIG. 7 is a diagram showing a calibration curve of nitrogen in Example 1.

FIG. 8 is a diagram showing a chromatogram of nitrogen when analysis was performed without adding a main component gas to an additive gas in Example 2.

FIG. 9 is a view showing a chromatogram of nitrogen when the main component gas is added to the additive gas and analyzed in Example 2.

FIG. 10 is a view showing a calibration curve of nitrogen in Example 2.

[Explanation of symbols]

Reference Signs List 11: gas chromatograph, 12: atmospheric pressure ionization mass spectrometer, 13: first purified gas supply path, 14: second purified gas supply path, 15: gas mixing section, 21: eight-way gas switching cock, 22: separation column, 23a, 23b ... measuring tubes,
Reference numeral 24: sample gas introduction path, 25: carrier gas introduction path, 26: flow meter, 27: exhaust path, 28: separation gas outflow path, 31: ion source section, 32: mass separation section / detection section, 33: gas introduction Path, 34 ... Gas derivation path, 35 ...
Pressure gauge, 36 ... Back pressure regulator, 37
... flow meters, 41, 42 ... pressure control valves, 43, 44 ... purifiers, 45 ... carrier gas supply paths, 46 ... flow controllers, 51, 52 ... flow controllers, 53, 54 ... valves, 55 ...
Additive gas supply path, 61: pressure control valve, 62: purifier,
63: flow controller, 64: third purified gas supply path

Claims (3)

[Claims] A main component and a trace impurity in a sample gas carried by a carrier gas are separated by a gas chromatograph, and an outflow gas from the gas chromatograph is introduced into an atmospheric pressure ionization mass spectrometer to remove the trace impurity. In the method for analyzing, trace amounts of impurities in the gas characterized by adding an additive gas containing a main component gas of the sample gas to the effluent gas from the gas chromatograph, and then introducing the additive gas to the atmospheric pressure ionization mass spectrometer. Analysis method. 2. The method for analyzing trace impurities in a gas according to claim 1, wherein said additive gas contains hydrogen gas. 3. The main component gas is one of hydrides such as oxygen, hydrogen, argon, carbon dioxide, carbon monoxide, and silane, a halogen-based gas such as hydrogen chloride, and a chlorofluorocarbon-based gas, or a mixture thereof. The method for analyzing trace impurities in a gas according to claim 1, wherein the gas is a gas.