JPH0674940A - Atmospheric pressure ionization mass spectrometry method and mass spectrometer - Google Patents

Abstract

PURPOSE:To provide an atmospheric pressure ionization mass spectrometry method and a mass spectrometer by which highly sensitive measurement can be attained on an extremely trace quantity of component on which the highly sensitive measurement could not carried out sufficiently. CONSTITUTION:In an atmospheric pressure ionization mass spectrometry method, the third component ion to ionize a trace component molecule at higher speed than a main component ion ionizes the trace component molecule is made to exist in an ionization area, and the trace component molecule is ionized efficiently by ion (-) molecule reaction between the third component ion and the trace component molecule. A molecular sheave trap 3 is arranged in a sample gas introducing pipe 7, and a gas introducing pipe having a molecular sheave trap 4 is branched off and connected to the sample gas introducing pipe 7 more downstream than the molecular sheave trap 3, and when sample gas is nitrogen, gas introduced from the branched and connected introducing pipe becomes argon.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an atmospheric pressure ionization mass spectrometric method and an analyzer for detecting trace impurities contained in a sample gas, and particularly to achieve high-efficiency ionization of ultratrace impurities for highly sensitive detection. Atmospheric pressure ionization mass spectrometry method and analyzer capable of performing

[0002]

2. Description of the Related Art Although a large amount of various gases are used in semiconductor manufacturing processes, the presence of trace impurities in the gases is one of the causes of deterioration of device characteristics. Accordingly, the purification of gas has become an important issue. Atmospheric pressure ionization mass spectrometry has been attracting attention and has begun to be used as an evaluation technique relating to the purification of gas, that is, a measuring method of a trace impurity contained in gas. This atmospheric pressure ionization mass spectrometry is an extremely sensitive analytical method and is described, for example, in JP-A-62-103954. It is important to increase the amount of detected ions in order to achieve high sensitivity in mass spectrometry, and for this purpose, it is necessary to perform highly efficient ionization. In atmospheric pressure ionization mass spectrometry, as a means for high-sensitivity analysis, high-efficiency ionization of trace components is performed by two-step ionization of primary ionization by corona discharge or radiation at 1 atmosphere and secondary ionization by ion-molecule reaction. Is adopted.

Further, a trace amount of carbon monoxide in a different substance having the same mass number, for example, nitrogen gas (N ₂ ) by atmospheric pressure ionization mass spectrometry.
(CO) (N ₂ and CO both have m / z of 28, where m
Is mass and z is the number of charges)
It is described in 217936. In this example, in order to distinguish two substances that generate two ions of the same mass number, a substance having an ionization potential intermediate between these two substances is allowed to exist, and one of them is charged by using a charge exchange reaction. The method of detecting the other ion by removing the ion is adopted. That relates to the detection of CO traces of N ₂ gas, the presence of krypton (Kr) as a third component having an intermediate ionization potential of N ₂ and CO, to remove the high N ₂ ionization potential than Kr CO ions are being detected.

[0004]

In the first known example described above, when the secondary ionization by the ion-molecule reaction is performed with high efficiency, high sensitivity is achieved because the amount of detected ions is large. The reaction process in the case of ionizing trace components in nitrogen gas will be described below. In addition, in the reaction formula, the indication of <+> indicates that the molecule or atom described on the left side of <+> is ionized.

Primary ionization (corona discharge ionization): N ₂ → N ₂ <+> (1) Secondary ionization (ion-molecule reaction ionization): N ₂ <+> + 2 N ₂ → N ₄ <+> + N ₂ (2) N ₄ <+> + CO ₂ → CO ₂ <+> + 2 N ₂ (3) N ₄ <+> + H ₂ O → H ₂ O <+> + 2 N ₂ (4) First, When N ₂ gas as a sample gas containing trace amounts of impurities such as CO ₂ and H ₂ O is introduced into the ionization section, primary ionization is performed by corona discharge. At this time, a part of the sample is ionized, but even in many ionization methods such as electron impact ionization, only a part of the sample is ionized. The composition of the generated ions is almost the same as the composition of the sample gas, consisting of most of N ₂ <+> and a few impurity ions. The impurity concentration that causes problems in the semiconductor manufacturing process is p
Since the concentration is extremely low from pm to ppt, the amount of impurity ions produced is extremely small, 1/10 ⁶ to 1/10 ^{12 of the} amount of N ₂ <+>, and even if ions in this state are introduced into the detector. However, impurity ions are difficult to detect. Therefore, in the atmospheric pressure ionization method, as the secondary ionization, ion-molecule reactions (3) and (4) are used to ionize the impurity molecules that are not ionized by the primary ionization to increase the number of impurity ions. That is, it is a method of converting main component ions, which occupy most of the ions generated by primary ionization, into impurity ions. When the pressure is high such as 1 atm, the mean free path is short, and the number of collisions between N ₄ <+> and impurity molecules is large, so that it is possible to increase the number of impurity ions. Also, for CO ₂ and H ₂ O in nitrogen gas, reactions (3), (4)
Has a large reaction rate constant (high-speed reaction) and the number of collisions increases, so that high efficiency ionization is achieved. Reaction (2) is a unique reaction under high pressure, because the probability that N ₂ <+> generated by discharge will cause three-body collision with two molecules of nitrogen, which is the main component, is high. Is. Reaction (2) is also a fast reaction.

However, if the reaction rate constant is small even if the mean free path is short, the impurity ions cannot be sufficiently increased, and high sensitivity cannot be achieved. A typical example in which this secondary ionization is not performed with high efficiency is when hydrogen is ionized in nitrogen gas. The ionization process for ionizing hydrogen in nitrogen gas is as follows. Primary ionization (corona discharge ionization): N ₂ → N ₂ <+> (5) Secondary ionization (ion-molecule reaction ionization): N ₂ <+> + 2 N ₂ → N ₄ <+> + N ₂ (6 ) N ₄ <+> + H ₂ → N ₂ H <+> + H + N ₂ (7) When detecting a trace amount of hydrogen in nitrogen gas, N ₂ H < ＋〉 is detected. Reaction (6) is (2)
Same as and faster. However, since the reaction (7) is slow (the reaction rate constant is small), hydrogen in nitrogen is not highly efficiently ionized.

The second known example is that CO 2
Kr is present for the purpose of removing interference by N ₂ ions (overlapping CO ₂ with N ₂ ions).
Is highly ionized with or without. Therefore, for reaction ionization that is originally low in efficiency,
The presence of a substance having an intermediate ionization potential is not effective in improving the ionization efficiency.

As described above, in the conventional atmospheric pressure ionization mass spectrometry, when the ion-molecule reaction used for secondary ionization is slow, high-efficiency ionization cannot be achieved and high-sensitivity detection cannot be performed. .

An object of the present invention is to solve the problems of the above-mentioned prior art, and to achieve high-sensitivity measurement of ultra-trace amount components for which sufficient high-sensitivity measurement could not be achieved. An object is to provide an analysis method and an analysis device.

[0010]

Means for Solving the Problems The above-mentioned object is to generate ions of trace components contained in a sample gas by primary ionization by corona discharge or radiation and secondary ionization by ion-molecule reaction at a pressure of about 1 atm. In a mass spectrometric method in which the generated ions are introduced into a high-vacuum mass spectrometer and mass-separated and detected, a third component ion that ionizes a minor component molecule faster than a major component ionizes a minor component molecule An atmospheric pressure ionization mass spectrometric method characterized in that it is present in an ionization region and ionizes a minor constituent molecule with an efficiency by an ion-molecule reaction between the third constituent ion and a minor constituent molecule, and 1 atm Included in sample gas by primary ionization by corona discharge or radiation at near pressure and secondary ionization by reaction of ions and molecules In a mass spectrometer in which ions of a quantitative component are generated, the generated ions are introduced into a high-vacuum mass spectrometer, and mass separation is performed, a molecular sieve trap is provided in a sample gas introduction pipe, and further a molecular sieve trap is provided. The sample gas introduction pipe on the downstream side has a configuration in which a gas introduction pipe having a molecular sieve trap is branched and coupled, and when the sample gas is nitrogen, the gas introduced from the branched coupling gas introduction pipe is argon. It can be achieved by using an atmospheric pressure ionization mass spectrometer characterized by:

[0011]

[Function] The ionization process in the atmospheric pressure ionization method can be expressed by the following equation, where C is a main component molecule and X is a trace component, that is, an impurity molecule.

Primary ionization (corona discharge ionization): C → C <+> (8) Secondary ionization (ion-molecule reaction ionization): C <+> + X → X <+> + C (9) Reaction (9 The production rate of X <+> in) is expressed by the following equation.

D [X <+>] / dt = k [C <+>] [X] (10) where [] represents the amount (density) of the substance in [], and k represents the reaction rate constant. . As can be seen from this equation, in order to achieve high efficiency of ionization using gas-phase ion-molecule reaction, the density of reacting ions and molecules is high (mean free path is short).
The two conditions are that the reaction is fast (reaction rate constant).

In the ionization section of a normal atmospheric pressure ionization mass spectrometer, the mean free path is extremely short at about 50 nm (particle density is high), and collisions occur far more often than under reduced pressure ionization. However, the magnitude of the reaction rate constant is determined by the combination of reacting ions and molecules. When the reaction rate constant is small, high-efficiency ionization does not occur even if the particle density is high, and high-sensitivity detection cannot be performed. Therefore, by converting a reaction having a small reaction rate constant into a reaction having a large reaction rate constant, high efficiency ionization can be performed. The above conditions can be achieved by mixing a third component gas (molecule A) different from C and X into the sample gas. Here, the conditions for A are A ion (A <+>) and X
The rate constant in the reaction with the main component ions C <+> and X
It is necessary to have a property larger than the rate constant in the reaction with. The reaction process when primary ionization by corona discharge or radiation is used for the formation of the third component ion A <+> is as follows.

Primary ionization (corona discharge or radiation ionization) C → C <+> (11) A → A <+> (12) Secondary ionization (ion-molecule reaction ionization) C <+> + X → X <+ > + C (13) A <+> + X → X <+> + A (14) When a sample gas mixed with A is introduced into the ionization section, C <+> and A <+> are generated by corona discharge or radiation. Is generated. Next, as a secondary ionization, C <+> and A <+> react with X as shown in reactions (13) and (14) to generate X <+>. At this time, if the reaction (14) is faster than the reaction (13), that is, the reaction rate constant is large, the production of X <+> is mainly performed by the reaction (14).

For example, considering the case where the same amount of A as C is mixed, first, almost the same amounts of C <+> and A <+> are produced by primary ionization. If mixing of A does not change the primary ionization efficiency, the reaction (13),
[C <+>] and [A <+>] in (14) are 1/2 of [C <+>] in reaction (9). When the rate constant of reaction (14) is 10 times larger than that of reaction (13), from equation (10), reaction (14) produces 10 times more X <+> than reaction (13). To generate. In addition, since the density of X in the sample is halved by mixing A, it is generated in reaction (9) [X
Compared to <+>], reaction (13) was 1/4 times, and reaction (14) was
Generate [X <+>] of 10/4 times. Therefore, by mixing A, the amount of X <+> produced increases 11/4 times.

As described above, high-efficiency ionization is achieved by mixing the third component gas into the sample gas, which can ionize the trace component molecules faster than the main component ions ionize the trace component molecules. Therefore, it is possible to perform high-sensitivity measurement of a sample that cannot be performed with the conventional atmospheric pressure ionization mass spectrometry.

[0018]

EXAMPLES The structures of the atmospheric pressure ionization mass spectrometry method and analyzer of the present invention will be specifically described below with reference to examples.

[Embodiment 1] FIG. 1 is a view showing a schematic configuration of an atmospheric pressure ionization mass spectrometer of the present invention. A sample gas cylinder 1, an argon gas cylinder 2, molecular sieve traps 3 and 4, flow meters 5 and 6, sample gas. Introducing pipe 7, argon gas introducing pipe 8, ionization part 9, differential evacuation part 10, analysis part 11, needle electrode 12, first orifice 13, second orifice 1
4, a quadrupole mass filter 15, a collector 16, a calculator 17, an amplifier 18, a recorder 19, exhaust pumps 20 and 21.

The operation of analysis using this apparatus will be described below. First, nitrogen gas containing a trace amount of hydrogen (impurities)
(Sample gas) is introduced from the sample gas cylinder 1 into the ionization section 9 through the sample gas introduction pipe 7. The introduced sample gas is ionized (primary ionization) by corona discharge at the tip of the needle electrode 12 to which a high voltage is applied. In the ionization section 9, the reactions (6) and (7) continue as secondary ionization, and impurity ions N ₂ H <+> are generated from a large amount of the main component nitrogen ions. Therefore, when detecting hydrogen in nitrogen gas, N ₂ H <+> is detected as hydrogen ions. Ions generated in the ionization section 9 are orifices 13,
High vacuum analysis section 11 through differential pumping section 10 and orifice 14.
After being introduced into the mass separator and mass separated by a mass separator,
Converted to current at 16. Since this current is extremely small, it is recorded in the calculator 17, the recorder 19, etc. after being amplified by the amplifier 18.

In order to achieve high sensitivity in the mass spectrometer, it is important to increase the amount of ions to reach the collector 16 to be detected, in this case, the amount of hydrogen ions. For this purpose, it is necessary to increase the amount of hydrogen ions generated in the ionization section 9. That is, the ionization unit 9
It is necessary to ionize as much hydrogen (high efficiency ionization) as possible in the sample gas introduced into the.

However, in the detection of hydrogen in nitrogen gas, the reaction rate constant of the reaction (7) is 2.4 / 10 ¹² cm ³ / molecule · s, and the ionization reaction of CO ₂ and H ₂ O in nitrogen gas ( Since the reaction rate constants of 3) and (4) are smaller than 7.5 / 10 ¹⁰ , 1.9 / 10 ⁹ cm ³ / molecule · s, high-efficiency ionization cannot be performed, and C
Highly sensitive detection like O ₂ and H ₂ O could not be performed.

Therefore, in the apparatus of the present invention, the reaction (7)
In order to generate hydrogen ions with high efficiency by using a reaction with a large reaction rate constant, a device that can mix argon gas into the sample gas was used. That is, it is an apparatus for mixing the argon gas from the argon gas cylinder 2 into the sample gas through the argon gas introduction pipe 8 connected to the sample introduction pipe 7. The control of the mixing ratio of the argon gas is performed by the flow meter 5 for adjusting the flow rate of the argon gas and the flow meter 6 for adjusting the total amount of gas. When this mixed gas is introduced into the ionization section 9, the following reactions occur.

Primary ionization (corona discharge ionization): N ₂ → N ₂ <+> (15) Ar → Ar <+> (16) Secondary ionization (ion-molecule reaction ionization): N ₂ <+> + 2 N ₂ → N ₄ <+> + N ₂ (17) N ₄ <+> + H ₂ → N ₂ H <+> + H + N ₂ K = 2.4 / 10 ¹² (18) Ar <+> + Ar + N ₂ → Ar ₂ <+> + N ₂ (19) Ar ₂ <+> + H ₂ → ArH <+> + H + Ar K = 4.7 / 10 ¹⁰ (20) ArH <+> + N ₂ → N ₂ H <+> + Ar K = 8.0 / 10 ¹⁰ (21) That is, first, nitrogen and argon, which are the main components of the mixed gas, are ionized by corona discharge. Next, reactions (17) to (21) occur as secondary ionization, and N ₂ H <+> is generated. The reactions (17) and (18) are the same as the reactions when argon is not mixed in, and the amount of N ₂ H <+> produced is small because the reaction (18) is slow. On the other hand, reactions (19) to (21) are reactions that occur when argon is mixed. Reaction (20) is a fast reaction and efficiently produces ArH <+>. ArH <+> further produces N ₂ H <+> by reaction (21). Reaction (21) is a fast reaction, and since N ₂ is the main component, the reaction
Nearly 100% of the ArH <+> generated in (20) is the ionization part 9
Is converted into N ₂ H <+> within the ion residence time (about 100 μsec) at. As shown in reactions (19) to (21), even if argon is mixed in the sample gas, the final hydrogen ion generated is N ₂ H <+>, which is detected when argon is not mixed. It is the same ion as hydrogen ion. Therefore, the only hydrogen ion to be noted is N ₂ H <+>, and even if argon is mixed in, the data processing when creating a calibration curve does not become complicated.

However, when argon gas is actually mixed with nitrogen gas, in the atmospheric pressure ionization mass spectrometer ion source, a large amount of (N ₂ Ar) <+ in addition to the above N ₄ <+> and Ar ₂ <+>.
> Is generated in the following reaction.

N ₄ <+> + Ar → (N ₂ Ar) <+> + N ₂ (22) Ar ₂ <+> + N ₂ → (N ₂ Ar) <+> + Ar (23) It shows the ionic strength change of the main ions observed when the mixing ratio was changed in an argon-nitrogen mixed system. Further, FIG. 3 is a diagram showing changes in the amount of N ₂ H <+> depending on the amount of argon mixed in the sample gas. Curve A in FIG. 3 is the total amount of N ₂ H <+> produced by the incorporation of argon. Curve B is N ₄ <+>, A produced by the ion source
The amount of N ₂ H <+> produced by the reactions (18), (20) and (21) was calculated from the amount of r ₂ <+> by the formula (10). Curve B is different from curve A. This difference (curve C) is generated from other than N ₄ <+> and Ar ₂ <+>. As shown in FIG. 2, in the nitrogen-argon mixed system, since the reactant ions that are considered to be involved in the generation of N ₂ H <+> are not observed other than the above (N ₂ Ar) <+>, the curve C is in the past. Although there is no reported example, it is considered that it was produced from (N ₂ Ar) <+> in the following reaction (24).

(N ₂ Ar) <+> + H ₂ → N ₂ H <+> + H + Ar (24) The reaction rate constant of the reaction (24) is the amount of (N ₂ Ar) <+> in FIG. an initial value, the curve C in FIG. 3 as N ₂ H <+> generation amount, the formula (1
0) can be calculated. The value is 1.4 / 1
It was 0 ¹⁰ cm ³ / molecule · s, and the reaction (24) was also a fast reaction.

Further, as shown in reactions (19) to (24),
The mixed argon neutral molecule finally returns to the argon neutral molecule, so that the spectrum is not complicated. That is, argon plays a catalytic role in the process of ionizing hydrogen in nitrogen gas.

N ₂ H finally detected by the collector 16
<+> Is the sum of N ₂ H <+> produced in reactions (18), (21) and (24). Reaction (18) response than (20), (24) Since the two orders of magnitude greater reaction rate constant, reaction among the detected _{N 2 H <+> (21} ), N 2 produced in (24) The proportion of H <+> is larger than that of N ₂ H <+> produced in reaction (18). further,
(N ₂ Ar) <+> production is larger than Ar ₂ <+> production, so reaction
The N ₂ H <+> generated in (24) is the most.

When argon is mixed in this way, the amount of N ₂ H <+> produced increases, but when the amount of argon mixed in is extremely large relative to the sample gas, hydrogen in the sample gas is diluted, so that it is produced. The amount of N ₂ H <+> that is removed decreases. As shown in FIG. 3, the amount of N ₂ H <+> becomes maximum when the amount of mixed argon is 20 to 30%. It is desirable that the amount of mixed argon is at least 50% or less.

FIG. 4 shows a calibration curve of hydrogen in nitrogen gas (relation curve between hydrogen concentration and N ₂ H <+> signal intensity) when 20% of argon is mixed, and a calibration curve when argon is not mixed. It is the figure which showed the line. For the same hydrogen concentration, the former shows that the signal intensity is increased by about one digit as compared with the latter, which shows that the high sensitivity of about one digit can be achieved.

The argon gas used in the above example is one of the gases that gives the highest purity, and the influence of impurities in the mixed gas on the reaction is small. However, as a means for coping with the case where the purity of the argon gas is low and the case where the cleaning of the argon gas introducing pipe 8 is insufficient, the present apparatus is provided with the molecular sieve trap 5. These impurities, especially water, are removed by the ionization section 9 and the differential pumping section.
The reaction in 10 produces N ₂ H <+>, which gives a background in the case of high-sensitivity detection, and therefore should be reduced as much as possible. For example, the water content of N ₂ H <+>
To generate.

H ₂ O <+> + N ₂ → N ₂ H <+> + OH Even if the water content is at the ppb level, the amount of N ₂ H <+> produced changes, so that an extremely low concentration of hydrogen is added to N ₂ When detecting with H <+>, it is necessary to reduce the water content as much as possible.

Further, in the above example, the case where the gas containing the cylinders 1 and 2 was used as the sample gas and the argon gas was explained, but these do not necessarily have to be the gas containing the cylinders. In a semiconductor clean room or the like, the line gas purity is often evaluated. In this case, the cylinder nitrogen gas becomes the line nitrogen gas, and the cylinder argon gas becomes the line argon gas.

Since argon is an inert gas,
Highly safe and extremely easy to handle.

In this embodiment, the case of detecting a trace amount of hydrogen contained in nitrogen gas has been described, but high sensitivity detection cannot be performed because the reaction rate is small due to other impurities contained in other gases. The idea of the present invention can be applied to the case.

[0037]

As described above, by using the atmospheric pressure ionization mass spectrometry method and the analyzing apparatus as the analyzing method and the analyzing apparatus having the constitution of the present invention, the problems of the prior art can be solved. It was possible to provide an atmospheric pressure ionization mass spectrometric method and apparatus capable of achieving high-sensitivity measurement for an ultra-trace amount component that could not be sufficiently and highly sensitively measured.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a schematic configuration of an atmospheric pressure ionization mass spectrometer of the present invention.

FIG. 2 is a diagram showing changes in ionic strength depending on the amount of main gas mixed with argon gas observed in a nitrogen-argon mixed system.

FIG. 3 is a diagram showing changes in hydrogen ion intensity depending on the amount of argon gas mixed.

FIG. 4 is a diagram showing a calibration curve of hydrogen in nitrogen gas.

[Explanation of symbols]

1 ... Sample gas cylinder, 2 ... Argon gas cylinder 3,4
... Molecular sieve trap, 5 ... Flowmeter for adjusting the flow rate of argon gas, 6 ... Flowmeter for adjusting the total amount of gas, 7 ... Sample gas introducing pipe, 8 ... Argon gas introducing pipe, 9 ... Ionization section,
10 ... Differential exhaust part, 11 ... Analysis part, 12 ... Needle electrode, 13
... first orifice, 14 ... second orifice, 15 ... quadrupole mass filter, 16 ... collector, 17 ... calculator, 18
... Amplifier, 19 ... Recorder, 20,21 ... Exhaust pump.

Claims (7)

[Claims] 1. A trace amount of ions contained in a sample gas are produced by primary ionization by corona discharge or radiation and secondary ionization by ion-molecule reaction at a pressure of about 1 atm, and the produced ions are produced in a high vacuum. In a mass spectrometric method of guiding to a mass spectrometer and performing mass separation and detection, a third component ion that ionizes a minor component molecule at a faster rate than a major component ion ionizes a minor component molecule is present in the ionization region, A method for atmospheric pressure ionization mass spectrometry, characterized in that a trace component molecule is highly efficiently ionized by an ion-molecule reaction between a ternary ion and a trace component molecule. 2. A mixed gas in which the above-mentioned third component is mixed in a sample gas is introduced into an ionization region, and primary component ionization by corona discharge or radiation and secondary component ionization by ion-molecule reaction are performed to achieve high efficiency of trace component molecules. The atmospheric pressure ionization mass spectrometric method according to claim 1, wherein the third component ion that is ionized is generated. 3. The atmospheric pressure ionization mass spectrometric method according to claim 1, wherein the third component is a catalytic component that ionizes a minor component molecule and then returns to a neutral third component molecule. 4. The atmospheric pressure ionization mass spectrometric method according to claim 1, wherein when the main component is nitrogen and the trace component is hydrogen in the sample gas, the third component is argon. 5. When the main component in the sample gas is nitrogen and the trace component is hydrogen, the third component ion is (N ₂ Ar).
Ions and Ar ₂ ions.
~ Atmospheric pressure ionization mass spectrometric method according to 3 above. 6. The atmospheric pressure ionization according to claim 4, wherein the mixing ratio of the argon gas as the third component is at least 50% or less, preferably 20 to 30% with respect to the sample gas. Mass spectrometric method. 7. A trace amount of ions contained in a sample gas are generated by primary ionization by corona discharge or radiation and secondary ionization by reaction of ions and molecules at a pressure of about 1 atm, and the generated ions are in a high vacuum. In a mass spectrometer for detecting mass-separated mass, the sample gas introduction pipe is provided with a molecular sieve trap, and further the sample gas introduction pipe downstream of the molecular sieve trap has a molecular sieve trap. An atmospheric pressure ionization mass spectrometer, comprising a configuration in which gas introduction pipes are branched and coupled, and when the sample gas is nitrogen, the gas introduced from the branch coupling and introduction pipe is argon.