Abstract:
A mass spectrometry apparatus is provided with an emitter for emitting metal ions, a reaction chamber where the detected gas is introduced and ionized by the metal ions, an aperture for guiding molecules of the ionized detected gas, and a mass spectrometer for measuring the guided molecules. The metal ions emitted from the emitter are caused to fly to the reaction chamber to ionize said detected gas. The detected gas is a halide compound. Further provision is made of a sample gas source for feeding a halide compound to the reaction chamber and an N 2  gas source for feeding to the reaction chamber a gas (N 2  etc.) to which the metal ions attach less easily than to the halide compound. It is therefore made possible to apply cation attachment of the Fujii system to mass spectrometry of a halide compound and enable precise measurement of fluoride compounds etc. having a large impact on global warming.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a mass spectrometry method and a mass spectrometry apparatus for a halide compound, and more particularly relates to a mass spectrometry method and a mass spectrometry apparatus for a halide compound using soft ionization utilizing ion attachment in ionizing the halide compound for the purpose of mass spectrometry.  
           [0003]    2. Description of the Related Art  
           [0004]    CF 4 , C 4 F 8 , and other perfluoro compounds (PFC), CH 3  and other hydrofluorocarbons (HFC), and SF 3 , NF 3  and other gases are utilized in various industries. These fluoride compounds are extremely stable, but have an extremely large effect on global warming. There have therefore been calls for reducing the amount of their emission on a global scale. In particular, in semiconductor and electronic component manufacturing facilities, gases are inevitably being discomposed and emitted. Therefore, emphasis is currently being placed on precisely measuring what types of ingredients of the gases are being emitted in exactly what amounts.  
           [0005]    In the past, the general practice had been to use mass spectrometry for measuring the ingredients and amounts of gas. Mass spectrometry measures the mass of the gas (or molecular weight) to identify the ingredients of the gas and measure the amount of the gas. In mass spectrometry, a positive charge or negative charge is given to the neutral gas molecules to ionize them, then the gas molecules are led into a specific electrical field or magnetic field space and the electrical force or magnetic force applied to the ions is changed to obtain a mass spectrum with mass as its abscissa and the amount of Ions as its ordinate. For this mass spectrometry, there have conventionally been several methods in accordance with the means of ionization used, that is, (1) the method of utilizing electron impact, (2) the method of utilizing electron attachment, and (3) the method of utilizing cation (positive ion) attachment. These mass spectrometry methods will be explained in brief below and their problems are pointed out.  
           [0006]    (1) Electron Impact Mass Spectrometry:  
           [0007]    In a mass spectrometry method utilizing electron impact (EI), electrons having an energy of, for example, 50 to 100 eV or so are made to collide with gas molecules to strip electrons from the gas molecules and convert the gas molecules to positive ions. This is the most generally used method because the hardware is simple.  
           [0008]    (2) Electron Attachment Mass Spectrometry:  
           [0009]    This mass spectrometry method has been developed in recent years and uses the action of electron attachment (EA). That is, electrons having a low energy of not more than 10 eV are made to attach to the gas molecules to give the gas molecules a negative charge as a whole and ionize them. This method has the advantage of involving less excess energy and resulting in less dissociation compared with electron impact. The method was developed with the intent of precise mass spectrometry of fluoride compounds. As a technical reference of the related art,  Oyo Butsuri (Applied Physics),  vol. 68, no. 10, p. 1148 (1999) and  Review of Scientific Instruments,  vol. 69, no. 1, p. 116 (1998) may be mentioned.  
           [0010]    (3) Cation Attachment Mass Spectrometry  
           [0011]    This mass spectrometry method causes cations (or positive ions) to be attached to the gas molecules for ionization instead of the afore-mentioned electron attachment. This method has been in existence for a comparatively long time, therefore has been mainly used in the field of organic spectrometry. The method alms at precise mass spectrometry of gas molecules without causing dissociation. The method of using positive charge metal ions of an alkali metal is effective. In practice, the efficacy has been confirmed for hydrocarbons with small electron affinity. Up until now, various systems have been proposed by Hodge, Bombick, Fujii, etc. These systems will be explained in brief next.  
           [0012]    The Hodge system Is described in  Analytical Chemistry,  vol. 48, no. 6, p. 825 (1976). The system proposed by Hodge utilizes Li + as the alkali metal ions. Li + is generated by heating an emitter including an Li oxide. The Li + generated from the emitter moves to the flight region, then is guided to the reaction chamber containing the gas molecules to be detected. Therefore, the emitter is placed in a region outside the reaction chamber. The ionization is performed in the reaction chamber. The gas molecules with the Li + attached as a result of the ionization are withdrawn from the reaction chamber and transported to a quadrapole mass spectrometer for mass spectrometry. This system is an indirect attachment method which causes Li + to be attached to the gas molecules being detected. It is said that direct attachment is difficult with this system. In indirect attachment, the Li + is first attached to a reaction gas, then the Li + is moved from the reaction gas with the Li + to the gas being detected. In direct attachment, the Li + is attached directly to the gas molecules being detected without interposition of a reaction gas. The above reference gives working data to explain the indirect attachment in the above way. The reasons given are that there is little difference in the sensitivity by the detected gas (ease of attachment of cations) and there is little dissociation. As opposed to this, it indicates that when using direct attachment, dissociation occurs when the detected gas is a fluoride compound etc. so direct attachment is difficult. In the above configuration, a large amount of reaction gas (isobutane or other hydrocarbon) is introduced into the reaction chamber.  
           [0013]    The system of Bombick is described in  Analytical Chemistry,  vol. 56, no. 3, p. 396 (1984). The system proposed by Bombick utilizes K + as the alkali metal ions. An emitter containing potassium (K) oxide is placed in the reaction chamber. Only the gas being detected is introduced into the reaction chamber. No other reaction gas is introduced. Therefore, the K + released from the emitter in the reaction chamber directly attaches to the molecules of the detected gas. That is, the system of Bombick is a direct attachment system.  
           [0014]    The system of Fujii uses Li + as the alkali metal ions. An emitter containing an Li oxide is placed in the reaction chamber. A repeller for repelling the ions is arranged behind the emitter. Inert gas (He, N 2 ) and the detected gas are introduced into the reaction chamber The pressure In the reaction chamber is set to a high pressure of about 133 Pa for example. In this system, inert gas is introduced instead of a reaction gas into the reaction chamber, so the system is a direct attachment system. The system of Fujii is described in, for example,  Analytical Chemistry,  vol. 64. no. 7, p. 775 (1992),  Journal of Applied Physics,  vol. 82, no. 5, p. 2095 (1997), etc. According to this system, by making the concentration of the detected gas in the reaction chamber low and making the pressure of the inert gas high, it is possible to prevent the dissociation of the detected gas or the occurrence of clusters even with a high emitter potential. The system of Fujii is a direct attachment system in the same way as the system of Bombick.  
           [0015]    Problems is Electron Impact Mass Spectrometry:  
           [0016]    In this method, the electron impact is a physical action, so a high energy is given to the detected gas. Therefore, if electron impact is used for gas molecules of a fluoride compound or other halide compound, since the binding energy of atoms in the gas molecules is usually small, the excess energy of the electron impact will cause the gas molecules to dissociate into fragments along with the ionization. Therefore, the peak due to the gas molecules not dissociating in the mass spectrum is called the molecule peak (or parent peak), while the peaks of the dissociated fragments are called the fragment peaks. With molecules with large number of atoms such as the above halide compounds, however, fragment peaks appear due to dissociation and the inherent molecule peak cannot be discerned well at all. FIG. 4 shows an example of a mass spectrum obtained by ionization of molecules of a fluoride compound, that is. C 4 Fe 8 , by electron impact by an electron energy of 70 eV. In FIG. 4, the abscissa indicates the mass, while the ordinate indicates the amount of ions. The mass of the C 4 F 8  molecule is 200 amu (atomic molecular units), but the molecule peak does not appear much at all in the mass spectrum. Only the fragment peaks of C 3 F 5 , C 2 F 4 , CF, etc. appear. In the past, the practice had been to deduce the inherent molecule peak, that is, the mass and amount of the molecules, based on the state of appearance of these fragment peaks. In this way, precise mass spectrometry of a halide compound was difficult with mass spectrometry using electron impact.  
           [0017]    Problems in Electron Attachment Mass Spectrometry:  
           [0018]    This method has currently reached a level of measurement sensitivity practical for halide compounds. Compared with the method of utilizing electron impact, there is indeed less dissociation. Even so, however, it is not possible to sufficiently suppress dissociation. Even with electron attachment mass spectrometry, precise mass spectrometry of a halide compound is difficult.  
           [0019]    Problems in Cation Attachment Mass Spectrometry:  
           [0020]    This method, as explained above, includes various systems such as the Hodge system, Bombick system, and Fujii system. The Hodge system Is an indirect attachment method as shown in the working data. In the case of the Hodge system, direct attachment does not allow precise measurement of the molecule peak of the gas molecules being detected, because, if using a fluoride compound, dissociation occurs along with the Li +  attachment and the Li + only attaches to the fragments after dissociation. Further, in the Bombick system, when the potential of the emitter becomes higher (more than 5V), dissociation occurs due to the reaction of the detected gas. Further, the concentration of the detection gas becomes high, so the problem arises of the molecules of the detected gas easily bonding together and forming clusters. In addition, there is the problem of emitter damage, that is, the problem of the emitter suffering large damage due to the active fragments dissociated at the surface of the emitter. Among the above systems, the Fujii system has the advantage in structure of eliminating the above problems relating to cation attachment mass spectrometry and enables precise measurement of hydrocarbons and their radicals and other unstable molecules. Further, the Fujii system is being improved and applied to inventions for ion attachment in atmospheric reaction chambers (Japanese Examined Patent Publication (Kokoku) No. 7-48371), inventions for attaching ions to neutral active species (Japanese Unexamined Patent Publication (Koyal) No. 6-11485), etc. As explained above, however, the cation attachment method of the Fujii system is not currently being applied to mass spectrometry for measuring the amount of emission of halide compounds, including fluoride compounds, having a large impact on global warming.  
         SUMMARY OF THE INVENTION  
         [0021]    An object of the present invention is to provide a mass spectrometry method and mass spectrometry apparatus for a halide compound which apply cation attachment of the Fujii system to mass spectrometry of a halide compound and enable precise measurement of fluoride compounds etc. having a large impact on global warming.  
           [0022]    The mass spectrometry method and mass spectrometry apparatus of a halide compound according to the present invention are configured as follows to achieve the above object. Below, first, the principle of the present invention will be explained, then the means of the invention and action of the same will be explained.  
           [0023]    Principle of Invention;  
           [0024]    The fact that electrons easily attach to fluoride compounds (large electron affinity) has been well known in the past. Therefore, electron attachment mass spectrometry had been applied to actual measurement of fluoride compounds. At the same time, the general belief had been the vague “positive charges or cations do not attach easily to fluoride compounds to which negative charges or electrons easily attach”. This had been the conventional belief judging from the fact that there is overwhelmingly less F + compared with the negative ions F + . Therefore, most persons involved in the mass spectrometry method had not even imagined applying the cation attachment mass spectrometry method to fluoride compounds.  
           [0025]    The present Inventor, however, engaged in a careful study and as a result predicted that cation attachment mass spectrometry could be applied to fluoride compounds. A precise explanation of the applicability should be made from the results of theoretical calculations by a supercomputer. Very simply, however, it can be said that, since the distribution of electrons of the compound Is lopsided, positive charges easily attach to the side opposite to where the negative charges easily attach. That is, the ease of attachment of the cations is determined by the lopsided distribution of electrons and is not inversely proportional to the ease of attachment of the electrons. Therefore, it is predicted that cations will sufficiently attach to even fluoride compounds to which electrons easily attach.  
           [0026]    Further, as clear from the above Hodge reference, it has been suggested that, basically, cation attachment mass spectrometry can be applied to a fluoride compound. In this case, however, as explained above, it had been understood that application was difficult because dissociation would occur when direct attachment was applied to a fluoride compound, and that indirect attachment using a reaction gas would be more practicable.  
           [0027]    As opposed to this, the present inventor did not believe that dissociation of the fluoride compound in the direct attachment method was in principle an unavoidable phenomenon. The reasons were as follows.  
           [0028]    In the Hodge system, the pressure of the region of placement of the emitter is a low 10 3  Pa, so the Li + produced from the emitter flies without absorption of energy by the atmospheric gas (reduction of translational energy). Further, the pressure of the reaction chamber is a somewhat high 1.3 Pa, but the mean free path at that pressure is about 5 mm and almost no absorption of energy by collisions can be expected. Therefore, the Li + collides with the detected gas at a high energy of 1 to 2 eV and dissociation easily occurs. Incidentally, due to the high energy impact, the probability of not again separating after impact (attachment efficiency) is also considerably degraded.  
           [0029]    Further, excess energy is produced in the molecules after attachment of Li + . Due to the attachment of Li + , the molecules as a whole stabilize in terms of energy. That is, the internal energy becomes low. The difference In internal energy before and after attachment is the excess energy. This causes dissociation. In particular, this excess energy is large in direct attachment. Further, in the Hodge system, the pressure of the reaction chamber is an insufficiently high 1.3 Pa, so excess energy remains in the molecules without being dispersed and absorbed by the atmospheric gas and causes dissociation.  
           [0030]    That is, the present inventor judged that dissociation occurs in fluoride compounds etc. by the direct attachment of the Hodge system since the pressure in the region of placement of the emitter and the reaction chamber is not sufficiently high. Therefore, the present Inventor judged that by making the pressure of the region of placement of the emitter and the reaction chamber sufficiently high, the translational energy of the Li + in the region of placement of the emitter can be reduced and the excess energy of the Li + attached detected gas can be dispersed and accordingly fluoride compounds can be measured without dissociation. Making the pressure of the region of placement of the emitter and the reaction chamber sufficiently high was predicted to be substantially identical to the cation attachment mass spectrometry of the Fujii system.  
           [0031]    Therefore, the present inventor conducted the following experiment to confirm the prediction that the Fujii system cation attachment mass spectrometry can be applied to a fluoride compound. The detected gas was made the representative PFC (perfluoro compound) of C 4 F 8 , the cations were made Li + , the pressure of the reaction chamber was made 133 Pa, and the inert gas was made N 2 . The hardware configuration will be explained in the second embodiment. As a result of the measurement test as shown in the mass spectrum of FIG. 3, peaks only appeared at the 207 amu of C 4 F 8 Li + , 35 amu of N 2 Li +  corresponding to the molecule peak of N 2 , 25 amu of H 2 OLi +  corresponding to the molecule peak of H 2 O included in the gas as an impurity, and 7 amu of unattached Li + . No fragment peaks at all appeared. That is, it was confirmed that the mass of the gas molecules of the detected gas was precisely measured without the occurrence of dissociation even when using cation attachment mass spectrometry for a fluoride compound. Note that 133 amu is the Cs impurity included in the emitter itself. Note further that making the pressure of the region of placement of the emitter and the reaction chamber higher is a preferable condition, but is not an essential condition when considering future advances. For example, if electrically giving conditions assisting the reaction, it may be possible to perform the mass spectrometry by cation attachment to a fluoride compound under relatively low pressure conditions.  
           [0032]    Configuration of Invention:  
           [0033]    A mass spectrometry method for a halide compound according to the present Invention is configured as follows.  
           [0034]    The mass spectrometry method is a method comprising ionizing a detected gas and then utilizing electromagnetic force to measure the mass of the molecules of the detected gas. The detected gas is a halide compound. Positive charge metal ions are attached directly on the halide compound to ionize it. According to this mass spectrometry method, the technique for mass spectrometry of a fluorine compound or other halide compound comprises directly and softly attaching positive charge metal ions on the halide compound, then transporting the molecules of the halide compound with the metal ions to a quadrapole mass spectrometer to measure the same, so it is possible to directly observe the molecule peaks in the mass spectrum without causing dissociation.  
           [0035]    In the above mass spectrometry method, preferably the main component in the atmosphere of the reaction region for the ionization is a gas to which metal ions attach less easily than to the halide compound. By making a gas to which the metal ions attach less easily the main component in the reaction region, metal ions attach directly more easily on the halide compound of the sample gas.  
           [0036]    In the mass spectrometry method, preferably the main component is a gas selected from any of He, Ar, Ne, H 2 , and N 2 . As the gas to which the metal ions attach less easily, a stable inert gas is used.  
           [0037]    In the above mass spectrometry methods, preferably a pressure of an atmosphere of at least the reaction region is maintained at a predetermined high pressure by the gas forming the main component. To set the pressure of the atmosphere of the reaction region high, as explained in the principle of the invention as well, is a preferable condition from the viewpoint of the prevention of dissociation when attaching ions on a halide compound by direct attachment. As explained earlier, however, there is a possibility that the pressure can be lowered as well, so this cannot be said to necessarily be a necessary condition.  
           [0038]    In the above mass spectrometry methods, preferably the pressure of the atmosphere of the reaction region is at least 10 Pa, more preferably at least 100 Pa.  
           [0039]    In the above mass spectrometry methods, preferably the pressure of the atmosphere of a region wherein the metal ions fly until entering the reaction region is at least 10 Pa. In making the pressure of the atmosphere of the reaction region high, the pressure of the atmosphere of the metal ion flight region between the emitter emitting the metal ions and the reaction region is preferably also made higher.  
           [0040]    In these mass spectrometry methods, preferably the halide compound includes a carbon atom and a fluorine atom.  
           [0041]    In these mass spectrometry methods, preferably the metal ion is any of LI, K, Na, Rb, Cs, Al, Ga, and In.  
           [0042]    Further, the mass spectrometry apparatus for a halide compound according to the present invention is configured as follows.  
           [0043]    This mass spectrometry apparatus is provided with an emitter for emitting metal ions, a reaction chamber where the detected gas is introduced and ionized by the metal ions, an electromagnetic guide (aperture, condensing lens, etc.) for guiding molecules of the ionized detected gas, and a mass spectrometer for measuring the molecules guided by the guide (quadrapole mass spectrometer etc.) and causes the metal ions emitted from the emitter to fly to the reaction chamber to ionize the detected gas. Further, the detected gas is a halide compound and further provision is made of a first gas source (sample gas source) for feeding a halide compound to the reaction chamber and a second gas source for feeding to the reaction chamber a gas to which the metal ions attach less easily than to the halide compound (N 2  gas source).  
           [0044]    According to the apparatus of the above configuration, it is possible to work the mass spectrometry method of a halide compound according to the present invention and possible to apply the cation attachment method to mass spectrometry of a halide compound and thereby accurately measure a fluoride compound etc.  
           [0045]    In the mass spectrometry apparatus, preferably the gas to which the metal ions attach less easily is any of He, Ar, Ne, H 2 , and N 2 . As the gas to which the metal ions attach less easily, an inert gas having stable properties is preferred.  
           [0046]    In the mass spectrometry apparatus, preferably further provision is made of a pressure adjusting means for adjusting an internal pressure of the reaction chamber to a predetermined high pressure and the internal pressure of the reaction chamber is held at the predetermined high pressure by the pressure adjusting means. By setting such pressure conditions, it becomes easy to make metal ions directly attach on the molecules of the halide compound, dissociation etc. is prevented, and a practical apparatus is obtained.  
           [0047]    In the mass spectrometry apparatus, preferably the internal pressure of the reaction chamber is at least 10 Pa. As the above high pressure, at least 10 Pa is preferable and at least 100 Pa is more preferable.  
           [0048]    The mass spectrometry apparatus preferably feeds the gas from the second gas source also to a region where the metal ions fly until striking the reaction region and sets the pressure of the region to at least 10 Pa.  
           [0049]    The mass spectrometry apparatus preferably does not provide the reaction chamber as a separate part but configures it using as common regions a region of placement of the emitter, the flight region, and a reaction region corresponding to the reaction chamber. Since the reaction chamber can be eliminated, the configuration becomes simpler and the manufacturing cost can be reduced.  
           [0050]    In the mass spectrometry apparatus, preferably the halide compound includes a carbon atom and a fluorine atom.  
           [0051]    In the mass spectrometry apparatus, preferably the metal ion is any of Li, K, Na, Rb, Cs, Al, Ga, and In.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]    These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, in which:  
         [0053]    [0053]FIG. 1 is a view of the configuration of a first embodiment of the present invention;  
         [0054]    [0054]FIG. 2 is a view of the configuration of a second embodiment of the present invention;  
         [0055]    [0055]FIG. 3 is a view of an example of a mass spectrum obtained by the mass spectrometry method according to the present invention; and  
         [0056]    [0056]FIG. 4 is view of an example of a mass spectrum obtained by the mass spectrometry method of ionization by electron impact.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0057]    Preferred embodiments of the present invention will be explained next with reference to the attached drawings.  
         [0058]    [0058]FIG. 1 shows a mass spectrometry apparatus according to a first embodiment of the present invention. The mass spectrometry apparatus of the first embodiment is configured with the emitter portion and reaction chamber separate.  
         [0059]    First, the configuration will be explained. In FIG. 1, an emitter  12  including an Li oxide is for example provided at the left end of the inside of the apparatus housing  11 . An emitter heating control power source  13  and an emitter potential control power source  14  are attached to the emitter  12 . A repeller  15  is placed behind the emitter  12 . A repeller potential control power source  16  is attached to the repeller  15 . An emitter lens  17  is placed in the emitter front space located to the right of the emitter  12 . A lens potential control power source  18  is similarly attached to the emitter lens  17 . An Li + flight region  19  is provided at the right of the emitter lens  17 . Further, a reaction chamber  20  is provided. Therefore, in the mass spectrometry apparatus, the region of placement of the emitter  12  and the reaction chamber  20  are separated. The reaction chamber  20  is provided to form an isolated reaction space in the left inside space  11 A of the apparatus housing. The left inside space  11 A contains the emitter placement region and the Li + flight region  19 . In the reaction chamber  20  are formed an emitter side wall and the opposite side wall and also n Li + inlet  20   a  and outlet  20  for withdrawing the Li + attached gas molecules respectively. Further, in the reaction chamber  20 , a sample gas source  21  and N 2  gas source  22  are connected by piping  21   a  and  22   a.  The sample gas source  21  feeds the detected gas of the halide compound as a sample gas through the piping  21   a  to the reaction chamber  20 . The N 2  gas source  22  feeds N 2  gas through the piping  22   a  to the Li + flight region  19  as well. Note that while N 2  gas was used as the inert gas introduced into the reaction chamber  20 , the present invention is not limited to this.  
         [0060]    In the above configuration, a vacuum pump  23  is provided at the left internal space  11 A (in particular, the Li + flight region  19 ) so as to evacuate that space to a predetermined pressure, while the reaction chamber  20  is provided with a potential control power source  24  and a vacuum pump  25  for evacuating the internal pressure to a predetermined pressure.  
         [0061]    At the right of the reaction chamber  20  in FIG. 1 are provided a first aperture  26  and a second aperture  27 . The first aperture  26  and the second aperture  27  are arranged separated by a required distance. A space (region)  28  is formed between the two. A vacuum pump  30  is attached for evacuating this space  28 . Further, potential control power sources  31  and  32  are provided at the first and second apertures  26  and  27 .  
         [0062]    At the right of the second aperture  27  is placed a quadrapole mass spectrometer  34 . A condensing lens potential control power source  35  is attached to the condensing lens  33 , while a quadrapole mass spectrometer control power source  36  is attached to the quadrapole mass spectrometer  34 . A vacuum pump  37  is attached for evacuating the space  11 B from the second aperture  27  to the quadrapole mass spectrometer  34 .  
         [0063]    Next, the operating conditions and actions of the above hardware will be explained. Li + ionized by a surface electrolytic action is directly emitted from the emitter  12  heated to about 600 C by the emitter heating control power source  13 . The Li + flies through the Li + flight region  19  toward the reaction chamber  20  based on the electrical field determined by four potentials, that is, (1) the potential of the emitter  12  applied from the emitter potential control power source  14 . (2) the potential of the repeller  15  applied from the repeller potential control power source  16 , (3) the potential of the emitter lens  17  applied from the emitter lens potential control power source  18 , and (4) the potential of the reaction chamber  20  applied from the reaction chamber potential control power source  24 .  
         [0064]    The distance in the axial direction of the apparatus housing  11  in the Li + flight region  19  constituted between the emitter  12  and the reaction chamber  20  is for example about 70 mm. The potential of the emitter  12  is for example 160V, while the potential of the reaction chamber  20  is for example 60V. The pressure in the Li + flight region  19  is determined by the flow rate of the N 2  gas introduced through the piping  22   a  for introducing N 2  gas from the N 2  gas source  22  to the Li + flight region  19  and the pumping speed of the vacuum pump  23  evacuating the Li + flight region  19  and is set to at least 10 Pa, preferably at least 100 Pa. In this embodiment, it is adjusted to for example about 100 Pa.  
         [0065]    At a pressure of 100 Pa, the mean free path in which the ions can proceed without colliding with the N 2  of the atmospheric gas is about 70 μm. Therefore, while the Li + is flying from the emitter  12  to the reaction chamber  20 , the Li +  collides with the N 2  as much as 1000 times (=70 mm/70 μm). If the Li + collides with the N 2 , seen overall, the Li + may lose its translational energy and stop. Therefore, the Li +  repeatedly progresses and stops for every 70 μm 1000 times. That is, the translational energy when the Li + collides with the N 2  only becomes a slight 0.1 eV (=160V−60V)/1000) even at the maximum.  
         [0066]    The potentials of the repeller  15  and the emitter lens  17  are for example made 150V and 50V, respectively. These are adjusted to as to concentrate the Li + and drive as much Li + as possible into the reaction chamber  20 . These do not have any effect on the translational energy in the axial direction, so have almost nothing to do with the collision energy of the Li + .  
         [0067]    The thickness (width) of the reaction chamber  20  in the axial direction is about 10 mm. The pressure in the pressure chamber  20  is determined by the flow rates of the sample gas and N 2  gas fed from the sample gas source  21  and the N 2  gas source  22  to the reaction chamber  20 , respectively, and the pumping speed of the vacuum pump  25  evacuating the reaction chamber  20 . The internal pressure of the reaction chamber  20  is set to at least 10 Pa, preferably at least 100 Pa. In this embodiment, it is for example adjusted to about 133 Pa. Further, the concentration of the detected gas to the N 2  gas in the reaction chamber is preferably adjusted to about 0.1% by adjusting the ratio of the flow rates of the sample gas and the N 2  gas.  
         [0068]    The Li + driven into the reaction chamber  20  only has a low translational energy of 0.1 eV at the most. Also, there is no strong electrical field in the reaction chamber  20 . Due to this, the motion of the Li + in the reaction chamber mainly becomes random motion due to heat. Therefore, the Li + repeatedly collides an extremely large number of times with the N 2  gas or the detected gas. One Li + ion collides as much as 10 7  times with the main N 2  gas and as much as 10 4  times with the 0.1% concentration detected gas. Further, the mean collision energy is only 0.04 eV due to the heat energy according to the kinetics of gas molecules. With such collisions, the Li + will attach efficiently to the detected gas and will not cause dissociation at the time of collision.  
         [0069]    The Li + attached detected gas has excess energy due to the internal energy difference. If this is left alone, dissociation will occur. However, the Li + attached detected gas repeatedly collides with the large number of N 2  gas atoms at a low energy, so the N 2  gas absorbs the energy and the excess energy of the Li + attached detected gas is dispersed. Due to this, the Li +  attached detected gas becomes stable and can stand without dissociation.  
         [0070]    The stablized Li + attached detected gas is withdrawn from the reaction chamber  20  by being drawn by the potential of the first aperture  26  applied from the first aperture potential control power source  31  and flies toward the first aperture  26 . The distance in the axial direction between the reaction chamber  20  and the first aperture  26  is for example about 30 mm. The potential of the first aperture  26  becomes for example 10V. The Li + attached detected gas proceeds while colliding with the N 2  gas by a low energy of about 0.1 eV in the same way as the Li + at the Li + flight region, but no dissociation or separation of the Li + occurs with such a degree of energy.  
         [0071]    Near the hole of the first aperture  26  (emitter side). the N 2  gas etc. flows in the direction of the low pressure second aperture  27 . Therefore, the Li + attached detected gas which had flown up to close to the hole of the first aperture  26  is entrained in this flow and passes through the hole of the first aperture  26  to reach the region (space  28 ) between the first aperture  26  and the second aperture  27 . Next, the Li + attached detected gas flies toward the second aperture  27  due to the electrical field determined by the potentials of the first aperture  26  and the second aperture  27 .  
         [0072]    The distance in the axial direction between the first aperture  26  and the second aperture  27  is for example 20 mm and the potential of the second aperture is −100V. The pressure in the space  28  between the first aperture  26  and the second aperture  27  is determined by the flow rate from the hole of the first aperture  26  and the pumping speed of the vacuum pump  30  evacuating the space  28  and is generally designed to become not more than 10 −1  Pa. The mean free path at 10 −1  Pa becomes about 70 mm, so the Li + attached detected gas reaches the second aperture  28  by a translational energy of 110 eV (=10−(−100) eV) without striking the atmospheric gas,  
         [0073]    The Li + attached detected gas moving entrained in the flow of the gas near the hole of the first aperture  26  moves due to only the electrical field irregardless of the flow of the gas after the first aperture  26 . That is, the movement of the Li + attached detected gas after the first aperture  26  becomes substantially the same as the mass spectrometer of the ordinary EI method. The Li + attached detected gas passes through the second aperture  27 , then is condensed by the condensing lens  33  controlled by the condensing lens potential control power source  35  and is transported to the quadrapole mass spectrometer  34 . The quadrapole mass spectrometer  34  is controlled by the quadrapole mass spectrometer control power source  36  and then the sample gas including the Li + attached detected gas is analyzed.  
         [0074]    The fact that there is no collision with the gas after the first aperture  26  means that even if there is a change in the potential in the intermediate region, the translational energy finally striking the quadrapole mass spectrometer  34  will be equal to the translational energy when the gas leaves the first aperture  26 . Further, the translational energy of the ions which can be analyzed by the quadrapole mass spectrometer  34  is known to be 5 to 15 eV. In the present embodiment, the potential of the first aperture  26  is 10V, so the translational energy of the ions of the Li + attached detected gas striking the quadrapole mass spectrometer  34  becomes 10 eV and normal mass spectrometry becomes possible.  
         [0075]    The pressure in the space  11 B from the second aperture  27  and containing the condensing lens  33  and the quadrapole mass spectrometer  34  is determined by the flow rate from the second aperture  28  and the pumping speed of the vacuum pump  37  evacuating the space  11 B from the second aperture  28  to the quadrapole mass spectrometer  34  and is designed to become generally less than 10 −3  Pa. Therefore, not only is the Li +  attached detected gas prevented from striking the atmospheric gas, but also the quadrapole mass spectrometer  34  can be operated stably over a long period without contamination by the halide compound.  
         [0076]    Next, a mass spectrometry apparatus according to a second embodiment of the present invention will be explained with reference to FIG. 2. The mass spectrometry apparatus of this second embodiment is configured with the emitter portion and the reaction chamber integral. That is, the emitter placement region, Li + flight region, and reaction chamber explained in the first embodiment are made integral. Therefore, a reaction chamber is not specially provided. The rest of the configuration is the same as that of the first embodiment. Elements shown in FIG. 2 will be explained assigning the same reference numerals as with the elements explained in the first embodiment. In the left internal space  11 A, the above-mentioned reaction chamber  20  and the related vacuum pump  25  etc. are eliminated. The sample gas source  21  and the N 2  gas source  25  introduce the sample gas and the N 2  gas directly into the Li +  flight region  19  through the piping  21   a  and  22   a . The conditions of the space etc. including the emitter  12 , repeller  15 , emitter lens  17 , and Li + flight region  19  are substantially the same as those explained in the first embodiment.  
         [0077]    According to the above second embodiment, it is possible to eliminate the parts relating to the reaction chamber, so the hardware configuration can be simplied and the manufacturing costs can be reduced.  
         [0078]    [0078]FIG. 3 shows the mass spectrum obtained as a result of a measurement test conducted using the apparatus according to the second embodiment of the present invention. In FIG. 3, the abscissa indicates the mass, while the ordinate indicates the amount of ions. In this measurement test, the detected gas was made C 4 F H , the cations were made Li + , the pressure in the reaction chamber was made 133 Pa, and the inert gas was made N 2 . In the obtained mass spectrum, only peaks of C 4 F H Li + (207 amu), N 2 Li +  corresponding to the molecule peak of N 2  (35 amu), H 2 OLi +  corresponding to the molecule peak of H 2 O included in the gas as an impurity (25 amu), and unattached Li + (7 amu) appear. That is, it was confirmed that even if using cation attachment mass spectrometry for a fluoride compound, the mass of the gas molecules of the detected gas is precisely measured without causing dissociation. Note that 133 amu is the Cs impurity contained in the emitter itself.  
         [0079]    The above embodiments can be modified in the following way. In the above embodiments, the detected gas was made the representative PFC of C 4 F 8 , but the present invention is not limited to a PFC. It can also be applied to all fluoride compounds having a large electron affinity and other similar characteristics as with PFC such as SF 6 —NF 3 . Further, it may also be applied to all halide compounds having characteristics similar to a fluoride compound.  
         [0080]    In the above embodiments, the main component of the atmosphere in the attachment region was made the most generally used N 2 , but the present invention is not limited to N 2 . It may also be applied to He, Ar, Ne, H 2 , etc. to which metal ions do not easily attach. Further, it may also be applied to any gas to which metal ions do not easily attach compared with the detected gas.  
         [0081]    In the above embodiments, use was made of the lightest Li + as the metal ions, but the present invention is not limited to Li + . It is also possible to use as the metal ions the easily handleable K + , Na + , Rb + , Cs + , Al + , Ga + , In + , etc. Further, it is possible to use any positive metal ions able to attach to a halide compound.  
         [0082]    The numbers and arrangement of the gas feed piping and vacuum pumps in the present invention are not limited to those of the above embodiments. Any system can be used so long as the pressure in the metal ion flight region and attachment reaction region is at least 10 Pa, preferably at least 100 Pa. These pressures may also be pressures smaller than 10 Pa if in a range giving the necessary measurement performance. Further, if it is possible to promote direct ion attachment in the reaction region by electrical or magnetic conditions, the pressure condition in the reaction region may also be set to a low pressure smaller than 10 Pa. Further, in the above embodiments, use was made of a quadrapole mass spectrometer for the mass spectrometry method and mass spectrometry apparatus, buy the present invention is not limited to this. It is also possible to use an ion trap ( 3 D) type mass spectrometer, magnetic field sector type mass spectrometer, TOF (time of flight) type mass spectrometer, etc.  
         [0083]    According to the present invention, provision is made of a mass spectrometry method and mass spectrometry apparatus applying Fujii system cation attachment to ionize gas molecules of a sample gas for mass spectrometry, so it becomes possible to ionize a fluoride compound or other halide compound by direct attachment without causing dissociation, it becomes possible to precisely observe a molecule peak of the halide compound being measured in the mass spectrum obtained by the mass spectrometry, and it becomes possible to precisely measure fluoride compounds etc. having a large impact on global warming.  
         [0084]    While the invention has been described with reference to specific embodiment chosen for purpose of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.