Patent Grant
7015461
1. Field of the Invention
The present invention relates to a method and apparatus for ion attachment mass spectrometry, and more particularly relates to a method and apparatus for ion attachment mass spectrometry suitable for measuring the ingredients and concentration of, for example, a low concentration detected gas without causing dissociation.
2. Description of the Related Art
An ion attachment mass spectrometry apparatus has the advantage of enabling quantitative analysis of a detected gas without causing dissociation. In the past, some ion attachment mass spectrometry apparatuses have been reported in Hodge, Analytical Chemistry, vol. 48, no. 6, p. 825 (1976); Bombick, Analytical Chemistry, vol. 56, no. 3, p. 396 (1984); and Fujii et al., Analytical Chemistry, vol. 1, no. 9, p. 1026 (1989), Chemical Physics Letters, vol. 191, no. 1.2, p. 162 (1992), and Japanese Unexamined Patent Publication (Kokai) No. 6-11485.
The ion emitter 18 of the emission mechanism 20 is made of a material including an oxide of an alkali metal. The material comprising the ion emitter 18 is for example a mixture of an Li oxide, Si oxide, and Al oxide. When the ion emitter 18 placed on the axis of the apparatus vessel 10 is heated to about 600° C. by electric power supplied from the emission mechanism control power source 21, Li+ or other positively charged metal ions are emitted into the space. These metal ions move toward an opening 16a of the first aperture 16 due to the electric field and flow of gas. During this moving period of time, the metal ions attach to the gas to be detected, which is introduced into the ionization chamber 11 as a sample gas by the sample gas introduction mechanism 22. In this way, the gas ionized by the attachment of metal ions is produced. For example, H2O becomes H2OLi+ of a mass number of the 18 amu (atomic mass units) of H2O plus the 7 amu of Li, that is, 25 amu. The positively charged ionized gas to be detected moves as it is and passes through the opening 16a. The above-mentioned path 29 shows the path of the metal ions and the gas with the metal ions attached.
When the metal ions attach to the molecules of the gas to be detected, they extremely gently attach to the locations of the charges biased on the gas molecules and almost no dissociation occurs. The smaller the bond energy, however, the easier the re-detachment of the Li+. To prevent this, it is necessary to raise the pressure in the ionization chamber 11 to the value included in the range of 10–1000 Pa (usually 100 Pa) by the third component gas introduction mechanism 23 and use collision with the gas in order to absorb the excess energy. The third component gas which also can be defined as excess-energy absorbing gas, is one of various inert gases, such as N2, which it relatively hard for the metal ions to attach to. The gas with the metal ions stably attached thereto passes through the differential evacuation chamber 12 where the focusing lens 28 is arranged. The gas subsequently enters the mass spectrometry chamber 13 where it is separated from the other gases so as to be detected in every mass through the Q-pole mass spectrometer 30.
When detecting a low concentration gas to be detected by use of the conventional ion attachment mass spectrometry apparatus shown in
Here, the “macromer” signifies a substance of two (dimer) or more gas molecules bonded together. For example, water is normally H2O, but becomes (H2O)2 as a dimer. Nitrogen is normally N2, but becomes (N2)2 as a dimer. In an ion attachment mass spectrometry method, there is the problem that even when there is actually no macromer, a slight amount of a macromer is finally produced in the process of ionization. For example, in the case of water, not only the usual H2OLi+, but also the dimer (H2O)2Li+ appears, while in the case of nitrogen, not only the usual N2Li+, but also the dimer (N2)2Li+ appears.
Further, the “surface ionization ions” signify ions produced by removing some atoms from the molecule of the gas when the gas comes in contact with a heated surface. In the ion attachment mass spectrometry method, there is the problem that after all the surface ionization ions are produced at the surface of the heated ion emitter 18 depending on the gas. For example, in the case of dimethylphthalate (C10H10O4=194 amu), ions of 163 amu being less than the inherent mass number by exactly OCH3 (31 amu) appear.
Further, the “isotope” signifies the same element with a different mass number. In the case of Li, most of them have the mass number of 7 amu, but an isotope with a mass number of 6 amu is also present in an amount of about 7.5%.
An object of the present invention is to provide a method and apparatus for ion attachment mass spectrometry capable of preventing the occurrence of the interference peak and carrying out accurate mass spectrometry when analyzing the mass of the gas to be detected by ionizing it using the ion attachment method.
The method and apparatus for ion attachment mass spectrometry according to the present invention are configured as follows so as to achieve the above-mentioned object.
A first method of ion attachment mass spectrometry is a method for attaching positively charged metal ions emitted from an ion emitter to a gas to be detected, which is introduced into an ionization chamber, in an atmosphere of a third component gas so as to ionize the gas by the metal ions, and then performing measurement of mass of the gas by mass spectrometry. In the method of the mass spectrometry measurement, a third component gas is selected from a plurality of third component gases prepared in advance.
A second method of ion attachment mass spectrometry is a method having the same ionization and measurement steps mentioned above, and further, in performing the measurement step, a plurality of measurements are performed respectively using different third component gases and interference peaks arising due to the third component gases are distinguished on the basis of the data obtained from the measurements.
A third method of ion attachment mass spectrometry is a method having the same ionization and measurement steps mentioned above, and further, in performing the measurement step, one ion emitter is selected from a plurality of ion emitters prepared in advance.
A fourth method of ion attachment mass spectrometry is a method having the same ionization and measurement steps mentioned above, and further the measurement step comprising steps of preparing a plurality of different ion emitters, performing measurement a plurality of times by different ion emitters, and distinguishing the interference peak arising due to the third component gas from the data.
A first apparatus for ion attachment mass spectrometry is provided with an ion emitter for emitting positively charged metal ions, an ionization chamber for attaching the metal ions to a gas to be detected, a third component gas introduction mechanism for preparing a plurality of types of third component gases in advance and introducing one type of third component gas selected from the plurality of types of third component gases into the ionization chamber, and a mass spectrometer for performing mass spectrometry to detect the gas to which the metal ions are attached.
A second apparatus for ion attachment mass spectrometry is provided with the ion emitter, the ionization chamber, the mass spectrometer, the third component gas introduction mechanism for introducing one type of third component gas selected from the plurality of types of third component gases prepared in advance into the ionization chamber, and a data processor for processing data given from the mass spectrometer in order to distinguish the interference peak arising due to the third component gas from a plurality of sets of measurement data based on a plurality of different types of third component gases.
A third apparatus for ion attachment mass spectrometry has the above-mentioned second apparatus configuration, and further has a plurality of types of ion emitters for emitting different types of positively charged metal ions, in which one of the plurality of types of ion emitters is selected for emission of the metal ions.
A fourth apparatus for ion attachment mass spectrometry has the above-mentioned second apparatus configuration, and is further configured so that a plurality of types of ion emitters for emitting different types of positively charged metal ions are prepared, one type of the ion emitters is selected for emission of the metal ions, and the data processor processes data given from the mass spectrometer for distinguishing the interference peak arising due to the ion emitter from a plurality of sets of measurement data based on the different ion emitters.
In the third and fourth apparatuses for ion attachment mass spectrometry, preferably, the plurality of types of ion emitters are arranged at positions offset from the axis.
In accordance with the method and apparatus for ion attachment mass spectrometry of the present invention, when Li+ being normally low in generation of fragments is used as the primary ions and the interference peaks occur, in order to eliminate the interference peaks due to the macromers of the third component gases with each other and the macromers of the third component gases and the high concentration ingredients, one type of third component gas among a plurality of types of third component gases prepared in advance is selectively used depending on the type of the gas and the objective of the measurement. Further, in order to eliminate the interference peaks due to ionization ions at the surface of the ion emitter and isotopes of the metal ions, similarly, one type of ion emitter among the plurality of types of ion emitters prepared in advance is selectively used depending on the type of the gas and the objective of the measurement.
When using the ion attachment mass spectrometry apparatus to detect for example a low concentration gas, as explained above, the macromers of third component gases with each other, the macromers of third component gases and high concentration ingredients, the surface ionization ions, and the isotopes of metal ions normally cause the interference peaks in the measurement data obtained by the mass spectrometry and make measurement of the signal of the gas impossible due to concealment by the interference peaks. With the present invention, however, the occurrence of interference peaks is eliminated to make measurement possible. The means of eliminating the occurrence of the interference peaks, in view of the causes of them, are, first, to change the mass of the dimer ions appearing at the same positions as the detected ions to shift their position, second, to shift only the peaks position of the ionized gas by attachment of metal ions, or, third, to prevent the generation of isotope ions.
In accordance with the present invention, when detecting a low concentration gas for example by using the ion attachment mass spectrometry apparatus, the basic idea of the detection is to switch the third component gas or the ion emitter so as to change the mass of the dimer ions appearing at the same positions as the detected ions to shift their position, or shift only the peaks position of the ionized gas by attachment of metal ions, or prevent the generation of isotope ions, in order to prevent the generation of interference peaks and perform accurate mass spectrometry.
The 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:
Preferred embodiments of the present invention will be explained next with reference to the attached drawings.
Embodiments of the method and apparatus for ion attachment mass spectrometry according to the present invention will be explained next with reference to
The basic configuration will be explained first. In the figure, an ionization chamber 11, a differential evacuation chamber 12, and a mass spectrometry chamber 13 are connected in cascade to form an apparatus vessel 10 as a whole. The differential evacuation chamber 12 and mass spectrometry chamber 13 are provided with vacuum pumps 14 and 15, respectively. A first aperture 16 is arranged between the ionization chamber 11 and differential evacuation chamber 12, while a second aperture 17 is arranged between the differential evacuation chamber 12 and mass spectrometry chamber 13. The ionization chamber 11 is provided with an emission mechanism 20 comprised of an ion emitter 18 and a repeller 19. Further, the emission mechanism 20 is provided with an emission mechanism control power source 21. The ionization chamber 11 has a sample gas introduction mechanism 22 connected to it. A sample gas is introduced from this. The sample gas introduction mechanism 22 includes a sample gas cylinder 24 and a valve 25. In this embodiment, the sample gas is a low concentration sample gas. The differential evacuation chamber 12 has a focusing lens 28 arranged in it. In the figure, the path 29 of the metal ions and the gas with the metal ions attached to it is shown. The mass spectrometry chamber 13 is provided with a Q-pole type mass spectrometer 30. The output section of the ion trap 31 is connected to a data processor 50. The data processor 50 has the function of processing the measurement data based on a detection signal given from the ion trap 31 and the function of controlling the valve opening and closing operation.
The ion emitter 18 of the emission mechanism 20, as explained in the section on the related art, is formed from a mixture of an Li oxide, Si oxide, and Al oxide, for example. When the ion emitter 18 placed on the axis of the apparatus vessel 10 is heated to about 600° C. by the electric power supplied from the emission mechanism control power source 21, Li+ or other positively charged metal ions are emitted into the space. These metal ions move toward the opening 16a of the first aperture 16 due to the electric field and flow of gas. During this, the metal ions attach to the gas to be detected introduced into the ionization chamber 11 by the sample gas introduction mechanism 22. In this way, the gas ionized by the attachment of metal ions is produced. The rest of the basic configuration and action are substantially the same as those of the conventional apparatus explained with reference to
The characteristic configuration of the ion attachment mass spectrometry apparatus having the above basic configuration will be explained next. In the ion attachment mass spectrometry apparatus according to the present embodiment, a third component gas introduction mechanism 51 configured to introduce into the ionization chamber 11 one type of third component gas among a plurality of types, for example, three types, of third component gases (A, B, and C), is provided. The third component gas introduction mechanism 51 is provided with three third component gas cylinders 52a, 52b, and 52c and valves 53a, 53b, and 53c provided at the gas introduction pipes of the third component gas cylinders. The third component gas cylinders 52a, 52b, and 52c contain different types of third component gases A, B, and C, respectively. The opening and closing operations of the valves 53a, 53b, and 53c for introducing the third component gases A, B, and C into the ionization chamber 11 or stopping the introduction are automatically controlled by the data processor 50. The valves 53a, 53b, and 53c are opened or closed at suitable timings in accordance with the measurement conditions. Due to this, one type of third component gas among the three types of the above-mentioned third component gases is suitably selected and introduced into the ionization chamber 11.
As explained above, the ion attachment mass spectrometry apparatus according to the present embodiment is exactly the same in its basic configuration and operation as the conventional apparatus explained with reference to
Further, when the interference occurs even if the third component gas is changed or when it is unknown whether the interference occurs, the opening or closing operations of the valves 53a, 53b, and 53c in the third component gas introduction mechanism 51 are controlled by the data processor 50 to successively introduce different third component gases into the ionization chamber 11, due to this, a plurality of measurements are performed using the different third component gases, and the interference peak arising due to the third component gas is identified based on the set of measurement data.
Next, specific examples of the method of analysis using the above ion attachment mass spectrometry apparatus will be explained in detail for different cases with reference to the drawings.
In
The spectrum after attachment of metal ions for actual measurement becomes as shown in FIG. 2(2). All of the peaks 54 and 55a to 55d shown in FIG. 2(2) are shifted to the high mass side (right side in the figure) by exactly the mass number (=i) of the metal ions attached. That is, the mass number on the abscissa of the peak 54 due to the third component gas A in FIG. 2(2) becomes a+i. For example, if the third component gas A is N2 and the metal ions are Li, the mass number becomes 28+7=35. If assuming no macromer occurs, no interference arises in this state so long as there is no detected gas of the same type as the third component gas A from the start.
In accordance with the method of ion attachment mass spectrometry, however, a macromer is actually formed in the process of ionization. The case where the third component gas A forms a dimer is shown in FIG. 2(3). Reference numeral 56 shows the peak of the dimer. If the position of appearance of the peak is indicated by the mass number, it becomes 2a+i. If there is a detected gas present right at the same position, an interference state (state of 57 in the figure, called “interference peak”) is caused. In general, the third component gas is relatively hard for metal ions to attach to and becomes a dimer even less frequently, but this becomes a major problem when measuring a low concentration gas to be detected.
Therefore, in the ion attachment mass spectrometry apparatus shown in
The type of the third component gas introduced into the ionization chamber 11 is usually selected in accordance with a predetermined routine when judging the interference has occurred by the data processor 50 for judging the measurement results obtained.
The change from the type A to B of the third component gas for avoiding the above interference 62 is, in the same way as the above, executed based on the control function of the data processor 50 in the ion attachment mass spectrometry apparatus shown in
In the ion attachment mass spectrometry apparatus according to the present embodiment, the spectrum of only the peaks of the detected gas are derived as follows from the two spectra measured as shown in (1) and (2) of
First, a peak where no interference occurs is selected from the spectrum of FIG. 4(1). The positions of appearance of the dimer and trimer are known from the molecular weight of the third component gas A, so while the magnitudes are unclear, it is possible to determine the peaks where the interferences 66 and 67 occur. Therefore, it is possible to determine that {circle around (3)}, {circle around (6)}, {circle around (8)}, and {circle around (9)} of FIG. 4(1) are peaks where interference does not occur. In FIG. 4(2) as well, similarly {circle around (3)}, {circle around (4)}, {circle around (6)}, and {circle around (8)} are determined as peaks where interference does not occur, so the peaks where interference does not occur in both are {circle around (3)}, {circle around (6)}, and {circle around (8)}. Comparing the magnitudes of the peaks of {circle around (3)}, {circle around (6)}, and {circle around (8)} of FIG. 4(1) and 4(2), the difference in sensitivity depending on the third component gases A and B is determined. Therefore, if using the peaks of FIG. 4(2) for {circle around (3)}, {circle around (4)}, {circle around (6)}, and {circle around (8)} and using the peak of {circle around (9)} of FIG. 4(1) calibrated by the difference of sensitivity for {circle around (9)}, it is possible to derive the peaks of all of the detected gas as shown in FIG. 4(3).
Next, another embodiment of the method and apparatus for ion attachment mass spectrometry according to the present invention will be explained with reference to
One ion emitter is suitably selected and used from two types of ion emitters 18a and 18b prepared in advance according to the type of the gas to be detected or the purpose of measurement, for example when measuring a low concentration gas, to prevent the occurrence of interference with the detected gas. That is, two ion emitters 18a and 18b are arranged offset from the axis of flow of the ions (line matching with path 29), one of these is heated by being powered from the emission mechanism control power source 21, and thereby only one type of metal ions is emitted. Even metal ions emitted from the ion emitter at a position offset from the axis move riding the flow of the gas, so no problem arises in measurement.
Further, when interference arises even when changing the ion emitter or when whether interference occurs is unclear, the data processor 50 controls the operation so as to switch the ion emitter powered by the emission mechanism control power source 21, make a plurality of measurements by successively different ion emitters, and identify an interference peak arising due to an ion emitter from these data.
Next, specific examples of the method of analysis using the ion attachment mass spectrometry apparatus according to another embodiment will be explained for different cases with reference to the figures.
To change the metal ions, the ion attachment mass spectrometry apparatus shown in
In the method of analysis for eliminating an interference peak arising due to an isotope of the metal ions, in the ion attachment mass spectrometry apparatus shown in
The spectrum of only the peaks of the detected gas is derived as explained below based on the two spectra measured shown in FIGS. 8(1) and 8(2). Different from the case of a macromer, it is unclear where the surface ionization peak appears. Therefore, the ratio of the magnitudes of the corresponding peaks at FIGS. 8(1) and 8(2) is calculated. In FIGS. 8(1) and 8(2), the peaks shift by exactly the difference of the molecular weight of the metal ions, so the corresponding peaks can be easily determined. All of the peaks {circle around (3)} and {circle around (4)} among {circle around (3)}, {circle around (4)}, {circle around (5)}, {circle around (6)}, and {circle around (7)} have generally the same ratios, so it is determined that interference occurs at {circle around (3)} and {circle around (4)}. With surface ionization, the peaks appear at the same locations, so it is judged that {circle around (3)} shows a detected peak in FIG. 8(1) and {circle around (4)} a detected peak in FIG. 8(2). In this way, as shown in FIG. 8(3), it is possible to derive the peaks of all of the detected gases.
In the first embodiment, a plurality of third component gas cylinders were prepared, but the invention is not limited to this. It is sufficient that a plurality of types of third component gases can be introduced into the reaction chamber. The method of switching the plurality of types of third component gases may, as mentioned above, be automatic or manual. As the third component gas, nitrogen (N2) or argon (Ar) is used.
In the second embodiment, the ion emitters were arranged at positions offset from the axis, but the invention is not limited to this, For example, they may be arranged changed in position along the axis. Further, heating was used to select the ion emitter used, but the invention is not limited to this. It is sufficient that the plurality of ion emitters can be select to emit the metal ions. For example, it is also possible to change the voltage applied to the repellers.
In the above embodiments, the explanation was made about separate apparatuses for dealing with the interference arising due to the third component gases and interference arising due to ion emitters, but these may also be combined to a single apparatus.
In the above embodiments, the explanation was given with respect to Li+ as the metal ions, but the invention is not limited to this. It is also possible to use K+, Na+, Rb+, Cs+, Al+, Ga+, In+, etc. As metal ions with no isotope (extremely low presence), Na may be used. Further, as the mass spectrometer, use was made of a Q-pole type mass spectrometer, but the invention is not limited to this. It is also possible to use a three-dimensional (3D) type, magnetic field sector type, time-of-flight (TOF) type, or ion cyclotron resonance (ICR) type mass spectrometer.
Further, in the above embodiments, the explanation was given with reference to samples to be measured all in the gaseous state, but the samples themselves may also be solids or liquids. It is possible to convert solid or liquid samples to a gaseous state by some means or another and then analyze that gas. Further, the apparatus of the present invention may also be connected to another component separation apparatus, for example, a gas chromatograph or liquid chromatograph, for use as a gas chromatograph/mass spectrometer (GC/MS) or liquid chromatograph/mass spectrometer (LC/MS).
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.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-401483, filed on Dec. 28, 2000, the disclosure of which is expressly incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2000-401483 | Dec 2000 | JP | national |
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| 6744041 | Sheehan et al. | Jun 2004 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20020084408 A1 | Jul 2002 | US |
