Mass spectrometer

Abstract

A mass spectrometer including: an ionization chamber (11) that generates ions from a sample, a collision cell (222) located downstream from the ionization chamber (11), a mass separation unit (2412) located downstream from the collision cell (222), an energy barrier unit (223) located between the collision cell (222) and the mass separation unit (2412), a voltage application unit (30) that applies a voltage to each of the ionization chamber (11), the collision cell (222), and the energy barrier unit (223), and a control unit (42) that controls the voltage application unit (30) such that a potential of the ionization chamber (11) is set to a first potential, a potential of the collision cell (222) is set to a second potential that is lower than the first potential, and a potential of the energy barrier unit (223) is set to a third potential between the first potential and the second potential.

Claims

1. A mass spectrometer comprising: a) an ionization chamber configured to generate ions from a sample; b) a collision cell located downstream from the ionization chamber; c) a mass separation unit located downstream from the collision cell; d) an energy barrier unit located between the collision cell and the mass separation unit; e) a voltage application unit configured to apply a voltage to each of the ionization chamber, the collision cell, and the energy barrier unit; and f) a control unit configured to control the voltage application unit such that a potential of the ionization chamber is set to a first potential, a potential of the collision cell is set to a second potential that is lower than the first potential, and a potential of the energy barrier unit is set to a third potential between the first potential and the second potential.

2. The mass spectrometer according to claim 1, comprising: g) a gas introduction means configured to introduce a predetermined kind of gas at a prescribed pressure into the collision cell, wherein the control unit further controls the gas introduction means to execute a first analysis in which the gas is introduced into the collision cell and a second analysis in which no gas is introduced into the collision cell.

3. The mass spectrometer according to claim 1, wherein the ionization chamber is grounded.

4. The mass spectrometer according to claim 1, wherein the ionization chamber includes an inductively coupled plasma ion source.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a principle configuration diagram of a conventional inductively coupled plasma mass spectrometer.

(2) FIG. 2 is a principle configuration diagram of an inductively coupled plasma mass spectrometer as one embodiment of a mass spectrometer according to the present invention.

(3) FIGS. 3A and 3B are diagrams illustrating a potential of each unit of the inductively coupled plasma mass spectrometer of this embodiment.

(4) FIG. 4 is a graph showing a relation between a potential difference of a barrier unit with a collision cell and a percentage of ions introduced into a mass separation unit.

DESCRIPTION OF EMBODIMENTS

(5) Hereinafter, one embodiment of the mass spectrometer according to the present invention will be described with reference to the drawings. The mass spectrometer of this embodiment is an inductively coupled plasma mass spectrometer (ICP-MS).

(6) FIG. 2 is a principle configuration diagram of an inductively coupled plasma mass spectrometer 1 of this embodiment. The inductively coupled plasma mass spectrometer 1 is roughly composed of an ionization unit 10, a mass spectrometry unit 20, a power supply unit 30, and a control unit 40.

(7) The ionization unit 10 has an ionization chamber 11 that is at approximately atmospheric pressure and grounded, and a plasma torch 12 is disposed inside the ionization chamber 11. The plasma torch 12 is composed of a sample tube through which a liquid sample atomized by a nebulizer gas flows, a plasma gas tube formed on an outer periphery of the sample tube, and a coolant gas tube formed on an outer periphery of the plasma gas tube. In addition, the plasma torch 12 also includes an autosampler 13 for introducing a liquid sample into the sample tube of the plasma torch 12, a nebulizer gas supply source 14 for supplying a nebulizer gas to the sample tube, a plasma gas supply source 15 for supplying a plasma gas (argon gas) to the plasma gas tube, and a coolant gas supply source (not shown) for supplying a coolant gas to the coolant gas tube.

(8) The mass spectrometry unit 20 includes, sequentially from the plasma torch 12 side, a first vacuum chamber 21, a second vacuum chamber 22, and a third vacuum chamber 24. The first vacuum chamber 21 is an interface with the ionization chamber 11. In the second vacuum chamber 22, an ion lens 221 for converging the flight trajectory of ions, a collision cell 222, and an energy barrier formation electrode 223 are disposed. The energy barrier formation electrode 223 is an electrode having an opening for the passage of ions, and is used to form the below-described energy barrier. In the third vacuum chamber 24, a quadrupole mass filter 241 (a pre-rod 2411 and a main rod 2412) and a detector 242 are disposed.

(9) The control unit 40 includes a storage unit 41 and also an analysis control unit 42 as a functional block. The entity of the control unit 40 is a personal computer, and a CPU executes a prescribed program (program for mass spectrometry) to realize the analysis control unit 42. In addition, an input unit 60, such as a keyboard or a mouse, and a display section 70, such as a liquid crystal display, are connected to the control unit 40. In the storage unit 41, analysis conditions used in the without-gas analysis and with-gas analysis described below are previously stored. In addition, output signals from the detector 242 are successively stored.

(10) In the inductively coupled plasma mass spectrometer 1 of this embodiment, based on the instructions from the user through the input unit 60, the analysis control unit 42 executes a first analysis in which mass spectrometry is performed with gas introduction into the collision cell 222 (with-gas analysis) and a second analysis in which mass spectrometry is performed without gas introduction into the collision cell 222 (without-gas analysis). The first analysis is an analysis in which the influence of interfering ions generated in the ionization chamber 11 is reduced using the kinetic energy discrimination (KED) method. Meanwhile, the second analysis is an analysis that does not use the KED method. Hereinafter, the case where the second analysis (without-gas analysis) is performed will be described as an example.

(11) When instructed by the user to perform a without-gas analysis through the input unit 60, the analysis control unit 42 applies predetermined voltages to the collision cell 222 and the energy barrier formation electrode 223, respectively. These voltages are predetermined such that the collision cell 222 is at a lower potential than the energy barrier formation electrode 223. For example, in the case where positive ions are measured, voltages are applied such that a negative potential (second potential: B) is formed in the collision cell 222, and a negative potential whose absolute value is lower than that of the second potential (third potential: C) is formed in the energy barrier formation electrode 223. In this embodiment, the ionization chamber 11 is grounded. A voltage may also be applied to the ionization chamber 11 to form a first potential (A). As a result, a first potential (A: in this embodiment, ground potential 0) is formed in the ionization chamber 11, a second potential (B) is formed in the collision cell 222, and a third potential (C) is formed in the energy barrier formation electrode 223.

(12) FIG. 3A schematically shows the potentials formed in the ionization chamber 11, the collision cell 222, and the energy barrier formation electrode 223 in the inductively coupled plasma mass spectrometer of this embodiment. In addition, for comparison, FIG. 3B shows the potentials of the respective units in a conventional inductively coupled plasma mass spectrometer.

(13) Before describing the behavior of ions in this embodiment, the conventional configuration will be described. In a conventional inductively coupled plasma mass spectrometer, at the time of a without-gas analysis, all the units are set at the same potential (typically, all at the ground potential). In this case, analyte ions generated in the ionization chamber and interfering ions generated in the collision cell are both introduced into the quadrupole mass filter (mass separation unit) 241 without being accelerated or decelerated. At the time of measuring the analyte ions, the interfering ions cause background noise. Therefore, even when the second analysis focusing on the measurement sensitivity (without-gas analysis) is performed, sufficient measurement sensitivity could not be sometimes obtained.

(14) In the inductively coupled plasma mass spectrometer 1 of this embodiment, in order to solve the above problems in a conventional device, the potentials of the ionization chamber 11, the collision cell 222, and the energy barrier formation electrode 223 are set as described above.

(15) To the analyte ions generated in the ionization chamber 11 which is at the ground potential, the initial kinetic energy is given at the time of generation. The analyte ions are, while moving toward the collision cell 222, accelerated with the energy corresponding to the potential difference (B) between the first potential (0) of the ionization chamber 11 and the second potential (B) of the collision cell 222. Subsequently, while moving toward the energy harrier formation electrode 223, the ions are decelerated with the energy corresponding to the potential difference (CB) between the second potential (B) of the collision cell 222 and the third potential (C) of the energy barrier formation electrode 223. Because the previous accelerating energy is larger than this decelerating energy, the analyte ions pass through the energy bar formation electrode 223 while possessing the kinetic energy.

(16) Also to the interfering ions generated in the collision cell the initial kinetic energy is given at the time of generation. The interfering ions are, after exiting from the collision cell 222, decelerated with the energy corresponding to the potential difference (CB) between the second potential (B) of the collision cell 222 and the third potential (C) of the energy partition wall formation electrode 223. Unlike the analyte ions, the interfering ions generated in the collision cell 222 are decelerated without being previously accelerated. Accordingly, most of the interfering ions are blocked by energy barrier formed between the collision cell 222 and the energy barrier formation electrode 223. That is, when the second potential (B) and the third potential (C) are predetermined such that the energy corresponding to the potential difference (CB) between the second potential (B) of the collision cell 222 and the third potential (C) of the energy barrier formation electrode 223 is larger than the initial kinetic energy of interfering ions generated in the collision cell 222, the analyte ions can be exclusively introduced into the quadrupole mass filter 241 located downstream from the energy barrier formation electrode 223.

(17) FIG. 4 shows the results of the simulation of the relation between the difference between the second voltage applied to the collision cell 222 and the third voltage applied to the energy barrier formation electrode 223 and the percentage of ions introduced into the quadrupole mass filter 241. The horizontal axis of the graph represents the difference between the third voltage of the energy barrier formation electrode 223 and the second voltage of the collision cell 222 (third voltagesecond voltage), and the longitudinal axis represents the percentage of ions introduced into the quadrupole mass filter 241 through the energy barrier formation electrode 223 (the percentage relative to the amount of ions introduced into the quadrupole mass filter 241 when the above voltage difference is 0). The solid line in the graph represents analyte ions generated in the ionization chamber 11. While the dashed line represents interfering ions generated in the collision cell 222. The horizontal axis 0 in FIG. 4 corresponds to the conventional configuration, and, taking the percentages of analyte ions and interfering ions introduced at this time as 100%, their introduction percentages at other potential differences were determined by simulation. The region on the right-hand side from the horizontal axis 0 in the graph (i.e., the region where the third potential is higher than the second potential) corresponds to the configuration of this embodiment. In this region, with an increase in the potential difference, the percentage of analyte ions introduced into the quadrupole mass filter 241 somewhat decreases, but the percentage of interfering ions introduced into the quadrupole mass filter 241 can be reduced even more; as a result, the S/N ratio can be improved to enhance the measurement sensitivity.

(18) In order to facilitate the understanding of the characteristics of the present invention, the case where the second analysis (without-gas analysis) is performed has been described as an example here. Also in the first analysis (with-gas analysis), the same configuration as above can be taken. Specifically, when the potential of the energy barrier formation electrode 223 is set lower by an amount corresponding to the kinetic energy that analyte ions lose upon collision with gas molecules in the collision cell 222, the same effects as above can be obtained. In this case, the introduction of interfering ions generated in the ionization chamber 11 into the quadrupole mass filter 241 in the KED method can also be prevented.

(19) The above embodiment is an example and can be suitably modified following the gist of the present invention. In the above embodiment, the energy barrier formation electrode 223 is disposed between the collision cell 222 and the partition wall 23. The energy barrier formation electrode 223 may also be disposed between the partition wall 23 and the pre-rod 2411 (position indicated by, the dashed line in FIG. 2). Alternatively, also when the energy barrier formation electrode 223 is not used, and the pre-rod 2411 is set at the third potential to form an energy barrier with the collision cell 222, the same effects as above can be obtained. Further, it is also possible that the outlet-side wall surface of the collision cell 222 is set at the third potential to form a potential difference with the inside of the collision cell 222, or the partition wall 23 is used as an energy barrier formation electrode and set at the third potential, for example. Like this, various configurations are possible. That is, as long as the energy barrier described above can be formed between the inside of the collision cell 222 and the main rod (mass separations unit) 2412, any suitable configuration can be taken.

(20) In addition, in the above embodiment, an inductively coupled plasma mass spectrometer has been described. Also in a different kind of mass spectrometer such as a triple quadrupole mass spectrometer, as long as it is a mass spectrometer including an ionization chamber, a collision cell, and a mass separation unit, the potentials of the ionization chamber, the collision cell, and the energy barrier unit can be set in the same manner as above, and the influence of interfering ions generated in the collision cell can be reduced.

REFERENCE SIGNS LIST

(21) 1 . . . Inductively Coupled Plasma Mass Spectrometer 10 . . . Ionization Unit 11 . . . Ionization Chamber 12 . . . Plasma Torch 13 . . . Autosampler 14 . . . Nebulizer Gas Supply Source 15 . . . Plasma Gas Supply Source 20 . . . Mass Spectrometry Unit 21 . . . First Vacuum Chamber 22 . . . Second Vacuum Chamber 221 . . . Ion Lens 222 . . . Collision Cell 223 . . . Energy Barrier Formation Electrode 23 . . . Partition Wall 24 . . . Third Vacuum Chamber 241 . . . Quadrupole Mass Filter 2411 . . . Pre-Rod 2412 . . . Main Rod 242 . . . Detector 30 . . . Power Supply Unit 40 . . . Control Unit 41 . . . Storage Unit 42 . . . Analysis Control Unit 60 . . . Input Unit 70 . . . Display Unit