INDUCTIVELY-COUPLED RADIO FREQUENCY PLASMA MASS SPECTROMETER

Abstract: An inductively-coupled radio frequency plasma mass spectrometer comprises an induction coil (2) for generating a high frequency magnetic field, a plasma torch (4) for causing a plasma by introducting an aerosol therein, and an electrostatic shield (10) interposed between the induction coil and the plasma torch, for shutting off the plasma from the electric field of the induction coil.

Description: BACKGROUND OF THE INVENTION The present invention relates to an inductively-coupled radio frequency plasma mass spectrometer for mass analysis with an inductively-coupled radio frequency plasma as an ion source as described in the first part of claim 1. Such a mass spectrometer is known from JP-A- 59-1052 57 which will be discussed later. Conventionally, an inductively-coupled radio frequency plasma mass spectrometer is more suitable for microanalysis than an inductively-coupled radio frequency plasma emission spectro-analyzer because of high sensitivity. Further, the former is suitable for analyzing isotope, so that wide applications have been recently developed. The inductively-coupled radio frequency plasma mass spectrometer is such that an induction coil is provided through which radio frequency current flows, where aerosol is introduced into a plasma torch to thereby generate an inductively-coupled radio frequency plasma (reffered to as "ICP" hereinbelow). Ions are thereby generated and introduced into a mass spectrometer, so that the mass of the ions is analyzed. In the conventional type of the above-described mass spectrometer, the energy of the ions developed by the ICP is too high to afford sufficient resolution in the mass spectrometer. FIG.3(a) shows a graph representing the spectrum of the energy of the ions. As shown in FIG.3(a), the spectrum of the energy of the ions is so wide that the ion beam cannot be enough forcused by a lens system leading the ion beam to the mass spectrometer, so that the signal output is not sufficient. Further, while the ion taken out of the plasma is introduced into a vacuum chamber containing the mass spectrometer therein via an orifice, the voltage of the plasma is varied so that a pinch discharge is caused between the ICP and the orifice. The orifice may be damaged. An ultraviolet ray noise may be caused because of the pinch discharge, so that the accuracy of the mass spectrometer may be influenced. A device has been proposed in which a tap is connected to the induction coil of the conventional mass spectrometer and the tap is kept grounded to reduce the voltage variation of the plasma (JP-A- 59-1052 57). However, in such a proposed apparatus, the grounding tap is directly connected to the induction coil supplied with a radio frequency current, so that the handling of the radio frequency current is complex in the sense of an electrical circuit. No improved mass spectrometer has been presented to resolve the above problems. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved inductively-coupled radio frequency plasma mass spectrometer for restricting the voltage variation of ions to thereby enhance the resolution of the mass spectrometer. It is another object of the present invention to provide an improved inductively-coupled radio frequency plasma mass spectrometer for efficiently preventing a pinch discharge between an inductively-coupled radio frequency plasma and an orifice leading ions to a vacuum chamber in which a mass spectrometer is disposed. Briefly described, in accordance with the present invention, an inductively-coupled radio frequency plasma mass spectrometer comprises an induction coil for generating a high frequency magnetic field, a plasma torch for introducing an aerosol therein and causing an inductively-coupled high frequency plasma therein with the aid of said induction coil, and an electrostatic shield interposed between the induction coil and the plasma torch for electrically shutting off the plasma from the electric field of the induction coil. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: FIG.1 is a sectional view of an inductively-coupled radio frequency plasma mass spectrometer according to the present invention; FIG.2 is a perspective view of an electrostatic shield used for the mass spectrometer of FIG.1; and FIG.3(a) and 3(b) are graphs representative of the energy distribution of ions provided by the conventional type of mass spectrometer and the mass spectrometer of the present invention, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENT FIG.1 shows the construction of an inductively-coupled radio frequency plasma mass spectrometer 1 according to the present invention. The mass spectrometer 1 comprises an induction coil 2 for generating a radio frequency magnetic field, a plasma torch 4 being a crystal tube to which aerosol is introduced, a radio frequency source 6 for providing radio frequency power to the induction coil, and a matching circuit 8 for affording an impedance matching. An electrostatic shield 10 is interposed, between the induction coil 2 and the plasma torch 4, for shutting off the electric field of the induction coil 2. FIG.2 is a perspective view of the electrostatic shield 10. The electrostatic shield 10 is provided with ring-shaped portions 12 with a predetermined distance from each other and a beam 14 for connecting the ring-shaped portions 12. Each of the ring-shaped portions 12 is cut away to thereby form an open end 16, so that the ring-shaped portions 12 function as an open loop to an induction current. When the electrostatic shield 10 is attached to the outside of the plasma torch 4, the electrostatic shield 10 is connected to a wall 36 of a first vacuum compartment 18 and stands thereby grounded. As far as the electrostatic shield 10 has no closed loop to the induction current, the structure of the electrostatic shield 10 should not be limited to the above-described one. First, second, and third vacuum compartments 18, 20, and 22 are provided. For example, the first vacuum compartment 18 is evacuated by a rotary pump while the second and the third vacuum compartments 20 and 22 are evacuated differentially by a diffusion pump. A lens system 24 is positioned within the second vacuum compartment 20. A quadrupole mass spectrometer 26 is positioned within the third vacuum compartment 22. An ion detector 28 is also positioned within the third vacuum compartment 22. A first orifice 30 is provided between the plasma torch 4 and the first vacuum compartment 18, a second orifice 32 is provided between the first vacuum compartment 18 and the second vacuum compartment 20, and a third orifice 34 is provided between the second vacuum compartment 20 and the third vacuum compartment 22. Within the wall 36 of the first vacuum compartment 18, on which the first orifice 30 is provided, a cooling water pathway 38 is formed to cool the wall 36 against the plasma of a high temperature. With the inductively-coupled radio frequency plasma mass spectrometer 1, a plasma 40 caused within the plasma torch 4 is shut off (out) from the electric field of the induction coil 2 with the help of the electrostatic shield 10, so that the voltage of the plasma 40 is kept substantially identical with the ground level of the electrostatic shield 10. Therefore, the voltage variation of the ions generated is prevented. The energy of the ions caused from the plasma 40 can be lowered. Further, as shown in FIG.3(b), the width of the energy distribution of the ions becomes narrow. Hence, the resolution of the mass spectrometer 26 can be improved. The pinch discharge caused between the plasma torch 4 and the first orifice 30 can be restricted to thereby prevent the generation of an ultraviolet ray noise. An induction current must flow within the plasma 40 due to a high frequency magnetic field generated with the induction coil 2 in order to maintain the plasma 40. If an electrostatic shield was shaped of a closed loop, an induction current might flow within the electrostatic shield, so that the high frequency magnetic field within the plasma torch 4 might be weakened to make it difficult to maintain the plasma 40. According to the present invention, the electrostatic shield 10 has the open end 16 to serve as an open loop to the induction current, so that no induction current can flow within the electrostatic shield 10. Therefore, the high frequency magnetic field within the plasma torch 4 cannot be influenced by the electrostatic shield 10. According to the inductively-coupled radio frequency plasma mass spectrometer of the present invention, the electrostatic shield is grounded, so that the plasma is shut off from the electric field of the induction coil with the help of the electrostatic shield. The voltage of the plasma is substantially grounded as in the electrostatic shield. The voltage variation of the ions caused is restricted, and the energy of the ions taken out of the plasma is lowered while the energy distribution width of the ions becomes narrow. Therefore, the resolution of the mass spectrometer can be improved. The pinch discharge between the ICP and the orifice can be prevented to thereby restrict the ultraviolet ray noise, resulting in the increase of the analysis accuracy and the prolongation of the life time of the orifice. While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the claims.

Claims: An inductively-coupled radio frequency plasma mass spectrometer comprising: induction coil means (2) for generating a high frequency magnetic field; plasma torch means (4) for introducing areosol therein and causing an inductively-coupled high frequency plasma therein with the aid of said induction coil means (2); characterized by electrostatic shield means (10) interposed between said induction coil means (2) and said plasma torch means (4), for electrically shutting off said plasma from the electric field of said induction coil means (2). The mass spectrometer as set forth in claim 1, wherein said electrostatic shield means (10) is shaped as an open loop. The mass spectrometer as set forth in claim 2, wherein said electrostatic shield means (10) is attached to the outside of said plasma torch means (4) and grounded. The mass spectrometer as set forth in claim 1, for electrically isolating said plasma via said electrostatic shield means (10) from the electric field caused by said induction coil means (2) and for maintaining the voltage of said plasma substantially identical with the ground level, so that the voltage variations of plasma ions are restricted. The mass spectrometer as set forth in claim 2, wherein said electrostatic shield means (10) comprises a plurality of ring-shaped portions (12) each having an open end (16), and a beam (14) for connecting the plurality of ring-shaped portions (12). The mass spectrometer as set forth in claim 3, wherein said electrostatic shield means (10) is coupled to a wall (36) of a vacuum compartment means (18) having orifice means (30) for taking out ions of said plasma (40) caused within said plasma torch means (4), through said orifice means (30).

Inventors: MISEKI, KOZO
Applicants: SHIMADZU CORPORATION
Application year: 1987
Main classification: H01J49
IPC codes: H01J49, G01N27