Microwave plasma spectrometer using dielectric resonator
A dielectric resonator is excited at its natural resonant frequency to produce a highly uniform electric field for the generation of plasma. The plasma may be used as a desolvator, atomizer excitation source and ionization source in an optical spectrometer or a mass spectrometer.
1. A spectrometer comprising a plasma generator, the plasma generator comprising:
a dielectric resonator structure of dielectric material extending around a central axis;
a port for introducing gas and a material to be studied into a region adjacent the dielectric resonator structure; and
a radiofrequency power source electrically coupled to the dielectric resonator structure to promote an alternating polarization current flow at a natural resonant frequency of the dielectric resonator structure and directed circumferentially about the central axis along a continuous circular path within the dielectric material of the dielectric resonator to generate plasma in the gas to interact with the material to be studied.
2. The spectrometer of claim 1 wherein the radiofrequency power source is electromagnetically coupled to the dielectric resonator structure.
3. The spectrometer of claim 2 wherein axial surfaces of the dielectric resonator are substantially free from electrical shielding.
4. The spectrometer of claim 1 wherein the dielectric resonator structure is electrically coupled to the plasma substantially only by induction, there being negligible capacitive coupling.
5. The spectrometer of claim 1 wherein the dielectric resonator has a quality factor of greater than 100.
6. The spectrometer of claim 1 wherein the dielectric resonator has electrical resistivity greater than 1×10 10 Ω·cm.
7. The spectrometer of claim 1 of wherein the dielectric resonator has a melting point greater than a melting point of copper.
8. The spectrometer of claim 1 wherein dielectric resonator has a dielectric constant with a loss tangent of less than 0.01.
9. The spectrometer of claim 1 wherein the dielectric resonator has a dielectric constant of greater than five.
10. The spectrometer of claim 1 wherein a dielectric material of the dielectric resonator is selected from the group consisting of alumina (Al 2 O 3 ) and calcium titanate (CaTiO 3 ).
11. The spectrometer of claim 1 wherein the gas is selected from the group consisting of nitrogen and air.
12. The spectrometer of claim 1 wherein the dielectric resonator is selected from the group consisting of a ring and a cylindrical annulus having a central opening along the axis.
13. The spectrometer of claim 12 wherein dielectric resonator has a central opening of at least one millimeter in diameter.
14. The spectrometer of claim 12 wherein the dielectric resonator has a central opening which is circular and has a diameter of between 15 mm and 25 mm.
15. The spectrometer of claim 12 wherein the port further includes a gas port introducing gas into the ring along an axis of the dielectric resonator.
16. The spectrometer of claim 12 wherein dielectric resonator has a central opening of at least one half inch (0.0127 m) in diameter.
17. The spectrometer of claim 1 wherein the radiofrequency power source is driven at a frequency which is within two full width at half maximum (FWHM) bandwidths of the resonant frequency of the dielectric resonator structure when the resonator is loaded.
18. The spectrometer of claim 1 wherein the radiofrequency power source automatically seeks the natural resonant frequency of the dielectric resonator structure to output radiofrequency power substantially at the natural resonant frequency of the dielectric resonator structure.
19. The spectrometer of claim 1 wherein the radiofrequency power source is selected from the group consisting of one or more of a magnetron, a solid state oscillator and a vacuum tube oscillator.
20. The spectrometer of claim 1 wherein the radiofrequency power source output frequency lies within a range from 20 to 1000 MegaHertz.
21. The spectrometer of claim 1 wherein the radiofrequency power source output frequency lies within a range selected from the group consisting of: 1 MHz-10 GHz, 30 MHz-300 MHz, and 300 MHz-3 GHz.
22. The spectrometer of claim 1 wherein the approximate radiofrequency power source output frequency is selected from the group consisting of 27 MHz, 60 MHz, 430 MHz, 915 MHz, 2450 MHz.
23. The spectrometer of claim 1 wherein the radiofrequency power source provides between 0.5 and 2 kW of power into the plasma.
24. The spectrometer of claim 1 further comprising:
an optical sensor for measuring frequency of light emitted by the material when heated by the plasma.
25. The spectrometer of claim 24 wherein the optical sensor comprises a dispersive element for dispersing light emitted by the plasma according to the wavelength of the light; and an optical detector for detecting the dispersed light.
26. The spectrometer of claim 25 wherein the dispersive element comprises a grating.
27. The spectrometer of claim 25 further including elements selected from the group consisting of one or more of: one or more optical focusing elements; mirrors for changing the direction of one or more beams of light; a focal plane array detector comprising multiple detecting elements for simultaneously detecting light dispersed by the dispersive element, the focal plane array detector forming at least part of the optical detector, a controller for controlling the spectrometer, and a controller for receiving an output from the optical detector.
28. The spectrometer of claim 1 further comprising:
a manifold comprising a gas port suitable for delivering sample material into the plasma generated by the plasma generator;
a sample cone and a skimmer cone;
at least one ion focusing element;
a mass analyzing element; and
an ion detector for detecting sample material ionized by the plasma.
29. The spectrometer of claim 28 further comprising a controller for controlling the spectrometer and a controller for receiving an output from the ion detector.
30. The spectrometer of claim 1 selected from the group consisting of an optical emission spectrometer and a mass spectrometer.
31. The spectrometer of claim 1 wherein the dielectric resonator and the radiofrequency power source are placed within a waveguide and the radiofrequency power source is electrically coupled to the dielectric resonator structure by radiation through the waveguide.