US3588594A

US3588594A - Device for bending a plasma flame

Info

Publication number: US3588594A
Application number: US791433*A
Authority: US
Inventors: Manabu Yamamoto; Hiromitsu Matsuno; Seiichi Murayama
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1968-01-19
Filing date: 1969-01-15
Publication date: 1971-06-28
Anticipated expiration: 1988-06-28
Also published as: DE1902307A1; DE1902307B2; FR2000460A1

Abstract

A PLASMA FLAME GENERATOR IS USED AS AN EXCITATION SOURCE FOR SPECTROSCOPIC ANALYSIS, THAT IS, A PLASMA FLAME GENERATOR HAVING A DEVICE FOR BENDING A PLASMA FLAME, IN WHICH A PAIR OF AUXILIARY ELECTRODES ARE DISPOSED SO AS TO ENCIRCLE THE PLASMA FLAME ALONG THE CENTRAL AXIS OF THE PLASMA FLAME AND A DC VOLTAGE IS APPLIED TO SAID AUXILIARY ELECTRODES FOR UTILIZING THE DRIFT OF IONS ALONG THE ELECTRIC LINES OF FORCE TO BEND THE PLASMA FLAME IN A PREDETERMINED DIRECTION.

Description

Inventors Manabu Yamamoto Odawara-shi; Seilchi Murayarna; Hlromitsu Matsuno, Hachlojl-shi, Japan Appl. No 791,433

Filed Jan. 15, 1969 Patented June 28, 1971 Assignee Hitachi, Ltd., Tokyo, Japan Priority Jan. 19, 1968 Japan 43/2629 DEVICE FOR BENDING A PLASMA FLAME 10 Claims, 11 Drawing Figs.

U.S.Cl 315/111,

219/121, 356/85 Int. Cl l-l0lj 7/24 Field of Search 219/121 [56] References Cited UNITED STATES PATENTS 2,945,119 7/1960 Blackman 219/123 3,277,265 10/1966 Reboux 219/121X 3,353,060 11/1967 Yamamoto et al. 315/111 FOREIGN PATENTS 1,345,152 10/1963 France 315/111 Primary Examiner-Raymond F. l-lossfeld Att0rneyCraig, Antonelli, Stewart and Hill ABSTRACT: A plasma flame generator is used as an excitation source for spectroscopic analysis, that is, a plasma flame generator having a device for bending a plasma flame, in which a pair of auxiliary electrodes are disposed so as to encircle the plasma flame along the central axis of the plasma flame and a DC voltage is applied to said auxiliary electrodes for utilizing the drift of ions along the electric lines of force to bend the plasma flame in a predetermined direction.

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SHEET 2 [IF 4 INVENTORS ATTORNIZY PATENTEU JUH28 19?:

SHEU R [If 4 INVENTORS nu llnumrrau M/Ifsmva BY M a?! ATTORNEYS DEVICE FOR BENDING A PLASMA FLAME BACKGROUND OF THE INVENTION FIELD 'OF THE INVENTION This invention relates to an excitation source for spectroscopic analysis and more particularly to a plasma flame generator in which auxiliary electrodes are disposed so as to encircle a plasma flameand a DC voltage is applied to said auxiliary electrodes to make ions in the plasma flame drift in a predetermined direction.

Spectroscopic analysis which makes a quantitative and qualitative analysis of a substance by means of emission or absorption spectra of elements to be analyzed in a substance introduced into a flame has generally a high accuracy and good reproducibility and is widely used. For spectroscopic analysis flames such as the following are used: the plasma flame produced by high frequency torch discharges, the oxyhydrogen flame, combustion flames and DC plasma jets.

BRIEF DESCRIPTION OF THE DRAWING FIG. la is a diagram illustrating the construction of a spectroscopic analysis device.

FIG. lb is an enlarged view illustrating the main portion of the relation between a plasma flame and a spectrometer of the spectroscopic analysis device shown in FIG. la.

FIG. 2 is an intensity of the spectral line vs. wavelength characteristic diagram of an element detected by the detector of the spectroscopic analysis device.

FIG. 3a is a diagram illustrating the observation position of the plasma flame.

FIG. 3b illustrates the main portion of a conventional high frequency plasma torch generator.

FIGS. 4a and 4b are diagrams illustrating the plasma flame state when the spectroscopic analysis is carried out by a conventional plasma flame generator.

FIG. 5 is a cross section of an embodiment of the plasma flame generator of the present invention.

FIG. 6 is a cross section of another embodiment of the plasma flame generator of the present invention.

FIGS. 7a and 7b are horizontal and vertical cross sections respectively showing the main portion of another embodiment of the present invention.

DESCRIPTION OF THE PRIOR ART First of all, the flame must be stabilized in order to improve the accuracy and detection limit of the spectroscopic analysis. For that purpose, the flame was hitherto jetted upward in the direction of gravity as shown in FIG. lb. Referring to FIG. 1a showing a horizontal cross section of a spectroscopic analysis device, the reference numeral I indicates a flame, 2 is a spectromcter, 3 and 4 are entrance and exit slits respectively, 5 is a light detector and 6 is a concave grating. When a spectrum is swept by rotating the concave grating of such a device, a wavelength vs. output waveform as shown in FIG. 2 appears at the light detector 5. Referring to FIG. 2, the reference mark S indicates the intensity of an emission line from an element to be analyzed (hereinafter referred to as the spectral-line intensity), N indicates the continuum emitted from the flame I (of course the band spectrum which cannot be resolved by the spectrometer used is also included), and AN indicates the fluctuation of the continuum caused by instability of the flame 1 (hereinafter referred to as noise). Now, the spectral-line intensity 5 decreases with a decrease in concentration of the elements to be analyzed. As described above the spectral-line intensity S is determined by the amount of elements to be analyzed, and the analysis cannot be carried out when the spectral-line intensity S becomes substantially equal to the noise AN of the continuum. Therefore, S/AN must be made large in order to analyze extremely small quantities of elements to be analyzed. For that purpose;

I. The flame must be stabilized in order to make AN small.

2. It is necessary to use an appropriate portion of the flame.

The foregoing can be explained by fixing the conditions of the spectrometer and light detector. The spectral-line intensity S of an element to be analyzed is a function of the temperature of the flame I. The noise AN depends on that portion of the flame l where the observation is made and is substantially proportional to N when the observation position is fixed, since the noise AN is caused by the fluctuation of the continuum intensity N due to, for example, flickering of the flame.

The central portion of the flame of the excitation source for spectroscopic analysis has a high temperature and its periphery has a lower temperature since the periphery is cooled. Therefore, S/AN differs depending upon the direction perpendicular to the central axis 0 of the flame (hereinafter referred to as the lateral observation position) as shown in FIG. 3a, for example, S/AN differs whether the center 0 or the periphery is observed. Moreover, the lateral observation position where S/AN becomes maximum differs depending upon the species of the elements to be analyzed or what form of compound of the elements is included in a sample, since parameters of the spectral-line intensity S (the dissociation energy of the sample compound, the excitation energy of the spectral-line, etc.) which are a function of the temperature differ depending upon the elements to be analyzed and the sample compounds, and the continuum intensity N differs dcpending upon the wavelength of the spectral-line intensity.

And in the case of an analysis using atomic absorption which is a method of spectroscopic analysis using the phenomenon wherein the spectral line emitted by an element in a high temperature portion is absorbed by an atom of the same element in a low temperature portion, there is also an optimum lateral observation position depending upon the kind of elements to be analyzed or of the compounds since in this case also the density of the absorbing atoms or others differ depending upon the position in the flame.

In short, when spectroscopic analysis is carried out, it is very important in order to increase the accuracy and detection limit of the spectroscopic analysis to suitably select the lateral observation position depending upon the kind of elements to be analyzed and of the compounds. However, it is very difficult in a conventional device to change the lateral observation position. That is, the image of the flame must first be focused on the slit of the spectrometer, then the flame must be moved in a lateral direction for that purpose, but the whole flame generator (such as magnetron, waveguides, water pipes and gas pipes in the case of high frequency torch discharges, and such as gas pipes in the case of combustion flames) must be moved in order to move the plasma flame in the lateral direction, and quick movement is impossible. Furthermore, it is generally very difficult to make a spectrometer.movable since a spectrometer is generally a large and heavy precision machine. In particular, as shown in FIG. 3b, a high frequency torch discharge generator is so constructed that the plasma flame l is surrounded by an outer conductor 9 by extending the outer conductor 9 of a coaxial waveguide upward beyond the end of an inner conductor 10 in order to lessen the high frequency electric power loss due to radiation from the discharge plasma and to prevent the plasma flame from becoming unstable by the turbulence of the gas stream. Then, observation is made through a small observation window 8 provided in the outer conductor 9. Thus it is very difficult to change the lateral observation position of the flame.

Thirdly, the resolving power of the spectrometer must be increased by making the width of

slits

3 and 4 of the spectrometer narrow in order to improve the detection limit. That is, first the detection limit can be improved by removing the effect of near lines. Next, S/N and then S/AN can be made large by making the width of the slits narrow enough since the spectralline intensity S is almost linearly proportional to the slit width and the continuum intensity N is almost proportional to the square of the slit width. Thus the detection limit can be improved.

However, when the excitation source flame is not bright enough, and particularly when the analysis of elements having spectral lines in the short wavelength region of about 2000 A where the absorption by the focusing system is large, for example, carbon, phosphorus, sulfur, etc. is carried out, the width of the slits cannot be made narrow enough since there is a limit in light detecting capacity of the light detecting portion 5. In particular, in the case of the lateral observation position, the light from the flame cannot be utilized effectively since the flame is long in the direction perpendicular to the optical axis and the light arriving at the spectrometer is weak.

However, if the flame is observed from the direction of the axis of the flame, the quantity of light increases by the ratio between the length of the flame in its axial direction and its diameter than when the flame is observed from the lateral direction. This ratio is about in the case of a usually used plasma flame by high frequency torch discharge or combustion flames and the light becomes about ten times stronger. In this case the width of the slit can be made narrow enough and the accuracy and detection limit can be improved.

As shown in FIG. 4a, the flame l is jetted perpendicular to the direction of gravity in order to observe the flame from its axial direction, but in this case a problem arises, that is, the flame is bent upward by the convection of gas as shown in FIG. 4b to cause instability and the detection limit decreases.

As has been described, the observation can be made suitably from the axial direction of the flame or from the direction perpendicular to the axis of the flame depending upon the intensity of the light, but in either case problems arise as described above and is not in practical use.

SUMMARY OF THE INVENTION One object of the present invention is to provide a plasma flame generator in which a desired portion of a plasma flame can be used suitably by bending the plasma flame in a desired direction.

Another object of the present invention is to provide a plasma flame generator in which the noise AN is made small and S/AN is made large by stabilizing the plasma flame.

The present invention consists of a plasma flame generator in which a pair of auxiliary electrodes are disposed so as to encircle a plasma flame and a DC voltage is applied to said auxiliary electrodes to bend ions in the plasma flame.

DESCRIPTION OF THE PREFERRED EMBODIMENT Now, a high frequency torch discharge generator shown in FIG. 5 is described as an example of the present invention. In the first place, a case is described where a plasma flame is stably jetted in the direction perpendicular to the direction of gravity. Referring to FIG. 5, reference numeral 1 is a plasma flame, 11 a rectangular waveguide, 12 a coaxial waveguide, 13 an inner conductor of the coaxial waveguide, 14 an electrode, 15 an inlet for a sample to be analyzed and plasma forming gas, 16 an insulator, 17 a lower correcting electrode, 18 an upper correcting electrode, 19 a DC power source, 20 a groove, 21 a variable shorting plunger, 22 an insulating plate which insulates the coaxial waveguide 12 from the correcting

electrodes

17 and 18 in response to a direct current but couples the waveguide 12 to the

electrodes

17 and 18 by capacitance in response to an alternating current, and 24 an insulating washer which insulates between a binding screw 23 and the correcting

electrodes

17 and 18. The coaxial waveguide in which the plasma flame is generated is so placed that its axis becomes perpendicular to the direction of gravity in order to jet the plasma flame in the direction perpendicular to the direction of gravity.

When an electromagnetic field is produced in the rectangular waveguide 11 by supplying microwave power from the lower side of the rectangular waveguide 11, an electromagnetic field is produced in the coaxial waveguide by a high frequency current is induced by the electromagnetic field in the inner conductor 13 of the coaxial waveguide 12. Then, the high frequency current and a high frequency voltage is induced at the tip of the inner conductor 13, that is at, the electrode 14. Here, the variable shorting plunger 21 is suitably moved to place it at a position where the high frequency power is effectively consumed in a region where the discharge plasma flame l is generated. On the other hand, the discharge plasma flame 1 is generated by introducing a plasma forming gas through the plasma forming gas inlet into the portion of the electrode 14. At this time, a sample to be analyzed is introduced into the discharge plasma flame by dispersing it into the plasma forming gas in the form of gas or spray. The plasma flame 1 thus generated is jetted into the space between the upper electrode 18 and lower electrode 17 which are electrically insulated from each other by the insulator 16 and the groove 20. When a voltage is applied to these electrodes by the DC power source 19 in such a polarity that the upper electrode 18 is positive and the lower electrode 17 is negative, positive ions in the plasma flame drift in the lower direction. Thereupon the positive ions collide with neutral molecules in the gases, then high temperature gases in the flame are pushed in the direction of motion of the ions, that is, in the downward direction. When the voltage of the DC power source 19 is suitably set up, the electric force which makes the ions move downward and a force by the convection of the gases which makes the plasma flame bend upward can be balanced, thereby the plasma flame 1 can be stabilized as if it were jetted upward in the direction of gravity.

Therefore, in a spectroscopic analysis device constructed by combining such a device for bending the flame for excitation source and a spectrometer and detector, the plasma flame can be observed from the direction of its axis without causing instability and a greater quantity of light can be obtained compared with a case of lateral observation position, therefore, the width of the slit of the spectrometer can be made narrow enough and an analysis of good accuracy and detection limit is possible.

The separate correcting

electrodes

17 and 18 are not limited to those shown in FIG. 5. That is, as is shown in FIG. 6, a plurality of sets of correcting electrodes (l7, 18; 17, I8; 17'', 18'') may be provided on the tip of the coaxial waveguide axially of the waveguide through rings (22, 22', 22") consisting of insulating material. Adjacent sets of the electrodes are clamped through the rings by a plurality of

screws

23, 23' and 23" to securely join them together. The respective sets of the correcting electrodes are supplied with the required voltages from

individual voltage sources

19, 19' and 19" to thereby bend the produced plasma flame in the desired direction with respect to the axis of the waveguide.

It has been explained that the lateral observation position can be changed without moving the plasma flame generator according to the present invention with reference to the high frequency torch discharge generator shown in FIGS. 7a and 7b as an example.

The construction of this device is the same as the device shown in FIG. 5 but the coaxial waveguide is placed upward in the direction of gravity so as to jet the plasma flame upward in the direction of gravity. The groove 20 serves also as observation port of light. The plasma flame 1 generated as in the above-mentioned case explained with reference to FIG. 5 is jetted in the space between the upper electrode 18 (in FIGS.

7a and 7b the left electrode) and lower electrode (in FIGS. 7a and 7b the right electrode) which are electrically insulated from each other by the insulator 16 and the groove 20. When a voltage is applied to these electrodes by the DC power source 19 in such a polarity that the upper electrode 18 is positive and the lower electrode 17 is negative, positive ions in the plasma flame drift in the right direction, that is, to the side of the lower electrode 17. Thereupon, the positive ions collide with neutral molecules in the gases, then high temperature gases in the flame are pushed in the direction of the motion of the ions, that is, in the right direction and the flame is bent in a shape 21 shown in FIG. 7. The degree of the bend of the plasma flame changes with the variation of voltage applied by the DC power source 19. Since the relative position of the observation window and the plasma flame in the direction of the radius of the plasma flame is changed by the variation of the degree of the bend of the plasma flame, the lateral observation position can be changed by changing the voltage without moving the discharging device such as a coaxial waveguide.

In a spectroscopic analysis device constructed by combining such a device for bending the flame for excitation source and a spectrometer and detector, the desired lateral observation position can be set up by only changing the DC voltage without making the flame generating portion move mechanically. Then, the device for changing the lateral observation position becomes simple and the optimum lateral observation position can be found quickly, thus the measurement can be carried out rapidly. Furthermore, if the correspondence between the kind of element or kind of sample compound and the optimum lateral observation position, that is, the DC voltage is once determined by carrying out such measurements, the analysis of many elements can be carried out rapidly with the best detection limit and accuracy. Though, the high frequency torch discharge generator has been described above as an example, the same effects as those described above can also be obtained in the case of DC plasma jets or combustion flames according to the present invention. Further, if the correcting electrodes are divided on both sides in the direction of the flames axis and DC voltages are applied as shown in FIG. 6, a stable plasma flame can be obtained by suitably adjusting each DC electric field even when the flame is substantially not uniform on both sides in the direction of its axis.

As has been described here above, the analysis can be carried out rapidly with good accuracy and detection limit according to the present invention and its practical effect is large.

We claim:

1. A plasma flame generator comprising:

a coaxial waveguide having an outer conductor portion and an inner conductor portion each concentrically disposed along a first axis;

means for supplying microwave power between said inner and outer conductor portions of said coaxial waveguide;

means for supplying a plasma forming gas between said inner and outer conductor portions so as to cause a plasma flame to be generated at said inner conductor portion upon the interaction of said microwave power with said gas; and

means for bending said plasma flame, comprising at least one means, surrounding said plasma flame, for generating at least one electric field in a direction transverse to said axis whereby the ions of said plasma flame will interact with said at least one electric field to cause said plasma flame to be bent in a predetermined direction with respect to said axis.

2. A plasma generator according to claim 1, wherein said at least one means for generating at least one electric field comprises a pair of electrodes concentrically disposed about said axis and a DC power source connected to said electrodes.

3. A plasma generator according to claim 2, wherein said pair of electrodes are separated from each other by a groove disposed parallel to said axis.

4. A plasma generator according to claim 3, further including an insulator disposed between said electrodes and coaxial waveguide.

5. A plasma generator according to claim 4, wherein said DC power source is variable, so as to vary the intensity of said one electric field thereby controlling the degree of bend of said plasma flame with respect to said axis.

6. A plasma generator according to claim 1, wherein said at least one means for generating at least one electric field comprises a plurality of means for generating a plurality of electric fields including a plurality of electrode pairs disposed along said axis and insulated from each other and from said coaxial waveguide, said electrode pairs each being provided with a separate DC power source for separately controlling the magnitude of the electric field generated by each electrode pair whereby said plasma flame may-be adjusted, even when sai flame is not substantially uniform on both sides of said axis.

7. A plasma generator according to claim 6, wherein each electrode of each pair is separated from the other electrode of the pair by a groove disposed parallel to said axis.

8. A plasma generator according to claim 7, wherein each of said separate DC power sources is variable.

9. A plasma generator according to claim 4, wherein said groove extends along said axis up to said insulator.

10. A plasma generator according to claim 9, wherein said inner conductor portion extends along said axis whereby its tip, at which said plasma flame is generated, is disposed at approximately the same position along said axis as said insulator.