Plasma light source for spectroscopic investigation

Abstract

A plasma of annular form is produced by passing a gas stream along the axis of a coil carrying high frequency-alternating current and a sample is injected through a low temperature central region of the plasma annulus into the tail flame and the resulting spectrum of the plasma is examined.

We claim: .[.1. An apparatus for carrying out spectroscopic investigation of a sample, the combination comprising: a first tubular member; a coil surrounding a portion of said first tubular member and thereby defining a plasma-forming region within said member; a high frequency generator for connection to said coil, said coil and its connections being wholly external to said first tubular member; a second tubular member coaxially arranged within said first tubular member to form a first cylindrical channel within said first tubular member; an inlet in said first tubular member for injecting an insulating gas into said first cylindrical channel; a third tubular member coaxially arranged within said second tubular member to form a second cylindrical channel within said second tubular member, said second and third tubular members terminating substantially at said plasma-forming region; an inlet in said second tubular member for injecting a plasma gas into said second cylindrical channel; an inlet in said third tubular member for injecting a carrier gas for a sample, whereby the insulating gas, the plasma-forming gas and the carrier gas flow towards the region of the coil in three coaxial laminar streams and the plasma formed by the plasma-forming gas is of annular form and has a tail flame; and means for passing a sample through one third tubular member to inject the sample into said plasma-forming region, whereby said carrier gas carries the sample through a low temperature 
     
        central region of said annulus into said tail flame..]. 2. Apparatus in accordance with claim .[.1,.]. .Iadd.13, .Iaddend.in which a capillary tube extends through that end of the third tubular member which is remote from the coil, said capillary tube extending substantially to the end of said third tubular member adjacent the plasma-forming region, and in which the end of the third tubular member adjacent the plasma-forming region has 
     
     
        a bore of restrictive cross-section to form a jet. 3. Apparatus in accordance with claim .[.1,.]. .Iadd.13, .Iaddend.in which said inlets in said first and second tubular members are tangentially arranged, whereby a rotary motion is imparted to said laminar streams of insulating gas and 
     
     
        plasma-forming gas. 4. Apparatus in accordance with claim .[.1,.]. .Iadd.13, .Iaddend.in which the coil has at least two turns and the high frequency generator has an operating frequency lying between 5 and 3000 
     
     
        mc./s. 5. Apparatus in accordance with .[.1,.]. .Iadd.13, .Iaddend.in which the insulating gas is the same as the gas used for the plasma 
     
     
        formation. 6. Apparatus in accordance with claim .[.1,.]. .Iadd.13, .Iaddend.additionally comprising a photoelectric device mounted so as to be exposed to said plasma-forming region; means for imposing a cyclic modulation on the amplitude of said high frequency output of said high frequency generator whereby the output of said photoelectric device is 
     
     
        correspondingly modulated. 7. Apparatus in accordance with claim 6, in which the frequency of modulation of the high frequency waveform is 
     
     
        between 100 and 360 c./s. 8. Apparatus in accordance with claim 6, additionally comprising means for examining radiation from the gas plasma downstream of the point at which the sample is introduced, together with an interrupter for intermittently interrupting the passage of the said radiation from the gas plasma to the examining means; and means synchronizing the operation of said interrupter with the modulation of the 
     
     
        high frequency waveform. 9. Apparatus in accordance with claim 6, including a limiter circuit for limiting the amplitude of the modulation 
     
     
        peaks of the high frequency waveform applied to the coil. 10. A method for the spectroscopic investigation of a sample, comprising: passing a high frequency alternating current through a coil surrounding a tubular member to define a plasma-forming region in said tubular member; passing along the axis of said coil within said tubular member three coaxial laminar gas flows, an outer insulating gas flow, an intermediate gas flow of a plasma-forming gas and an inner gas flow for forming a passage through the plasma formed by the intermediate gas flow, whereby the plasma is in the form of an annulus coaxial with the coil and has a tail flame; maintaining the three coaxial laminar gas flows separate from one another until they reach the plasma-forming region; introducing into the plasma-forming region means for initiating the formation of a plasma therein when the high frequency alternating current is flowing in said coil; withdrawing the plasma initiating means from said plasma region once the plasma has been started; introducing a sample into said inner gas flow and thereby injecting said sample through a low temperature central region of said annulus into said tail flame; and examining the spectrum of the plasma downstream of the point at which said sample was introduced. .Iadd. 11. A method for the spectroscropic investigation of a sample, comprising: passing a high frequency alternating current through a coil surrounding a tubular member to define a plasma-forming region in said tubular member; forming an annular plasma surrounding a low temperature substantially central region and a tail flame by: passing along the axis of said coil within said tubular member three coaxial laminar gas flows which include an outer insulating gas flow, an intermediate gas flow of a plasma-forming gas and an inner gas flow for forming a passage through the plasma formed by the intermediate gas flow, whereby the plasma is in the form of an annulus coaxial with the coil and has a tail flame; maintaining the three coaxial laminar gas flows separate from one another until they reach the plasma-forming region; introducing into the plasma-forming region means for initiating the formation of a plasma in the plasma-forming region when the high frequency alternate current is flowing in said coil; withdrawing the plasma initiating means from said plasma region once the plasma has been started; introducing a sample into said inner gas flow and thereby injecting said sample through the low temperature central region surrounded by said annulus into said tail flame; and examining the spectrum of the plasma downstream of the point at which said sample was introduced. .Iaddend. .Iadd. 12. A method in accordance with claim 11, in which the step of forming the annular plasma includes tangentially introducing the plasma-forming gas and the insulating gas relative to the axis of said coil and tubular member, to impart a rotary motion to the laminar streams of plasma-forming gas and insulating gas. .Iaddend. .Iadd. 13. Apparatus for carrying out spectroscopic investigation of a sample, comprising: a first tubular member;   a coil which surrounds a portion of said first tubular member and defines a plasma-forming region within said first tubular member;   a high frequency generator coupled to said coil for supplying electrical energy to said coil at a predetermined frequency and power, said coil and its connections being wholly external to said first tubular member;   a second tubular member coaxially arranged within said first tubular member to form a first cylindrical channel within said first tubular member;   an inlet in said first tubular member for injecting an insulating gas at a given rate into said first cylindrical channel;   a third member coaxially arranged within said second tubular member to form a second cylindrical channel within said second tubular member, said second and third tubular members terminating substantially at said plasma-forming region;   means for producing an annular plasma having a low temperature substantially central region and a tail flame, including:   an inlet in a side wall portion of said second tubular member remote from said plasma-forming region for injecting a plasma gas at a given rate into said second cylindrical channel such that said injected plasma gas flows along said second cylindrical channel toward said plasma-forming region;   an inlet in a side wall portion of said third tubular member remote from said plasma-forming region for injecting a carrier gas for a sample;   said given rates of injection of said insulating gas and plasma gas, and said predetermined frequency and power of said high frequency generator being such that: (i) the insulating gas, the plasma gas and the carrier gas flow in their respective channels towards the region of the coil in three coaxial laminar streams, and (ii) the plasma formed by the plasma gas at the plasma-forming region is of annular form and has a tail flame; and   means for passing a sample through said third tubular member with said carrier gas to inject the sample into the central region of said annular plasma in said plasma-forming region, whereby said carrier gas carries the sample through said low temperature substantially central region surrounded by said annular plasma and then into said tail flame. .Iaddend.   
     
     
        .Iadd. 14.  Apparatus in accordance with claim 13 further comprising spectroscopic examination means positioned to determine the spectrum of said sample in the region of the tail flame. .Iaddend.