Zeeman Modulated Spectral Source

Abstract

Zeeman splitting of spectral lines may be used as a technique for background correction in analytical atomic spectroscopy. Conventional spectral sources suffer two main deficiencies when using this method in that the plasma in conventional lamps becomes unstable and eventually extinguishes when a magnetic field is applied, an unacceptably high magnetic field strength would be required to produce useful Zeeman splitting. In order to alleviate the above, lamps have been constructed in which emission of atomic resonance lines is achieved by sputtering or the volatilisation of sample atoms by the cathodic region of a dc discharge, followed by the excitation and emission of those atoms within the discharge and in which a magnetic field may be applied over the discharge region in parallel with the plasma causing electric field, resulting in magnetic stability.

Description

This invention relates to spectral sources and in particular to novel spectral lamps which may effectively be Zeeman modulated.

Zeeman splitting of spectral lines may be used as a technique for background correction in analytical atomic spectroscopy. The method assumes a comparable background absorption of perturbed and non-perturbed components of the original spectral line, whereas atomic absorption only occurs on the unperturbed component due to the narrow absoption profiles of atomic spectral lines. Thus the perturbed components carry information on the noise levels of an analytical atomic absorption signal which can be used to correct and reduce such noise levels, correspondingly improving analytical sensitivity.

The difficulty of applying this method at the present time lies in the difficulty of building Zeeman modulated spectral sources. Due to the interactions between normal plasmas and magnetic fields, conventional hollow cathode lamps cannot be used, since the application of the field simple extinguishes the plasma.

In addition, existing hollow cathode lamps would demand unacceptable large magnets to give sufficiently high field strengths to produce useful Zeeman splittings.

These problems are discussed in more detail in a paper by N. Ioli, P. Minguzzi and F. Strumia entitled "Operation of HIgh-Intensity Spectral Lamps in a Strong Magnetic Field" which appeared in the Journal of the Optical Society of America, Volume 60, Number 9 -- September 1970. Thus, Zeeman modulated sources are generally built at present using high frequency discharges. These require high power R.F. or microwave generators, and usually very large magnets to produce useable Zeeman splitting.

It is therefore an object of this invention to provide a novel spectral lamp.

It is a further object of this invention to provide a spectral lamp which may readily be Zeeman modulated.

It is yet another object of this invention to provide a spectral lamp which may be Zeeman modulated using permanent or low power electro-magnets.

It is further object of this invention to provide a novel spectral lamp which is d.c. discharged.

It is yet another object of this invention to provide a novel spectral lamp in which either of the two electrodes may be used as the cathode.

These and other objects are generally achieved in the novel spectral lamp by producing a plasma causing electric field having substantially straight lines of electric force in a predetermined discharged region between two electrodes. A magnetic field may then be applied to the discharge region in the lamp such that the axis of the magnetic field is in parallel to these lines, avoiding plasma-field interactions. This results in a plasma which is stable in the presence of the magnetic field.

The construction of the electrode assembly in the novel spectral lamps will vary depending on the spectral lines to be produced, though all assemblies are governed by the above basic principle. Embodiments will be described for cathode materials with melting points between 600.degree. and 1,200.degree.C, for materials with low melting points, i.e. between 200.degree. and 600.degree.C, for materials with high melting points, i.e., above 1,200.degree.C, for liquid materials and finally for alkali and alkaline earth materials.

In the drawings,

FIG. 1 is a partial cross-section of the novel spectral lamp with one embodiment of the electrode assembly;

FIG. 2 is a cross-section of the electrode assembly taken along line A--A in FIG. 1;

FIG. 3 is a view of one type of electrode used in the novel lamp;

FIG. 4 is a view of a second type of electrode;

FIG. 5 is a view of a third type of electrode;

FIG. 6 is a cross-section of an electrode assembly including a liquid material;

FIG. 7 is a cross-section of an electrode assembly including alkali or alkaline earth materials, and

FIG. 8 is a cross-section of the electrode assembly taken along line B--B in FIG. 7.

As shown in FIGS. 1 and 2, the spectral lamp 1 includes a conventional glass envelope 2 with a quartz window 3 sealed in the front end and a vacuum take-off and seal tube 4 located at the other end. The novelty of the present spectral lamp rests with the electrode assembly 5.

The electrode assembly includes two electrodes 6 which are mounted substantially in parallel to one another. The assembly is sealed in place at the back end of the envelope such that the tube may be evacuated through tube 4 and filled with a rare gas such as argon or neon at a pressure usually between 5 and 50 torr. The electrodes are thus substantially perpendicular to the envelope windows so that when an appropriate voltage is applied between the electrodes, an electric field having substantially straight lines of electric force is created, causing a discharge between the electrodes which emits a radiation beam with predetermined spectral lines through the quartz window 3. A d.c. source is preferred, however a R.F. or a microwave generator may also be used as potential sources.

The outer case of the electrodes assembly 5 may be made entirely of materials such as soft iron so as to transmit a magnetic field through the assembly as effectively as possible. However only walls 7 need be made of soft iron since the poles M of the magnet used in Zeeman splitting will be located adjacent these walls. For Zeeman splitting, either a permanent magnet or an electromagnet may be used.

Finally a reflective surface 8 may be mounted at the end of the electrode assembly 5, or as shown in FIG. 1, it may form the end wall 8 of the assembly. The surface will reflect radiation emitted in this direction towards the front window 3.

As in all spectral line sources, the spectral lines produced depends on the materials used in the construction of the cathode. As the different materials have different melting temperatures, the electrode assembly will vary to take this into account and have been divided into five categories.

Category 1 includes materials having a melting point between 600.degree.C and 1,200.degree.C such as silver, copper and magnesium. This embodiment includes electrodes 6 as shown in FIG. 3.

Electrode 6 may consist of a plane strip having a thickness t from 0.001 inches to 0.01 inches, though only the section in the lower portion 6' immediately adjacent the discharge region need be flat. This section may also be necked, as shown, to raise the cathode temperature. The upper portion 6" may be necessary in some instances for very high power operation and will act both as a cooling fin and as an electrode connector. However, normally 6" is not necessary and may consist of two terminal leads. Two similarly constructed electrodes 6 are mounted within the electrode assembly 5 as shown in FIGS. 1 and 2. An asbestos/glass combination may be used for thermal and electrical insulation.

The electrodes are mounted substantially in parallel using spacers 9 consisting of glass. However, for optimum operation, the electrodes themselves should be in physical contact with only a good thermal insulator such as asbestos to avoid overheating the glass insulators 9 (causing them to crack) or the epoxy seals between the soft iron outer case and the glass envelope (causing vacuum failure). Thus asbestos spacers 10 are located between the glass spacers 9 and the electrodes 6. In addition, asbestos strips 10' are located between the electrodes 6 and the outer soft iron wall 7 of the electrode assembly.

In order to permit a maximum view of the cathode surface in the forward direction, a slight ridge such as a fold 11 in the asbestos material 10 (FIG. 1) or a ridge in the electrode (not shown) may be inserted at the front of each electrode. This forces the electrode faces slightly out of parallel.

The glass and metal portions of the electrode assembly may be sealed using an epoxy resin, or a single casing construction may be used such as an all metal jacket.

Finally the faces of the poles M, used to provide a desired magnetic field, are made to correspond to the width w and height h (FIG. 3) of the necked portion of the electrode.

As seen in FIGS. 1 and 2, the lamp described is symmetric having identical electrode construction. The electrodes are therefore interchangeable, and, if made from different materials, will provide for dual element operation of selecting the appropriate lamp polarity.

Category 2 includes materials having a melting point between 200.degree.C and 600.degree.C such as lead, cadmium and zinc. The electrode assembly is similar to that described above except for the electrode structure which is shown in FIG. 4. The electrode 6 is made from a good heat conductor such as brass. The low melting point material 12 is deposited over an area of the lower portion 6' of the electrode. This area again corresponds to the area of the faces of the magnet poles M. The lower portion 6' may, in addition, be extented downward and connected to heat sinks on the exterior of the electrode assembly. This is particularly useful if the material concerned has a low wavelength resonance line, requiring high excitation energy and a correspondingly high energy cathode discharge.

The parallel electrodes may be spaced as described with regard to category 1, and dual element operating lamps may be constructed using the above electrodes because of the symmetry of the lamp.

Category 3 includes materials having a melting point above 1,200.degree.C such as iron, cobalt and nickel.

The cathode for these materials should have as high a temperature as possible during operation and therefore as shown in FIG. 5, the electrode 6 is not extended outside the electrode assembly. The electrode material 13 is electrically connected to a rigid wire 14, such as tungsten, having a diameter of from 0.001 inch and 0.01 inch. The wire 14 minimises heat loss and also provides electrical contact. The electrode material should be as thin as possible consistent with mechanical stability. However metals such as chromium or manganese, whose mechanical characteristics do not permit them to be readily formed into thin sheets or foil, can be deposited on a base having a high melting point. Finally the electrodes 6 are mounted in parallel with spacers as in FIG. 1. The asbestos insulator 10' may be cut away at the centre. Once again, because of the symmetry of the two electrode lamp, dual element operating lamps may be constructed.

Category 4 includes liquid elements such as mercury. The electrodes for such a material are shown in FIG. 6. The lamp includes a quartz jacket 15 having a pool of the liquid material 16. Two electrodes 17, 18 made of refractory wire such as tungsten are sealed within quartz capillaries 19. These capillaries provide for a discharge only in the desired region. These electrodes are then sealed within the quartz jacket 15 in parallel to each other and with one electrode in electrical contact with the liquid material 16. The quartz jacket may have partial sleeves of soft iron, aligned such that the poles of a magnet may be applied to the lamp, maintaining the magnetic and electric fields in parallel. The lamp may also include a reflective surface on the inside or outside of the quartz jacket 15 to direct the radiation.

As an alternative, thin metal sheet in parallel may be used as electrodes in place of the wires, to improve lamp intensity and stability. The cathode 17 which is dipped in the liquid material 16, vaporises the liquid into the discharge region between the electrodes providing the predetermined radiation.

Category 5 includes the alkali and alkaline earth elements. Electrodes for some of these elements may be made in the same fashion as in category 2, FIG. 4, where suitable support materials exist, however generally an electrode assembly as shown in FIGS. 7 and 8 will be used. The electrode assembly 5 includes an outer casing which may have at least two walls 7 made of soft iron so as to transmit a magnetic field as effectively as possible. The cathode is formed by packing the cathode material 22 into the end of a bored metal rod 21, such as copper, which provides mechanical support. The bore having a diameter of approximately 1/60 inch. The rod 21 is therefore in intimate contact with the element 22 and acts as a thermal heat sink as well as an electrical contact terminal. A pyrex or ceramic tube 20 is fitted over the entire length of the rod 21 such as to prevent discharge from any part of the rod except the end where the cathode material 22 is exposed. The cathode is mounted within the assembly 5. The anode consists of a refractory wire 24, such as tungsten covered by a second pyrex or ceramic tube 23 which also prevents undesired discharge. The end of the wire is bent so as to have a face with a width equal to the diameter of element 22, as shown on FIG. 8. The anode is mounted within the assembly 5, such that a line discharge of width equal to the diameter of element 22 is effected between the electrodes. The electrodes are also aligned such that the line discharge is perpendicular to walls 7.

In all the lamps described, emission of atomic reasonance lines is achieved by sputtering or volatisation of sample atoms by the cathodic region of a discharge, followed by excitation and emission of those atoms within the discharge. Magnetic stability is achieved in all cases by arranging the axis of the applied magnet field to be in parallel with electric field causing the plasma, over the discharge region. This eliminates net plasma-field interactions.

The lamps are normally `run in` for a period of up to about 5 hours. This consists of initial high current operation under argon or neon for 2 or 3 minutes, followed by evacuation and re-filling. The cycle is repeating at gradually reduced lamp currents and increasing running times until a stable output is obtained. This `conditions` the cathode, and the lamp is then sealed. For the lamps which are symmetric, i.e., two identical electrodes, either electrode may act as cathode for dual operation and therefore each electrode should be conditioned separately while it is acting as cathode. Finally, with very volatile elements some anodic sputtering may occur, leading to the emission of a mixture of lines from both the anode and the cathode.

Claims

I claim:

1. A spectral source comprising:

first and second spaced electrodes mounted within said spectral source, said first and second electrodes adapted to be connected across a potential source to produce a plasma causing electric field having substantially parallel straight lines of electric force in a predetermined discharge region between said electrodes and at least one of said electrodes including a material adapted to emit radiation having predetermined spectral lines; and

means adapted to apply a magnetic field to said discharge region with the axis of the magnetic field substantially parallel to said lines of electric force, for producing Zeeman splitting of said spectral lines.

2. A spectral source as claimed in claim 1 in which at least a portion of each of said electrodes includes a plane surface; said electrodes positioned to provide the discharge region between substantially parallel plane surfaces.

3. A spectral source as claimed in claim 2 in which each of said electrodes consists entirely of said predetermined radiation emitting material.

4. A spectral source as claimed in claim 2 in which said plane surface of each electrode is coated with said predetermined radiation emitting material.

5. A spectral source as claimed in claim 2 in which the first electrode includes a first predetermined radiation emitting material and the second electrode includes a second predetermined radiation emitting material.

6. A spectral source as claimed in claim 2 in which the plane surface of the electrodes are positioned slightly out of parallel to provide a maximum view of the surfaces in one direction.

7. A spectral source as claimed in claim 1 wherein:

said predetermined radiation emitting material is liquid,

said first electrode includes a rigid wire with one end in electrical contact with said material;

said second electrode includes a second rigid wire mounted in parallel to the first wire, to provide a discharge region between said electrodes.

8. A spectral source as claimed in claim 1 wherein:

said predetermined radiation emitting material is an alkali or an alkaline earth element;

said first electrode includes a first non-conducting cylinder; and an electrically conducting rod, axially bored along a portion of its length, positioned within said first cylinder, with said material packed within said bore;

said second electrode includes a second non-conducting cylinder; and a rigid electrically conducting wire positioned within said second cylinder;

said electrodes mounted within the spectral source to provide a line discharge region between the end of said first electrode and the end of said second electrode.

9. A spectral source as claimed in claim 1 wherein:

said predetermined radiation emitting material is liquid;

said first electrode includes a thin rigid metal sheet with one end in electrical contact with said material and

said second electrode includes a second rigid metal sheet mounted in parallel to said first sheet, to provide a discharge region between said electrodes.