Arc electrode of graphite with tantalum ball ti

Gordon, W. A.

English

NASA Technical Reports Server

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON, D.C. 20546 
REPLY TO 
ATTNOF GP March 30, 1971 
TO : USI/Scientific & Technical Information Division 
Attention: Miss Winnie M. Morgan 
FROM 2 GP/Office of Assistant General 
Counsel for Paterlt Matters 
SUBJECT: Announcement of NASA-Owned 
U.S. Patents in STAR 
In accordance with the procedures contained in the Code GP 
to code US1 memorandum on this subject, dated June 8 ,  1970, 
the attached NASA-owned U.S. patent is being forwarded for 
abstracting and announcement in NASA STAR. 
The following information is provided: 
U.S. Patent No. : 3,396,303 
Corporate Source : Lewis Research Center 
Supplementary 
Corporate Source . 
Gayle Parker 
Enclosure: 
Copy of Patent 
(NASA CR OR TMX OR AD NUMBER) 
NASA-HQ 
https://ntrs.nasa.gov/search.jsp?R=19710013511 2020-03-17T02:52:37+00:00Z
Aug. 6, 1968 W . A  GORDON 3,396,303 
ARC ELECTRODE OF GRAPHITE WITH BALL T I P  
Filed March 21, 1966 
3,396,303 
William A. Gor io, assignor to the 
United States of America as represented by the Ad- 
ministrator of the National Aeronautics and Space 
Administration 
Piled Mar. 21, 1966, Ser. NO. 53’7,617 
3 Clspims. (a, 313-32) 
An arc source for emission spectroscopic analyses in 
which the cathode electrode comprises a graphite post 
having a globular tantalum tip. 
The invention described herein was made by an em- 
ployee of the United States Government and may be 
manufactured and used by or for the Covernment for 
governmental purposes without the payment of any royal- 
ties thereon or therefor. 
This invention relates generally to apparatus for con- 
trolling an energy source for spectral analysis and more 
particularly to such apparatus far  oontrolling a D.C. arc 
source for an emission spectrogaph or spectrometer. 
In making trace analyss by emission spectroscopy, a 
major source of difficulty has been the arc source used to 
excite the sample material. Commonly, the sample is held 
in a graphite crucible to providk the anode of the arc 
source, and a suitable potential is applied between this 
anode and the cathode of the arc source to produce an 
arc discharge for vaporizing the sample and for exciting 
the atomic spectra of the sample to produce light from 
which the trace analysis may be made by known tech- 
niques. 
Ideally, the arc source in such equipment should have 
both high sensitivity and high precision. It has been recog- 
nized that a D.C. arc source is extremely sensitive, but 
prior to the present invention there has been no entirely 
satisfactory way to achieve the desired precise control 
over the output energy of the D.C. arc source. In practice 
this has meant that the analytical spectroscopist must 
have available to him a variety of different arc sources, 
including a D.C. arc source, a spark source, various coni- 
binations of the two, and high voltage and low voltage 
A.C. arc sources. In the use of such arc sources the 
analytical spectroscopist must, in any specific instance, 
sacrifice precision for sensitivity, or vice versa. 
The present invention is directed to an apparatus which 
avoids these difficulties and disadvantages by providing 
an improved arrangement for precisely controlling the 
output energy of an energy source for spectral analysis. 
As applied to emission spectroscopy, the present control 
regulates the output energy of a D.C. arc source, so that 
the user may take advantage of the high sensitivity in- 
herent in the D.C. arc source, without, however, sacri- 
ficing the desired precision. 
In accordance with the preferred embodiment of this 
invention, these objectives are achieved by controlling 
the vaporization rate of the sample, and therefore the 
output energy of the D.C. arc source, in accordance 
with a programmed signal, either a constant or a time- 
dependent command signal, thereby malrjng the output 
energy of the D.C. arc source independent of transient 
events which may take place in the arc source. The out- 
put energy of the D.C. arc source is compared with a 
programmed command signal and the difference between 
them produces an error signal which regulates the direct 
current supplied to the arc source, so as to regulate the 
latter’s output energy in accordance with the programmed 
command signal. 
Another aspect of the present invention is directed to 
stabilizing the arc of the D.C. arc source so that it is a 
steady, rather than a flickering, source of light. This is 
achieved by providing a nwel and improved cathode 
structure for the arc source having a refractory metal tip 
10 which enables a more stable arc discharge to be main- 
tained. 
While the present invention is particularly applicable 
to the analysis of trace constituents in a sample by emis- 
sion spectroscopy, it is also advantageous for use in deter- 
15 mining major constituents of the sample, such as alloy 
constituents, by emission spectroscopy. Also, it may be 
used in mass spectrometry and in atomic absmption spec- 
troscopy, if desired. 
It is a principal object of this invention to provide a 
20 novel and improved apparatus for controlling the output 
energy of an energy source for spectral analysis. 
Another object of this invention is to provide such 
an apparatus which controls the output energy of the 
energy source in accordance with a constant or time- 
25 programmed command signal, thereby eliminating the 
effects of transient events in the energy source. 
Another object of this invention is to provide such an 
apparatus which is particularly useful for control!ing the 
output energy of a D.C. arc source for a spectrograph or 
Another objcct of this invention is to provide such an 
apparatus having a novel and improved cathode con- 
struction for stabilizing the arc in a D.C. arc source for 
a spectrograph or spectrometer. 
Further objects and advantages of this invention will 
be apparent from the following detailed description of a 
presently-preferred embodiment thereof which is shown 
in the accompanying drawing. 
5 
30 spectrometer. 
35 
In the drawing: 
FIGURE 1 is a schematic circuit diagram of a pres- 
ently-preferred embodiment of the present apparatus; and 
FIGURE 2 is an enlarged fragmentary longitudinal 
section of the cathode in the arc source in the FIG. 1 
apparatus, in accordance with the preferred embodiment 
Referring first to FIG. 1, an energy source in the 
form of a D.C. arc source BO is associated with a direct 
reading spectrometer PI of known construction, the de- 
tails of which will be understood by those skilled in the 
The D.C. arc source 80 includes a cathode 12 and an 
anode constituted by a graphite holder 13 and the sample 
14 in the holder. The cathode and anode are in spaced 
confronting relationship with respect to one another. A 
D.C. source for producing the arc across these electrodes 
is provided by an A.C. power supply source 15 and a 
phase-sensitive rectifier 16 connected between the output 
of this A.C. power supply source and the electrodes. 
The spectrometer 11 is provided with a photomultiplier 
6o tube 17 of known construction which senses the light out- 
put of a selccted spectral line which is emitted by the arc 
produced by the source 10. That is, the light energy pro- 
duced by atomic emission of a selected element in the arc 
is the input signal to this photomultiplier tube 17. The 
65 output signa! from the photomultiplier tube 17 is an elec- 
trical signal on line 18 having a magnitude substantially 
proportional to the light energy producd by the arc. This 
output signal is applied to one input terminal of a diger- 
entia1 amplifier 19 of known design. 
40 
45 of ~ j s  invention. 
50 art and therefore need not be described in detail. 
of the latter’s output, which provides the current ?or the 
arc source PO and therefore controls the rate at which the 
sample 14 is vaporized. 
The arrangement is such that if the output signal from 
the photomultiplier tube 17 has a higher amplitude than 
that of the signal from the command signal geierator 20 
(indicating that the output energy of the arc source BO, 
which is determined by the rate at which the sample 14 is 
being vaporized into the arc, is higher than that called 
for by the command signal), then the error signal applied 
to the phase-sensitive rectifier 16 will reduce the latter’s 
output, thereby reduciug the direct current applied to the 
arc source PO so as to reduce the latter’s output energy 
to the value called for by the command signal. Converse- 
ly, if the output signal from the photomultiplier tube I7 
has a smaller amplitude than that of the signal from the 
command signal generator 20 (indicating that the output 
energy of the arc source is lower than what is called for 
by the command signal), then the error signal applied 
to the phase-sensitive rectifier 16 will increase the latter’s 
output, thereby increasing the direct current applied to the 
arc source BO so as to increase the latter’s output enerey 
to the value called for by the command signal. 
With this arrangement, therefore, the output energy of 
the arc source PO is regulated in accordance with the signal 
of the command signal generator 21). This gives extremely 
precise control over the output energy of the arc source 
while at the same time retaining the advantages of the high 
sensitivity which is inherent in the D.C. arc source, as 
mmpared with other types of arc sources. 
From the foregoing it will be evident that the present 
control arrangement achieves the desired objective of con- 
trolling the vaporization rate of the sample in a manner 
which enables the reproduction of that same programmed 
vaporization rate for subsequent samples. This is in con- 
trast to previous proposals to provide a constant input cur- 
rent to the arc, which failed to achieve the desired ob- 
jective because the vaporization of the sample can itself 
change the arc current. Similarly, prior proposals to hold 
the arc current constant regardless of the rate at which 
vapors enter into the arc fail to achieve the objective of 
providing a reproducible vaporization rate of the sample. 
While the preferred embodiment is aimed at controlling 
the vaporization rate of the analytical specimen, it may be 
desirous to control the excitation of elements subsequent 
to their entering the arc column. This may be accom- 
plished by imposing control on the effective excitation tem- 
perature of the arc column. A further embodiment, there- 
fore, of the control arrangement is where the reference 
radiation from the arc in the arc source 10 is comprised 
of two spectral lines, the ratio of which is a function of 
the effective excitation temperature of the arc. The elec- 
3,396,303 
3 4 
In accordance with the present invention a second in-  trical ratio of the two lines is the input signal on line 18 
put signal to this differential amplifier is generated by a to the differential amplifier 19, and this signal is com- 
command signal generator 288 which may be either a time- pared to the signal from the command signal generator 
programmed function generator or a constant signal gen- 20. The matching of the ratioed reference signal to the 
erator of known design, which produces either a time- 5 command signal may then be accomplished by appropriate 
dependent or constant command signal in accordancc with adjustments in arc current, gas pressure, gas composition 
a predetermined program. The time-dependent function or other parameters. 
generator may be a curve following programmer of any While the present control arrangement has been de- 
suitable design, or it may be a function potentiometer of scribed with reference to the regulation of a D.C. arc 
the general type shown at page 253 of the book. “Elec- 10 source for an emission spectrograph or spectrometer, it 
tronic Analog Computers” by Korn et al., published by may also be used to control the variable energy source in 
McGraw-Hill Book Company, New York, 19.52, for ex- atomic absorption spectroscopy. For example, if the sam- 
ample. ple being analyzed is in the form of a solution being 
In the differential amplifier 19 the output signal from aspirated into a flame which provides the excitation of the 
the photomultiplier tube 17 is compared with the output sample, the present control may be used to regulate the 
signal from the command signal generator 20. The dif- aspiration pressure or in some other way to  control the 
ference between the output signal from the photomultiplier rate of aspiration of the sample in accordance with a 
tube 97 and the signal from the command signal gen- programmed control signal. 
erator 20 produces an error signal on the output line 21 Another aspect Qf the present invention is directed to 
from the differential amplifier 19. The sign and the mag- 2o stabilizing the arc in the source PO. For trace analysis of 
nitude of this error signal depend upon the respective mag- a sample in a static inert atmosphere, such as argon, the 
nitudes of the command signal and the output signal from free running D.C. arc has been found to be unstable, 
the photomultiplier tube. This error signal is applied to compared to its performance in air. In argon the arc 
the Phase-sensitive rectifier I6 to control the magnitude produces a noticeable ‘anode-flame, composed of plasma 
9r streaming toward the anode. This anode-flame tends to 
1 Y  
revolve erratically about the axis of the electrode be- 
cause of convection and buoyancy effects arising from 
the thermal gradients between the arc column and the 
surrounding atmosphere. This instability of the arc col- 
30 umn leads to poor precision of measurement. 
Referring to FIG. 2, in accordance with the present 
invention this difficulty is overcome by the provision of a 
novel cathode in the arc source PO of FIG. 1. This 
cathode includes a graphite rod or post 22 having a 
g j  tapered end 23 in which an axial recess 24 is drilled. In  
the arc source PO (FIG. 1) this tapered end of the post 
is disposed toward the anode. The cathode ‘also has a re- 
fractory metal tip, preferably of tantalum, which presents 
a generally cylindrical globular bead 25 extending across 
40 this end of the graphite rod and an integral stem 26 which 
is received tightly in the recess 24 to secure the refrac- 
tory metal tip to the graphite post. In one practical em- 
bodiment the refractCory metal bead 25 is about 2 mm. 
in diameter. 
When used in combination with a conventional anode 
arrangement in the D.C. arc source, it has been found 
that the cathode spot, when the arc is maintained, is 
spread over substantially the entire surface of the bead 
25. Consequently, the current density and the magnetic 
50 fields are reduced and the plasma stre’aming, or anode- 
flame, is greatly diminished or completely eliminated, 
thus providing a highly stabilized arc column. 
When used in emission spectrographic trace analysis, 
this novel cathode improves the reproducibility of line 
55 and background intensities and substantially eliminates 
arc wandering or fluctuations as a source of error with- 
out reducing the sensitivity of measurement for either 
solid or liquid samples. The cathode, constructed as de- 
scribed, may be used for hundreds of trace analyses with- 
The arc is initiated preferably by using a spark igniter 
to ignite the graphite post 22. After about two seconds 
the refractory metal bead 25 will be sufficiently hot to 
thermally emit, at which time it takes over as the cathode. 
65 While a presently-preferred embodiment of this in- 
vention has been described in detail with reference to the 
accompanying drawings, it is to be understood that vari- 
ous modifications, omissions and refinements which de- 
part from the particular embodiment disclosed may be 
70 adopted without departing from the spirit and scope of 
the present invention. 
I ct aim : 
I .  In a D.C. arc source for a spectralmeasuring instru- 
ment having an anode and a cathode in spaced, confront- 
75 ing relationship to each other, the improvement which 
43 
60 out the need for special treatment between exposures. 
3,396,303 
comprises a cathode structure comprising a graphite post 
and a refractory metal tip secured to said post and extend- 
ing across the latter’s end which is disposed toward the 
anode, said post being of progressively reduced cross 
section towards said end, and said refractory metal tip 
presenting a globular bead extending across said end of 
the post. 
2. The cathode of claim 1 wherein an integral stem is 
formed on said globular bead, said post having a recess 
in said end, said stem being disposed in said recess where- 
by said globular bead is retained on said post. 
3. The cathmode structure of claim 1, wherein said re- 
fractory metal tip is of tantalum. 
5 
10 
728,210 
2,055,530 
2,303,514 
2,433,624 
2,850,680 
973,520 
DAVID J. 
References Cited 
UNITED STATES PATENTS 
511903 Dean _ _ _ _ _ _ _ _ _ _ _ _  313-252 X 
611937 Inman et al. _____- 313-252 X 
121 1942 Toepfer __-_______ 3 13-252 X 
1211947 Rabinowitz _ _ _ _ _ _ _ _ _  315-151 
911958 Woll _ _ _ _ _ _ _ _ _ _ _ _ _ _  315-199 
FOREIGN PATENTS 
1011964 Great Britain. 
GALVIN, Primary Examiner. 


Arc electrode of graphite with ball tip Patent

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