1-s2.0-0371195154800038-mainThe spectrochemical determination of impurities in copper and copper alloys by means of a spark-ignited arc-like discharge

7/29/2019 1-s2.0-0371195154800038-mainThe spectrochemical determination of impurities in copper and copper alloys by

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Spectrochimiea Actn, 1954, ol. ,pp. 198 to 210. Pergamon Press td., ondon

The spectrochemical determination of impurities incopper and copper alloys by means of a spark-ignited arc-like dischargeFREDERICK V. SCHATZ

Research Department, Revere Copper and Brass, Inc., Rome, New York(Receivecl13 November 1953)

Summary-A spectrochemical method of analysis of copper and copper alloys with a sensi-tivity capable of detecting lead, tin, iron, nickel, silicon, bismuth and aluminium at O.OOl%,tellurium, arsenic and phosphorus at 0.0047& and zinc at 0.01% is described. This methoddiffers from the conventional arc analysis in that it is based on the excitation in a selectedregion in the vicinity of the cathode when using a special triggered discharge of the Multi-source. If the circuit constants of the Multisource are adjusted to give a heavily, over-dampeddischarge, and if the sample is made negative, a narrow region of enhanced sensitivity existsfor certain elements in the vicinity of the cathode. This region has been demonstrated to beequivalent to the cathode layer in D.C. arc. Variations in the relative sensitivities of certainelements, as well as variations in the excitation temperature, have been investigated for variouspoints across the discharge gap.

1. IntroductionRecent trends in spectrochemical analysis, especially on the part of the largemetal producing industries, have been toward fast automatic methods of analysis,utilizing the direct-reading spectrometer. This trend has introduced problemswhich, though not serious in a spectrographic laboratory where non-routineanalyses predominate, have become increasingly important to the dire$ readinganalyst. Most of these problems centre around simplifying and standardizinganalysis procedures, and at the same time increasing precision and sensitivity.

Applied specifically to the copper and brass industry, the problem involvescombining into a simplified procedure a method of high precision, capable ofdetermining 40% zinc with an error of 1% of content or less, and a method ofextreme sensitivity, capable of determining impurity elements present in amountsof O.OOlo/o or lower. In a previous paper [l] the author described a procedurewhich handled the above problem with considerable success. Briefly, this pro-cedure utilized several point-to-plane discharges, with a flat cast-metal specimenas the plane and a graphite electrode as the point. Different discharge character-istics were obtained by adjusting the capacity, inducta.nce and resistance in thepower circuit of the Applied Research Laboratories Multisource [a]. By meansof these spark igriited discharges, 30% zinc in 70-30 brass was analyzed with anerror of &0.35% of content, and impurity elements were detected down to con-centrations of 0.01-0.02 in yellow brass and 0.02-0.05/0 in red brass and copper.This original work was repeated by others [3] and has provided the basis for asuccessful direct-reading method of analysis. Although these sensitivities wereadequate for the analytical control of a large number of alloys, an equally largenumber of high purity alloys and refined copper remained to be analyzed by othermethods.

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The spectrochemical determination of impurities in copper and copper alloysThese other methods, based principally on D.C. arc excitat,ion, use self-elec-

trodes, solutions, oxides, or metal globules. Results based on D.C. arc excitationin general are very good. The procedures, however, are often very unsatisfactoryor even impossible when viewed from the standpoint of fast analytical control.In addition to disrupting working schedules, they mult,iply the basic analysis costper sample by large and unpredictable factors. This situation is aggravated to theextreme when direct-reading spectrometers are used.

The method described in this paper utilizes a spark-ignited, arc-like dischargeof high sensitivity, and is based on an unusual experimental arrangement. Thecomponents of the arrangement, though not new, do yield unique results whencombined in the experiment to be described. Also once installed and adjusted, t,hemethod has the same simplicity and speed of operation as the conventional analysesusing solid samples and point-to-plane sparking techniques. Applied to copperalloys it has an inherent relative sensitivity for some elements equal to all but themost refined D.C. arc type of analysis. The sensitivity of detection of phosphorusin copper, for example, is better than any previous method investigated by theauthor. Elements investigated in detail were lead, tin, iron, nickel, silicon, bismuthand aluminium, all detected at 0.001%; tellerium, arsenic and phosphorus,detected at 0.004$1/,; and zinc, detected at 0.01%.

2. Experimental arrangementThe method in its ultimate form was a natural outgrowth from an investigationof the distribution of certain element line intensities across the discharge. Theelectrode system was a conventional point-to-plane type consisting of a machinedmetal surface and a pointed graphite counter electrode. The spectrograph was alarge Bausch and Lomb Littrow model using quartz optics. The excitationsource was an Applied Research Laboratories Multisource [2].

In order to examine line intensities emanating from the vapour cloud atvarious distances from the sample, the discharge was considered to be composedof a set of lamellae parallel to the sample plane, each lamella emitting radiationcharacteristics of the state of affairs within its small volume. The optical arrange-ment for isolating the lamellae along the length of the discharge employed twocrossed cylindrical condensing lens combined in a manner suggested by HANSEN[4]. One lens with its axis horizontal was placed at the spectrograph slit to focusthe image of the discharge vert,ically on the collimator opening with a magnifi-cation of 6.5. The second lens with its axis vertical was placed in front of thedischarge to focus the image of the discharge horizontally on the slit with amagnification of 4.0.

I f the axis of the spark discharge is made horizontal, t,he radiation received bythe system will be from the lamellar volume described above. The resolution ofthe system, i.e. the thickness of the lamella, is defined by the slit width. In thepresent instance a 40 p slit was used, defining a discharge cross-section 10 ,u inwidth. Consequently, horizontal adjustment of the discharge position will enablethe system to analyze 10 p sections across the discharge from the sample plane tothe counter electrode.

A horizontal arrangement of the conventional Petrey stand was devised to199


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FREDERICK V. SCHATZ

facilitate the handling of flat samples (see Fig. 1). To prevent the projectingsurface of the sample from partially blocking the horizontally divergent rays. thevertical plane of the sample was placed at an angle of 54 to a vertical plane alongthe optic axis. The stand and electrode holder were mounted on a piece of t.ransite,a hard, easily machinable, asbestos board. The whole assembly was then mount,edon the optical bench in a stand with a fine screw thread for accurate horizontaladjustment,. The vertical adjustment was relatively coarse.

To facilitate horizontal adjustment and to furnish convenient reference point)s.an auxiliary lens was placed on the optic axis at the rear of the system and anenlarged image of the counter electrode was projected on a screen. Exact place-ment of the discharge gap was accomplished by aligning the image of the counterelectrode after the position of its point with reference to the sample plane had beenaccurately set by means of a 2.5 mm gap spacer, placed in the position of theanalysis sample. The spark stand was split horizontally for this purpose. Initialadjustments of the optical system were made with a line-filament microphoto-meter lamp placed with t.he filament vertical at the location of the spark gap. Thehorizontal and vertical images were then centred at the slit and the collimatorrespectively. The position of the discharge gap with respect to the optic axiscan be maintained if fiducial marks and an appropriate scale are placed on theauxiliary screen once the init,ial alignment is accomplished.

3. Investigation of intensity variations of selected elementsacross the discharge gapThree types of discharges commonly used in copper alloy analysis were studiedfor line intensity variations across the discharge gap (see Table 1). With the sampleposit,ive none of the discharges showed any unusual variation in line intensities

Table 1. Types of dischargej Sample ~ I

polarity ; Precision Sensitizdy Typical settingI/

I1. Slightly overdamped + 2.0% Error Poor 10 ,uF., 200 /H.,* 10 Cl2. Heavily overdamped i - 10 O/cError Excellent 60 /IF., 400 ,uH.,* 20 R3. Slightly underdamped + 2.0% Error Good 10 /LF., 200 /H.,* 3 fl

* Settings include 25 ,uH distributed inductance.in the region adjacent to the sample electrode. However, with the sample negative,an enhancement of line intensities appeared in the region adjacent to the cathode.This cathode enhancement increased from a small value in the slightly under-clamped discharge to a very pronounced effect in the heavily overdamped dis-charge. As the heavily overdamped discharge had the most pronounced effect incombination with a 20 to 1 increase in overall sensitivity, it was selected for study.

In the heavily overdamped or Type 2 discharge line intensit,ies of certain ele-ment,s were found to be greatly enhanced in a narrow region close to the negative

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sample electrode. Visually, the image of the discharge at the slit also presented anunusual appearance in that in this same narrow region there was a pronouncedgreenish glow, due to copper emission. The image of Ohis region was easily discernedsince it. was spread out in the verGea direction. This glow was at its narrowest,when using a copper sample and t


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FREDERICK. SCHATZin copper. Bismuth was checked only in brasses where its cathode enhancement isvery pronounced. Elements, behaving like iron, in that cat)hode enhancementbecomes very slight when zinc content increases, are arsenic and antimony.Phosphorus and zinc have a pronounced cathode enhancement in copper. Alu-minium showed Wle or no cathode enllancement in either 70-30 brass or copper.In Fig. 3 the intensity ratios of a few copper atom and ion lines are plotted asatom-atom and atom-ion combinations versus distance across the gap to demon-strate the variation in cathode enhancement for different lines of the same element.

The above material suggests a similarity bet,ween Type 2 discharge and theD.C. copper arc, as described by MILBOURN [5] [S]. In his papers, MILBOURNdemonstrated qualitatively the validit,y of the following points:

6.0

040 0.5 I.0 I.5 2.0 2.5

/ /Cu2634.91

0.210 05 I.0 I.5 2.0 25 mmmm - DISTANCE AC ROSS DISCHARGE CAP-

Fig. 2. Variation in relative sensitivity Fig. 3. Variation in intensity ratios ofof lead and iron mross t,he discharge selected copper line pairs amoss dis-gap. charge gap.

1. In a D.C. copper arc the negative electrode is consumed while the anodeis untouched.2. Oxygen is necessary for the maintenance of the arc, the reaction with

copper occurring at the cathode.3. Impurity lines behave generally like copper lines in that their arc radiations

are strongest near the cathode and diminish in intensity across the arc.4. Ion-line radiations are emitted only near to the electrodes.5. When the size of the sample at the cathode is reduced to a small globule,

vapour concentration in the arc rises, the cathode layer disappears and the lineenhancement spreads across the arc gap.

6. The principles applying to copper are valid in general for it,s alloys.202


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-Fig. 1. View of spark stand and sample.


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Fig. 4. Horizontal D.C. arc, showing pin samples.


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The spectrochemical determination of impurities in copper and copper alloys

In order to make a direct comparison between the two kinds of discharge, ahorizontal D.C. arc was devised. Some experimental difficulties were encountered,due to the t,endency of a D.C. arc to either hang on a point or to wander erratically.However, &-in. diameter pin samples with hemispherical tips were found to beadequate for the purpose of comparison, in that the arc wandering could be keptreasonably within bounds in the optical system and still not hang at one point(see Fig. 4). The gap distance was adjusted to 2.5 mm and a 3.5 ampere arccurrent, equivalent to the average current, in the Type 2 discharge, was used. An85-15 brass alloy containing 0.016% lead was used for bot#h anode and cat,hode,

mm -Fig. 5. Comparison of the relative sensitivities of Pb 1833 across the discharge

in Type f and the D.C. arc.

and a series of spectra at varying distances from the cathode was made, usingboth types of excitation. The variation in relative sensitivity of the Pb 28338line versus distance across the gap is shown for both cases in Fig. 5, where theyare superimposed for the sake of comparison. As indicated in the figure, t,hecathode layer is present in both discharges and in the D.C. arc shows a relativesensitivity approximately ten times greater than the corresponding layer in theType 2 discharge. This suggests the use of cathode layer in the above describedD.C. arc excitation for t,he detection of impurities at extremely high sensitivities.The increase in relative sensitivity at the anode in each case is not due t#o anincrease in the effective intensity of the lead line, but to a decrease in backgroundwhich became quite low and erratic in the region of the anode. In the Type 2discharge, the standard deviation for the intensity ratio Pb 2833/Cu 3030 was

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3.X0,/ in the centre of the discharge, and ranged from approximately 107; atthe cathode to 157 and greater at the anode. In the D.C. arc, standard devia-tions were not computed but appeared to be somewhat larger. Proceedingfrom the cathode to the anode, the reduction in effective intensity of the lead lineis approximately 10 to 1 in the case of the D.C. arc and 5.5 to 1 in the case of theType 2 discharge. Ion lines, which in both cases are strongly enhanced at thecathode, extended across the Type 2 discharge. Also in both cases the cathodewas consumed and the anode was relatively untouched. The similarities in thespectra, in the electrode attack, and in the cathode enhancement of the lead line,indicate that Type 2 excitation is close to that of a D.C. arc, and that the cathodelayers are similar in both types of discharge.

4. Investigation of excitation temperature across the discharge gapThe variation in the arc-spark line pair in Fig. 3 raises the question of whether

the discharge is in thermal equilibrium at all points across the gap. I f t#emperatureis the only parameter, the relative intensity of lines 1 and 2 is given by

I , (4 - &),G,v, - KT-_~_ e12 = A,G,v,where I = measured intensity

A = transition probabilit*yG = statistical weightv = frequency

E = energy of init,ial transition levelK = Boltzmann constantT = absolute temperature

L_~KGSTROTH nd MCRAE [7] have determined the transition probabilities ofselected tin lines in the course of an investigation of temperatures existing in aD.C. and A.C. spark and the D.C. arc. Using values determined by them, we havefor three tin line pairs:

Sn 3262 = 3.06 e- 5;Sn 3034Sn 2850--__ = 0,95 e_ ??$?!?Sn 3034Sn 2840Sn 2706 = 1.90

A flat copper sample containing approximately 0.2% tin was excit,ed. using agraphite counter electrode, and relative intensities of the above tin line pairs,corrected for background, were determined at various locations across the dis-charge gap. The relative intensities of the same tin line pairs were also determinedin t,he horizontal D.C. arc using pin samples of the dimensions described before.

The t#emperature computed from the relat,ive intensibies of these tin line pairs204


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The spectrochernical determination of impurities in copper and copper alloysare summarized in Table 2. I t is apparent from the large divergences in tem-perature in the region next to t,he negative sample electrode for both the Type 2discharge and the D.C. arc: that, in this region other factors in addition to tem-perature are operating. Consequently, a Boltzmann dist,ribution of the electronpopulation among t,he energy levels is not possible in the region of the catjhodelayer. On the other hand, at the centre of the arc, at about 1 to 1.5 mm from theelectrode, temperatures computed from the two line pairs approach each other,and thermal equilibrium can be considered to exist.

Table 2. Temperatures across discharge gapDistance across gap i n mm

RI i TKI1.23 7,2000.98 5,8001.09 , 6,4001.13 ( 6,6001.12 6,500

IRI TK ~ RI 1 TK1.34 8,000 1.16 6,800I I1.39 ( 8,400 1.19 6,9001.46 8,900 1 1.21 7,100_____~_ IF-1.29 i,600 1.09 6,4001.27 7,500 1.13 6,600

RI ( TK RII TK RI ) TK !1.97 I 14,700 115,000+1.88 / 2.172.0 i 13,400 , 1.97 14,700, 15,000 1.88 13,400

Hn 326Sn3034 Type 2DC &PC

6,300 0.137,000 0.146,700 0.146,500 ~0.14 6,700 0.12 6,3006,700 0.14 6,700 0.11 6,0006,700 0.15 7,000 0.12 ~6,3006,000 0.12 )__-___6,300 ! 0.11 6,0006,300 0.11 ~6,000 i 0.10 1 5,800I

0.11 ~ 6,000 0.120.13 6,500 0.150.1 6,300 0.140.10 5,800 0.110.11 /, 6,000 0.11

sn 2850 Type 2Sn 3034 ----DC arc 6,000 0.116,000 0.12 0.11 6,OO0.11 6,000

12.56 12.33 ( I2.30 2.16 2.02 : 2.172.32 2.16 2.22 I 2.17 2.34 2.332.17 ( 2.25 2.16 2.12 ; I.15 2.482.12 1.95 , 2.06 ,162.08 1 1.95 2.03 2.16

BRACDO, CRAGGS, and WILLIAMS [R] observed divergences in temperaturealmost idenOica1 to those in the cathode region in the course of an investigationof t,he excitation temperatures in a low-voltage, high-power, triggered spark-arcsource. They used Ba I, Ba II? and Mg I line pairs. Though not statSed explicitly,the t,otal light flux from the discharge was probably examined. As part of theirexplanation of this temperature discrepancy, reference was made to the possi-bility that different zones of the discharge produced different populations indifferent energy levels. However. this point was not examined in detail.

Relative intensities of the tin line pair 2840/2706, though not temperaturesensitive, were also determined to check departures from the theoretical valueof I .9. This determination was not too successful for the spark-ignited arc-likedischarge since the photographic density of Xn 2840 was greater than 2.0, and

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FREDERICKV. SCHATZaccurate microphotometer readings were not possible. Using the values of thetin line pair 284012706 in the centre of the discharge as an emulsion contrastcontrol, slight corrections were applied to the relative intensities of the other linepairs. When this was done, temperatures were found to be single valued in theregion 1.5mm from the cathode to the anode in both cases, and to have a valueof 6900"-7100Kor the Type 2 discharge and 6300"-6500K or the D.C. arc.Since radiation from the cathode layer is several times more intense than thatfrom the centre of the discharge, its contribution to the total light flux would tendto be dominant in any optical condensing system that collects radiation from theentire discharge. Consequently, any plate calibration or excitation standardi-zation based on selected line pairs is of dubious value if the above factors areignored. Errors arising from this situation are aggravated if slight changes in

IO

A7ttrgtt--_j 1 i/iii]]

% LEAD -Fig. 6. Determination of lead in brass and copper.

t,he adjustment of a critical optical system tend to throw the radiation from thecathode layer in or outside the ent,rance pupil of the optical system of thespectrograph.

5. A sensitive analysis of copper and copper alloysThe enhanced relative sensitivity of many of the elements in the vicinity of thecathode or sample electrode, using the overdamped discharge, opened up thepossibil ity of exciting large metal samples with a spark-ignited, arc-like dischargeequivalent to the conventional D.C. arc in sensitivity. This possibil ity was en-couraged by the fact that many of the highly enhanced elements are the sameelements which need to be detected in low concentration in copper and copperalloys.However, if an analysis is to be practical for fast routine work, optical adjust-ments and the position of the sample should not be so critical that it is difficultto maintain reproducibility. In the investigation just described, line intensitieswith respect to background change with great rapidity in the cathode region.

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Consequently, grave doubts were entertained regarding the suitability of a spectro-chemical analysis based on enhancement at the cathode. However, in testsover a period of two years, during which the system was dismantled and re-assembled many times, no difficulty was experienced in reproducing the w-orkingcurves. Day-to-day checks were easily accomplished not only by means of theauxiliary lens system. but also by means of spectrographic line pairs whose relativeintensities change rapidly in the cathode region.

Figs. 6 to 10 show working curves that have been included to demonstratethe various analyses possible on copper alloys. In many cases the most sensitivelines hare not been used. Since most of the curves are non-linear and approach

3.004 0 Cl c,o:! 0.04 ,:: IRON -Fig. 7. Determinat.ion of iron in brass and copper

background relative int,ensity as a horizontal asymptote, background correctionsare made.

The auxiliary curves designated by primed letters, included in the variousfigures are obtained by correction of the background of the analysis line only.In all cases these curves are linear, and their slopes, wit,h few exceptions, areapproximately 45. Once these auxiliary curves are obtained they can be usedfor extrapolating values on the uncorre&ed curves by adding the backgroundrelat*ive intensity to values along their extended range. This device is quite usefulwhen a limit,ed number of standards are available and the uncorrected workingcurve is to be used for analysis purposes. However. for careful work near thelimits of relat,ive sensitivity, background corrections should a,lways be made.

Shifts of the working curves? due either to a change in concentration of the207


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FREDERICK V. SCHATZ

matrix element or to the presence of other elements, were observed and areindicated on many of the curves. Studies of these shifts were hampered by aninsufficient number of accurately analyzed standards, especially in the very lowranges of impurity concentration. Consequently, many of the curves are incom-plete. In some cases shifts were so small that an average curve was drawn forthe uncorrected curves. Such is the case in Fig. 8 where a single uncorrectedcurve is drawn for tin.

The following list is a brief resume of the observed causes for curve shifts,many of which are illustrated in the figures:

1. Changes in the matrix element concentration. In many cases this shiftoccurs in a straightforward manner, but it is often complicated by the effectslisted below.

o 67-33 BRASS, 0.5 % Pb0 70-30 BRASS90-10 BRASS

0.3 I0.01 0.02 0.04 0.1 0.2 0.4% TIN -

Fig. 8. Determination of tin in brass and copper.

2. Changes in the length of the exposure time, Curve shifts due to differentrates of evaporation with time of the element in question and the internal standardcopper may result from a change in exposure time.3. Changes in background intensity relative to the line intensity. Curveshifts of this type have been observed in copper where the presence or absence ofminor impurity elements has an effect on the background. This effect can beeliminated by correcting both impurity and matrix lines for background.

4. Changes in relative intensity of an impurity line due to the presence ofother elements. Changes of this nature, which are all out of proportion to theamount of their additions, have been observed when tin, silicon, and aluminiumare added to copper.

5. Changes in relative intensity due to the effect of a change in concentrationof the specific impurity on the intensity of the matrix element line. This rather

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peculiar shift was observed in the determination of small amounts of iron in copperwhere increasing amounts of iron so enhanced the copper lines that the curve forrelative intensity versus concentration was quite flat. [l]6. Changes in the volatility of the brass alloy with increasing amounts of zinc.

5zEz

0.1001 0.1 I.0%ZINC -

Fig. 9. Determination of zinc in copper and silicon-manganese bronze.

70 PHOSPHORUS -Fig. 10. Determination of phosphorus in phosphor bronze and copper.

Since copper is quite different from zinc in its susceptibility to attack by the dis-charge, relative intensities of different elements vary differently with increasingzinc content.

7. Changes due to metallurgical effects. This extremely annoying curve shift209

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FREDERICK V. SCHATZ

has its origin from causes which are often obscure and difficult to locate. Also itdiffers from the preceding shifts in that it is often random in its occurrence.

In general the method has displayed the greatest improvement in sensitivityover conventional spark methods in the analysis of red brass, gilding metal andcopper. In the conventional spark methods, using the light emitted over theentire discharge, background builds up with increasing copper content veryrapidly, and is so heavy for pure copper that the entire spectrum is quite black.However, in the method described, which uses a selected portion of a special typeof discharge from the Multisource, copper is handled with the same ease as theyellow brasses of high impurity content. As an example, this method has beenespecially noteworthy in the determination of phosphorus in copper. Beforeusing the present procedure, no excitation conditions could be found that gaveconsistently good results for a phosphorus content of 0.01 to 0.05% in copperdue to variations in the relatively strong background present. Any D.C. arcexcitation methods were also complicated by the fact that when O.OOl~/, Fe waspresent, Fe 2535.6 interferes with P 2535.7, the most sensitive phosphorus line.However, in the present method Fe 2535.6 is suppressed and does not appearuntil Fe reaches O-O15o/o, well above the Fe content of most refined copper.

References[l] SCHATZ, F. V.; J . Inst. Met. 1952 80 77. [2] HASLER, M. F., and KEI+~P, J. IT:.; J. Opt.Sot. Amer. 1944 34 21. [3] Applied Research Laboratories Newsletter 1951 4 2. [4] HANSEN,G.; Z. Phys. 1924 29 356. [5] MILBOURN, M.; J. Inst. Met. 1943 99 441. [6] MILBOURN, M.;Proc. Phys. Sot. Lond. 1947 59 273. [7] LANGSTROTH, G. O., and MCRAE, D. R.; Can. J. Res.1938 16A 17. [8] BRAUDO, G. J.; CRAGGS, J. D., and WILLIAMS, A. C.; Spectrochim. Acta1949 3 546.

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1-s2.0-0371195154800038-mainThe spectrochemical determination of impurities in copper and copper alloys by means of a spark-ignited arc-like discharge