THE ADVANTAGES OF CONTROLLED ATMOSPHERES FOR

ARC SPECTROSCOPY

By RALPH E. THIERS

Pratt Trace Analysis Laboraotry,

Department of Chemistry, University of Virginia

Effective spectrochemical analysis depends upon three components of the apparatus. Two of them, the detector (including such elements as plates and photocells), and the light disperser (including prisms, gratings, and now echelles) are relatively well understood and perfected. The third and most significant of the three, the light source, has proved a more difficult problem. One line of attack used fairly recently, however, has been on the atmosphere around the arc. Air, the usual medium, has one major failing, namely, the interference of the cyanogen bands accompanying the reaction between atmospheric nitrogen and the electrode carbon. Other gases used in place of air not only overcome this disadvantage, but provide sev- eral additional advantages which may prove to be as im- portant as they were unexpected. The reasons for these gains are still not understood, and painstaking trial-and- error selection of the best medium for a particular analysis is still necessary. Nevertheless the gains realized from operating a source in artificial atmospheres are frequently great enough to justify each analyst in trying all avail- able gases for his own peculiar circumstances, problems, and apparatus. In addition to the control of source en- vironment by varying the gas around the arc another approach to the problem is possible: that of varying the pattern of the gas flow around the arc.

Although a few papers have been published on special aspects of using gases other than air, no review of publications and work in this field has been available. It is therefore the purpose of the present paper to "present a summary of the progress to date, not only in the field of selecting gases other than air, but also in the very new and comparatively unknown area of controlled flow around the arc.

Specifically, at least five advantages may accrue to the spectroscopist from substituting carbon dioxide, oxy- gen, helium, argon, or some other gas for normal air: (1) Cyanogen bands are suppressed and in effect elimi-

nated, thus restoring a large portion of the spectrum which had previously been unusable. (2) The over-all background interference is reduced so that the usual back- ground corrections may be unnecessary. (3) Changes are

157

produced in the relative volatility of elements in the source, implying not only the possibility of varying burn- off times, but of allowing the wider selection of internal standard elements. (4) Increased sensitivity of methods may result often by a factor of ten or more. (5) Newer methods of analysis may ensue for elements such as nitro- gen, whose determination has hitherto been impossible spec- trochemically because of the interference of atmospheric nitrogen. The determination of nitrogen alone by spectro- chemical methods will of course be a considerable achieve- ment.

Still further gains may be possible through stabilization of the arc position by control of the pattern of gas flow around the arc. While this approach to the problem of re- ducing errors and lack of reproducibility is quite radical and few specific results can be shown so far, the method does point toward isolating the various factors of error and thus eventually increasing significantly the precision of arc determinations.

Effects of Gas Selection Several devices have been used to eliminate cyanogen

bands. The simplest one with the required effectiveness is perhaps that of Owen (1) as shown in Fig. 1. Important advantages of the Owen design are that the mechanical

FIc. 1 The Owen Chamber

routine of handling the electrodes and their holders need not be changed at all, and the chamber can be clamped in place in the arc stand or removed in two or three minutes. The electrodes are introduced through somewhat larger holes in a cylindrical fused silica envelope having a good quality window at one or both ends. The desired gas is led in at the window and escapes both around the electrodes and through the exit tube. The gas flow is sufficiently rapid so that material from the arc is not allowed to de- posit on the window during exposures. The model shown has a window at each end to extend its use even after one of the windows has been damaged or fogged. Other chambers of various design and somewhat greater com- plexity have been described by Vallee (2) (shown in Fig. 2), Emery (3), Temple (4), and others (5,6,7,17,18,19).

4215 ,CA4227

AIR

Mo3864 Mo 3798 3833

GAS OUTLI

GASTIGHT CONTAINER

)E

AIR

FIG. 3 Line Spectra With and Without Cyanogen Bands

Laboratory 2 M Applied Research Laboratories spectro- graph through a step-sector, under identical conditions ex- cept that the arc was first in air then in pure oxygen, using as sample some impure AI~O3). As Fig. 3 indicates,

LENS HOLC

EASE OF ARC STAN

)

TABLE I

INTENSITIES OF RESIDUAL C Y A N O G E N BAND H E A D AT 3883 .A.

Approixmate Relative Gas Band Head Intensity

Air 1000 Nitrogen 3000

Carbon Dioxide 35 Oxygen 20 Steam 15 Helium 10 Argon 10

FIG. 2 The Vallee Chamber

The simpler Owen type seems to accomplish the desired end, that of exhausting all of the air with a reasonably small gas flow (less than five liters per minute). Further complication seems to be unnecessary, if not undersirable.

The great practical advantage of removing cyanogen bands is shown by the two examples of Fig. 3. In the first, the observability of the strontium line in oxygen (above) as against its complete obliteration in air (below) is quite noticeable. In the second broader example, the lines of molybdenum and magnesium are all quite usable in oxy- gen but again are blocked out in normal atmosphere. (The spectra of Fig. 3 were photographed on the Pratt

in the presence of a sample, under proper cond.itions, the elimination of the cyanogen bands is for all practical pur- poses complete.

In the absence of a sample, even with high purity graphite electrodes in pure gases, residual bands can still be found. For this reason an attempt was made to deter- mine the intensity of the band head at 3883 .~ using the above 2 M spectrograph. Nitrogen, air, steam, carbon di- oxide, oxygen, helium, and argon were used as contnolled atmospheres, with high purity graphite electrodes pre- burned in the appropriate gas. Spectra from the center four mm. of a nine mm.-nine ampere arc were photo- graphed with the lower electrode positive. The approximate relative intensities were determined by densitometry of step-sectored spectra and are g ivenin Table I. The spectra themselves are shown in Fig. 4 together with a comparison

158

spectrum of the A1203 sample discussed above. Note that the characteristic residual band disappears entirely when such a sample is present on the electrode. One other in- teresting interpretation of the data of Table I is possible: Excluding nitrogen and air, the limited intensity variation

, . I'

T

H E 1'

A

~ : ~ 1 ~ ,

Fz6. 4 Cyanogen Band Head at 3883 A in Six Gases

(by a factor of only 4) in other gases implies a com- paratively constant source of nitrogen such as impurity in the graphite electrodes or adsorption on the electrodes or chamber wall. The author leans toward the electrodes as a probable source of nitrogen, and this inclination is sup- ported by the electrode manufacturers.

The second major advantage, that of decreasing the effect of background radiation, is shown in Fig. 5. As can be seen, the spectral lines in oxygen alone can be measured directly without need for subtracting the background, which is obviously not the case with the corresponding spectrum in air. Such a reduction in background intensity is of course important to anyone whose methods require background correction, for such corrections are at best tedious and tlme-consuming and at worst inaccurate.

The causes of background radiation are not completely clear. Assuming that no light from the electrodes reaches the grating or prism the background radiation must be a

159

0 2

BACKGROUND REDUCiD .,~,. "

~ AIR . . . . . . FzG. 5 Over-All background Decreased by

Arcing in Oxygen

conglomeration of lines and bands too weak for individual definition, plus radiation from condensed materials in the arc. Vallee (8) contends that the continuous background has the energy distribution of black body radiation prob- ably from transient solid particles in the arc. This raises the question of why air should be a worse offender than oxygen; for the more vigorous reaction occuring at the electrode with oxygen would lead one to expect more, not fewer solid particles. This question remains unanswered. What is certain, however, is that the reduction in back- ground obtained by the use of a gas other than air is often sufficient to eliminate the necessity for background cor- rections, a boon to any analyst whose work generally requires such corrections (9), (10).

Advantage three, control of volatility factors and burn-off time in the arc, is illustrated by the data in

TABLE II

V A R I A T I O N OF "BURN-OFF" TIME A N D GAP VOLTAGE IN SIX GASES

Gas "Burn-Off" Time Voltage Air 180 seconds 33 volts Argon 300 21 Helium 240 27 Carbon Dioxide 180 35 Oxygen 60 3 5 Steam 5 5 7 5

Table II. These show, by way of example, that for 10 mg. samples of calcium carbonate arced on one-quarter inch electrodes at 6.0 amperes, the time required to burn com- pletely can be varied from 55 to 300 seconds by the appropriate selection of ambient gas. More interesting still is the effect different gases have on the volatility patterns of individual elements (11). To present a more precise example of the effects to be expected, the cadmium line from an impurity in the calcium carbonate sample of Table II was photographed on moving film under identi- cal conditions in different gases and the intensity plotted against time. Fig. 6 shows the resulting curves, with the duration varying from 8 to over 80 seconds for the same sample. Not only is the ability to control in this way the burn-off time for an individual element helpful, but as can be readily seen the area under each curve represents the total amount of light emitted. Obviously the light total in argon, which maintained a fairly high intensity for

G 4 Yi--

Cd3261,

,~~!~ , SECS.o FIG. 6 Light Intensity vs. Time for Cd 3261 .~i

a long period of time, is far greater than in the other gases. This gain of intensity and duration is one of the factors

causing increased sensitivity, advantage four. Sensitivity is of course a function of the matrix, the line, the elec- trodes and the arc parameters as well as of the gas, and while it would be nice if the gas had a predictable effect, independent of the other conditions, it most certainly does not. Table III shows the effect of the numerous variables

TABLE IV

V A R I A T I O N IN Be SENSITIVITY W I T H AMBIENT GAS

Line Intensity Gas Adjacent Bkgd. Intensity Increase

Ratios Relative to Air Be 3131 Be 3321 Be 3131 Be 3321

Air 15 20 1.0 1.0 A 5000 500 330. 25.0 He 2000 200 130. 10.0 CO2 70 50 4.7 5.0 H 2 0 60 50 4.0 5.0 02 10 10 0.7 0.5

containing beryllium. The very great enhancement of in- tensity (some 300 times from air to argon) is most pro- nounced, though to a lesser extent this same behavior can be found with any elements for some combination of matrix and gas (12). This table also points out the in- consistency of the enhancement effect on different lines of the same element. Be 3131 X is an ion line, whereas Be 3321 A is an atom line. In this case the greater en- hancement of the ion line might be correlated with the metastable high energy states occuring in rare gases, but in general no such reason can be postulated. Fig. 7 gives

TABLE 1II

V A R I A T I O N IN Co 3261 • SENSITIVITIES W I T H DIFFERENT MATRICES A N D GASES

Line Intensity of Co 3261 A

Adjacent Background Intensity Matrix Gas A1203 CaCO3 Li2CO3 C

CO2 110 10 80 35 Air 100 40 40 5 02 100 40 30 45 A 50 40 5 30 H 2 0 40 15 20 10 He 30 40 - - 30

and demonstrates no correlation, unifying influence, or pattern. Ratios of line intensity to adjacent background (as a measure of sensitivity) for Co 3261 .& in four matrices show that for a matrix of A1203 carbon dioxide is the gas which provides the greatest sensitivity. If the matrix is calcium carbonate then carbon dioxide is the worst gas to use. Conversely if one m u s t run in carbon dioxide, A1203 is the best matrix; in helium, calcium carbonate is the best. These data seem to show that the analyst's only course is to try various gas-matrix combina- tions until the best one for his determination is found. Because the matrix is generally forced upon the analyst, his real recourse is to try various gases under his specified conditions to see what advantages can be gained. A spec- tacular example of such results is given in Table IV, which presents the line-to-background intensity ratio for A1203

FIG. 7

8 5 3131

:, 1.: A R G O N

AIR Be 3131 .A. Enhancement by Arcing in Argon

in visual form the data in Table IV. From argon above compared to air below the intensity difference is perfectly

clear. The fifth and last advantage derived from varying the

gas around the arc is the ability to determine nitrogen (and possibly other gases in analogous fashion). Obviously, once the interfering nitrogen has been removed from the gases which surround the electrodes, the intensity of the cyanogen bands should be a function of the nitrogen con- centration in the sample. Utilizing this relationship, the spectrochemical determination of nitrogen in organic com- pounds for instance, would require only about ten per cent of the time needed by microchemical techniques. Such a method should prove extremely useful once its precision has been adequately improved ( ± 3 % ) . For the present, results published by Smith and Frederickson (13) are only semi-quantitative in nature, but the promise of fur ther development is strong.

160

Effects of Controlled Flow Even further than the choice of environmental gases,

the possibilities of stabilizing the arc and thus of investi- gating and possibly reducing the lack of reproducibility are enhanced by various means of directing the flow of the particular gas around and past the arc. Several means of control have been tried. Gillis and Eeckhout (14) passed a rapid cylindrical stream of gas upward around the electrode during the arcing process, and contended that the precision of observations was enhanced by doing so. Other similar recommendations include those of Scribner (15) who advocated a stream of gas across the gap in the direction away from the slit during the excitation spark to reduce "alloy effects", and Stallwood (16) who used a conical stream of cooling gas converging just below the tip of the lower electrode as a means of controlling vola- tilization of the sample and of making the different com- pondents vaporize together. With this arrangement vertical air currents around the arc were produced similar to those of Gillis and Eeckhout; and reduction of matrix effect, straightening of working curves, and some stabilization of the arc position were obtained.

The writer tried a somewhat different method of con- trolling flow by introducing carbon dioxide gas tangen- tially into a cylindrical chamber, thus producing a rapid circular flow, with outflow around the electrodes, which were on the axis of the cylinder. By using four jets directed into the chamber horizontally and tangentially a very strong vortex was obtained; the arc was found to remain very precisely in the center of this vortex.

The resulting stabilization of the arc position is so spectacular that based on the usual assumption that the wandering of the arc is a major factor in lack of repro- ducibility, a higher precision might be expected for all arc methods. After considerable experimentation, however, the only unequivocal evidence of increased precision has been for C 2497 X versus the cyanogen band head at 3883 •. Here the precision achieved with plain graphite electrodes in a vortex ( ~ 3%) was twice that obtained in still air under otherwise identical conditions. On the other hand, subsequent work on a variety of methods has shown the vortex to have either inconstant or insignificant effect. The mere aesthetic desirability of positional stabilization, however, as well as the great hopes which have been so long held out for higher precision if arc wandering could only be prevented, would seem to warrant further experi- mentation on such a promising method.

One apparently limiting factor of the vortex flow is the varying ability of the different gases to blow back the arc when it wanders out of the vortex center. The varia- tion proved to be much greater than might be expected; for example, a cross wind of sufficiently high velocity to extinguish the arc in steam or carbon dioxide causes no perceptible bowing or movement in argon atmosphere. In Table V, therefore, we have listed gas rates required to extinguish an arc when a jet one millimeter in diameter is directed at the center of the arc from two centimeters

161

away. The extreme variation appeared to surprise even persons familiar with the field, who were unable to pro- vide any explanation for the phenomenon in terms of the properties of the gases involved.

TABLE V

W I N D VELOCITIES W H I C H EXTINGUISH ARC Gas Relative Velocity

Steam approx. 0.1 Carbon Dioxide 0.3 Air 1.8 Oxygen 1.8 Nitrogen 2.3 Helium greater than 5. Argon greater than 5.

These variations in stabilizing property would of course have to be taken into account when the atmosphere is specified in advance by the requirements of the analysis, but something more than a limitation may be involved here. From these varying effects on extinction and stabili- zation of the arc, study of the characteristics of gases may well lead to more fundamental notions about the nature of the arc and the whole problem of the light source than have been produced so far.

Conclusions Very helpful indeed will be any means of getting hold

of, or defining these complex electro-physico-chemical phenomena which are involved in the functioning and nature of the light source. One reason for the importance of such a summary as this is the presently poor and in- complete understanding of light-source conditions. It is the writer's hope that the various unrelated facts and ob- servations here presented: the causes and elimination of the cyanogen bands, the effects of various gases on background radiation on the volatility and burn-off time, and on sensi- tivity, as well as the stabilizing effect of various gases on arc movement, all and any of these data and their relation- ships may be the clues which will eventually lead to a more nearly complete understanding of the light source problem, and from that understanding to a higher degree of control for spectrochemical analysis.

While the empirical methods so far used for spectro- chemical analysis have been very successful, the only really satisfying attack on the problem is promised by funda- mental experimental and theoretical approach. When this has been done by relating such factors as the observations and experiments herein reviewed, the impact of emission spectroscopy on science and technology will be increased manyfold. I t is toward this day that the present paper looks forward.

Appendix The following two tables are not related directly to

the discussion. In the first one are listed the producers of unfamiliar lines and bands which may appear in arc spectra

from controlled atmospheres, and for which the analyst must look out. In the second are listed the appearance and other unfamiliar characteristics of the arc in various gases. In both cases pure graphite electrodes with no sample were used.

T A B L E V I

L I G H T - E M I T T I N G SPECIES OF T H E G R A P H I T E A R C I N VARIOUS GASES

Air carbon atoms, carbon molecules (C2), cyanogen

Ni t rogen

Oxygen Carbon Dioxide

Helium

Argon

Steam

cyanogen, carbon atoms, nitrogen mole-

cule ions (N2 + ) carbon atoms, oxygen atoms carbon atoms, carbon molecules (C2) carbon atoms, helium atoms, carbon ions

( C + ) carbon atoms, argon atoms, carbon ions

(c+) carbon atoms, carbon molecules (C2),

hydrogen atoms, hydroxyl molecules, oxygen atoms

GAS COLOR OF ARC

Air Whi te with purple sheath

Carbon Green Dioxide

Ni t rogen Purple

Oxygen

Steam

Argon

Helium

Very bright pale blue

Extremely bright blue

No color, arc almost in- visible

Several different types oc- cur with purple, pink, green, and invisible col- ors as the amperage is in- creased

T A B L E V I I

DESCRIPTION OF A R C IN VARIOUS GASES

C H A R A C T E R O F POSITIVE

C O L U M N A N O D E SPOT C A T H O D E SPOT

Clearly defined, uniform, Diameter 1 mm, wanders Diameter ½ mm, wan- about 1 mm in diameter rapidly ders slowly

Slightly diffuse, about 2 mm in diameter

Diameter ½ mm, wan- ders very slowly

Diameter ¼ mm, wan- ders very rapidly in small area and this small area wanders slowly

Very diffuse, about 6 mm Diameter 3 mm, wanders Diameter 2 mm, wanders in diameter slowly slowly

Very well defined, with a uniform core about 2 mm in diameter and a sheath 5 mm in diameter

Diameter 2 mm, wanders slowly

Diameter 1 mm, wanders slowly

Very condensed column, well defined, about ½ mm in diameter

Diameter ½ mm, wan- Diameter 3 mm, v e r y ders rapidly stable; electrode burns

away very rapidly at spot (Very high current-vol tage gradient)

Condensed, about 1 mm in diameter

P u r p l e - - v e r y diffuse, 4 mm diameter Pink---Cone shaped from ¼ " at anode to 0.1 mm at cathode Green - - very diffuse

Size varies directly with Diameter 1/10 mm wan- current from 0.1 mm di- ders in small area ameter to full electrode d i a m e t e r . E x t r e m e l y stable

(Very low current-vol tage gradient)

Spots are inconsistent; sometimes large, small, station- ary or wandering

162

Bibliography

1. L. E. Owen, J. Opt. Soc. Amer., 41, 139 (1951) 2. B. L. Vallee, C. B. Reimer and J. R. Loofbourow, J.

Opt. Soc. Amer., 40, 751-4 (1950) 3. R. St. J. Emery, Spectrochim. Acta, IV, 513 (1952) 4. G. W. J. Kingsbury and R. B. Temple, Spectrochim.

Acta, IV, 473 (1952) 5. L. S. Ornstein and H. Brinkman, Physica, I, 797

(1933-4) 6. R. Schmidt and F. Woutman, Congr. groupe, avance.

method, anal. spectrog, produits metal. 13th Congr., 13, 163-5

7. G. M. Wiggins, Analyst, 74, 101-4 (1949) 8. B. L. Vallee, Unpublished communication. 9. J. Y. Ellenburg and Louis E. Owen, Anal. Chem., 23,

1512-4 (1951) 10. Louis E. Owen, J. Opt. Soc. Amer., 41,709-11 (1951)

11. B. L. Vallee and R. W. Peattie, Anal. Chem., 24, 434-44 (1952)

12. Louis E. Owen, J. C. Delaney and C. H. Neff, Am. Ind. Hyg. Assoc. Quart., 12, 112-14 (1951)

13. L. D. Frederickson, Jr. and Lathrop Smith, Anal. Chem., 23,742-4 (1951)

14. J. Gillis and J. Eeckhout, Mededeel. Koninkl. Acad. Wetenschap. Belgie, 9, 5-9 (1949)

15. B. F. Scribner, National Bureau of Standards, unpub- lished work.

16. B. J. Stallwood, Paper given at the Pittsburgh Con- ference on Analytical Chemistry and Applied Spec- troscopy, March, 1951

17. G. W. Marks and M. Gardner, Rev. Sci. Inst., 18, 370 (1947).

18. L. T. Steadman, Phys. Rev., 63, 322 (1943) 19. T. Hugo, S. M. Naud~ and H. Verleger, Rev. Sci. Inst.,

22, 210 (1951).

DETERMINATION OF CERIUM IN FERROUS ALLOYS

By RALPH H. STEINBERG

Chemical Laboratory, South Works, U. S. Steel Corp.

Abstract The method is applicable to the determination of ceri-

um in cast iron, carbon and stainless steel. The cerium is co-precipitated with barium as fluorides in an hydrochloric- perchloric acid solution. The ignited precipitate is analyzed spectrographically using the D.C. arc. The lowest detect- able amount is of the order of .003% cerium for stainless and somewhat lower for iron and steel.

Synthetic standards for comparison are made by the addition of a standardized cerium solution to a cerium-free alloy similar to the unknown and then carried through the procedure simultaneously.

Introduction Cerium metal and its alloys and compounds are now

being used frequently in the production of ductile iron, cast steel and some austenitic stainless steels. The amounts used are generally of the order of .01 to .10%. Chemically, the separation and determination of small amounts of ceri- um in ferrous metals are very difficult. The spectral sensi- tivity of cerium in the presence of large amounts of iron and chromium is exceedingly poor. The following method, therefore, employs chemical concentration, followed by a spectrographic determination.

Chemical Separation Weigh a 2.00 g. sample and 0.100 g. of barium car-

bonate into a 400 ml pyrex beaker. Dissolve in 20 ml 1-1 hydrochloric acid and 27 ml 72% perchloric acid. Fume well. Cool and add 20 ml water. Bring to boil. Ignore any precipitate at this point.

Caution: Use rubber gloves when handling hydro- fluoric acid. Measure 30 ml of 48% hydrofluoric acid into a polyethylene beaker of approximately 200-400 ml ca- pacity. Now add the hot solution of sample. Let stand for 1 hour. Filter through ashless paper and wash 5 times with 5% hydrofluoric acid solution. (A discarded hydrofluoric acid polyethylene container with spout makes an excellent wash bottle). Transfer precipitate to platinum crucible and ignite at red heat in muffle furnace. The entire chemi- cal procedure should be performed in an exhaust hood.

Standard Samples Synthetic standards are prepared by using cerium-free

samples to which a standard cerium chloride solution is added. The cerium-free samples should be similar in alloy content to the unknown. A stock standard solution may be prepared by weighing out high purity cerium oxide, dis- solving in hot conc. sulphuric acid, diluting with water, precipitating with an excess of ammonium hydroxide, fil- tering, redissolving in 1-1 hydrochloric acid and diluting to volume. If high purity cerium metal is available, it can be weighed and dissolved directly in 1-1 hydrochloric acid. The metal reacts violently with acid. As a precaution, place the metal in water and add the acid a drop or two at a time. After solution, dilute to volume.

Equipment The spectroraph is a 1.5 meter instrument with a

24,000 lines per inch grating, providing a uniform dis- persion of 7~. per mm.

Spectrum Analysis No. 1 is used.

163