160 APPLIED SPECTROSCOVY

example a thallium impurity in the lead sample reduced sensitivity at mass positions 203 and 205. Occasionally compound ions also reduce sensitivity. A peak at mass 76 in the arsenic spectrum is attributed to AsH + and led to a poorer limit for As 7°.

The upper limits for existence in nature of Mn 53, C d 1°4, S n 110, S m 142, S m 146, Pt as8, Pt 2°°, Pb 2°~, Pb 2°5, and Bi "0°8 has also been reduced below previously reported ex- perimental data. These isotopes have been listed by Stehn (15) as possibly stable and of low abundance.

Literature Cited

(1) F. A. White, T. L. Collins, and F. M. Rourke, PHYS. REv. 101, 1786 (1956)

(2) L. Kerwln, CAN. J. PHYs. 34, 1080 (1956) (3) A. O. Nier, PHYS. Rrv. 52, 933 (1937) (4) M. B. Sampson and W. Bleakney, IBID. 50, 732

(1936)

(5) J. J. Mitchell, H. S. Brown, and R. D. Fowler, IBID. 60, 359L (1941)

(6) L. Kerwin, CAN. J. PHYS. 34, 1497 (1956) (7) A. A. Cohen, PHYS. REV. 63, 219A (1943) (8) H. Kintenberger, J. Mattauch, and W. Seelmann-

Eggebert, Z. NATURFORSCH. 3A, 413 (1948) (9) P. Kusch, A. Husteulld, and J. T. Tare, PHYS. REV.

52, 840 (1937) (10) M. G. Inghram, D. C. Hess, Jr., and R. J. Hayden,

IBID. 73, 180L (1948) (11) M. G. Inghram, IBID. 70, 653 (1946) (12) A. J. Dempster, NATURE 136, 65 (1935) (13) A. O. Nier, J. AM. CHEM. Soc. 60, 1571 (1938) (14) A. O. Nier, PHYS. REV. 54, 275 (1938) (15) J. R. Stehn, Private Communication, January 1954 (16) H. E. Duckworth, R. F. Black, and R. F. Woodcock,

PHYS. REV. 75, 1616 (1949) (17) H. E. Duckworth, et al., IBID. 85, 929 (1952) Submitted March 18, 1963

Spectrographic Analysis of Tantalum and Tantalum Oxide* Roger D. Laib

U. S. Industrial Chemicals Company, Cincinnati, Ohio

Abstract Tanta lum metal is analyzed for 12 xmpurxty elements by using

a carr*er dlstdlatmn techmque wi th a dc a r c . The metal *s ox,d*zed m a muffle oven at 900°C. The oxide *s m*xed wi th a specml carrier made of silver metal, salver chloride, and barmm fluorxde. A set of standards is included on each plate, and the analytical curves are hnear for the concentratmn range used. The coeflic,ent of vana tmn vanes from 3 0 to 8 7 %

Introduction

Increased interest in tantalum metal in the past decade has necessitated the development of an accurate and re- liable method of analysis of impurities in the metal and its compounds. Little has been published to date on the spectrographic analysis of high purity tantalum. A paper by Cypress (1) adapts a method developed for the analy- sis of thormm to zirconium and tantalum. He found that a 3 % mixture of Ga2Oa with the metal oxide was suffi- cient to distill the impurities into the arc in 35 see. The standard used by him contained 3600 ppm of impurities. However, in duplicating this procedure, it was impossible to obtain the same results. Two standards were use~--- one containing 6000 ppm of impurities and one containing 900 ppm of impurities. A moving plate study showed that the impurities were not completely evolved even after 150 sec. Andrychuk and Massengale (2) report that 5% GeOe mixed with tantalum metal powder is a satis- factory carrier using a 15 amp discharge. But germanium oxide was not found to be a good carrier in these studies.

Reference is made in a paper by Fornwalt and Healy (3) that their complete burn method, which they adopted for the analysis of niobium, could also be used for the

*Presented at the 11th Annual Symposmm on Spectroscopy, Chxcago, Ilhnms, June, 1960.

"determination of impurities in similar metals such as tantalum". This method was investigated, and it was found that it can be used. However, by vaporizing the sample completely, the authors are not taking advantage of the high boiling point of the m~/trix, the suppression of which would enhance the intensity of the impurity elements such as is done in th,s present work. Another advantage over the reference method is the elimination of the internal standard addition and subsequent calculations without sacrificing precision.

Zakharev, Lipis, and Petrov (4), concentrate five vola- tde elements in TacOs by distilling them at 1400°C from the matrix onto a copper electrode. This distdlation can be done directly into the arc of a spectrograph without the extra apparatus and manipulation that these authors used.

Lomonosova (5) describes a method for the determina- tion of six elements by partially burning the sample pellet of Ta205 in a nine--amp dc arc. They report 6 to 10% precision for those elements determined. However, with- out a spectrographic flux, they encountered enhancement of Fe and Si by variations in the concentration of titani- um.

The carrier distillation method used in the analysis of zirconium was investigated. This assumes the volatiliza- tion and excitation of all the impurity elements within a reasonable length of time while burning as little of the base material as possible. Tantalum oxide has those proper- ties which make this procedure desirable since tantalum has a high boiling point (6) and a complex spectrum (7). Whereas zirconium uses a single carrier, the tantalum carrier had to be made up of a combination of several chemicals which together gave the desired results.

VOL. 17, No. 6, 1963

Experimental Carrier Selection

The proper carrier combination was difficult to find, since a single exposure is desired for the analysis of all the elements both volatile and refractory. After single compounds were used in various proportions with the tantalum oxide with no success, combinations of those compounds were employed. It was found that a mixture of AgC1, BaFe, and Ag powder would solve the problem. Selection of the right proportions was critical, since too much AgC1 inhibited the evolution of silicon and an ex- cess of BaF2 produced a slow evolution of all the other elements, particularly Mn and Cr. The final proportions were Ag (-300 mesh) 30%, BaF2 20%, and AgC1 50%. The silver and BaFe used were high purity reagent grade chemicals. The AgCI was prepared in our laboratory by precipitation from a silver nitrate solution with the addi- tion of ammonium chloride. The cake was carefully dried in the dark with methanol to obtain a soft, friable prod- uct. The ingredients of the carrier are weighed into a plastic vial and ground three rain on a Wig-L-Bug Vi- brator (Crescent Dental Mfg. Co., Chicago, Ill.). The vial is then wrapped with black tape and stored until needed.

T A B L E I . C O R R E C T E D S Y N T H E T I C T A N A T A L U M S T A N -

DARDS~ P P M

Element T a - l l Ta-12 Ta-13 Ta-14 Ta-15 Ta-16 Blank

AI 1020 520 270 145 82 51 20 Cr 1015 515 265 140 78 46 15 Cu 102 52 26 14 8 5 2 Fe 4025 2025 1025 525 275 150 25 Mg 383 196 102 55 31 20 8 Mn 378 190 97 50 26 15 3 Nt 760 385 198 104 57 33 10 Pb 770 395 208 114 67 43 20 St 950 550 350 250 200 175 150 Sn 805 405 205 105 55 30 5 T1 928 474 247 134 77 48 20 V 385 198 104 57 33 22 10

Preparation of Standards High purity tantalum oxide was mixed with a dilute

sulfuric acid solution of soluble compounds of the follow- ing elements: A1, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Si, Sn, Ti, and V. A set of seven standards was made, an oxide blank plus six others of varying concentrations. The slurrys of the standards were carefully dried in platinum dishes and ground in plastic vials until thoroughly mixed. Preliminary spectrographic examination (8) consisted of a moving plate study using thirty sec exposure intervals to determine the evolution pattern of the elements. Using the most concentrated standard, it was found that a ninety see exposure was adequate for all elements. Next, the set of standards was arced in duplicate, and the analytical curves were checked for linearity and extra- polated for the concentration of impurities in the base material (9, 10). The amount of the elements added to each standard was adjusted by the amount found in the blank. The corrected values of the composition of the tantalum standards are listed in Table I.

Sample Preparation

Since tantalum sponge is pyrophoric, the oxide is easily prepared by heating the sample in a platinum dish with a Meeker burner. After the metal has ignited, the dish is placed in a muffle furnace at 900°C for ½ to

161

TABLE II. A P P A R A T U S AND S P E C T R O G R A P H I C O P E R A T I N G

C O N D I T I O N S

Spectrograph 3 m Baird Gra t ing Spectrograph h a w n g a dis- persion of 5.55 A / m m m the first order

Exci ta t ion Un i t Baird Source Power U m t Cur ren t , dc amp 10 (shor t c~rcutt) Prearc, sec 0 Exposure, sec 90 Sector wheel 6 % transmission Fd te r None Slit, /.~ 25 Analy tmal gap, m m 4 Sample electrode UCP N o 1990 on pedestal electrode UCP

No. 1964 Counte r electrode UCP No 1992 Spectral range, A 2200-3500 Photographic emulsion Eas tman SA-1 Developer Kodak D-19, 68°F, 5 rain Plate cahbra t ion Two-s tep method, ~ron arc exposure Dens~tometer Batrd Dens~tometer - Compara to r

2 hr depending on the size of the sample. After cooling, the TaeO.~ is pulverized in a polystyrene vial in the Wig-L-Bug Mixer. If lumps remain that appear to be in- completely oxidized, these are returned to the muffle for additional heating. Very hard lumps of oxide are ground in an agate mortar; however, this contaminates the sam- ple with silicon.

For maximum efficiency in a carrier distillation pro- cedure, the sample and carrier must be intimately mixed in the correct proportions. To accomplish this, 210 mg of the sample and 90 mg of the carrier mix are weighed into a one in. polystyrene vial. This is ground for one rain on the Wig-L-Bug Vibrator. One hundred mg of the mixture is then transferred into deep-cup electrodes. The balance 0an is made of platinum to avoid attack by the silver chloride with subsequent contamination of the sample. Samples are arced in duplicate. Standards are mixed and weighed in the same manner as described for the samples. After the characteristics of the analytical curves are checked initially, one spectrum of each stan- dard is put on each plate. This eliminates the need for internal standards with subsequent calculation of intensity ratios.

The filled electrodes are packed with a rod machined to fit inside the electrode cup. To allow the gases to escape during excitation without disturbing the sample, a hole is made through the center of the packed cup with a large steel pin (11).

The operating conditions and apparatus are shown in Table II.

Results

Minimum Detectable Limit

Maximum sensitivity was not a factor in the analysis, since the level of impurities was usually well above the detection limit of the method. By extrapolating the ana- lytical curves to an optical density of three units above background (with a Baird Microphotometer), we ob- tained the minimum detectable amounts of each element. A list of lines used for each of the elements is presented in Table III.

The sensitivity may be increased, if necessary, by choosing more sensitive lines of the elements or by vary- ing the ratio of carrier to sample or, in some cases, by decreasing the amount of light filtered by the sector wheel so that background is present.

162

TABLE IIL MINIMUM DETECTABLE LIMITS AND PRECISION OF ANALYTICAL LINES

Concn. Coe#. Wavdength, Mm. Detectable Range, of Var.,

Element A L~m~t, ppm ppm

AI 3082 5 50-600 2575 50-1000 7.6 2660 150-1000

C r 3021 20 30-500 2843 50-1000 6.1 2780 250-1000

Cu 3247 1 5-50 3.0 Fe 3021 10 10-500

2832 500-5000 2744 50-4000 6.4 2598 1000-5000

Mg 2802 8 8-200 5.8 Mn 2576 5 15-400 7.6 Ni 3003 20 30-1000 7.6 Pb 2833 20 30-800 8.6 $1 2881 150-1000

2519 100-1500 2524 150-1000 4.0

Sn 2840 10 15-1000 6.1 T l 3086 20 20-500 5.9

3078 40-1000 2599 500-5000

V 3185 7 20-400 8.7 3102 50-1000

Densitometry and Calculations

From the density readings of the lines, new analytical curves are drawn each time from the complete set of standards. This is preferred to the usual manner of cor- recting previously drawn curves with one or two stan- dards placed on each plate. It is possible to use element lines that are very close to tantalum lines because of the dispersion of the Baird Spectrograph. An example of this is A1 3082.1 and Ta 3081.8 A lines.

The analytical curves are plotted on two cycle semi- log paper with the concentrations as ordinate and the density readings plotted directly on the abscissa, which has been modified to correspond to the calibration curve. The sample concentrations are read from the curves, and the duplicate values are averaged.

Precision Reproducibility of the method was checked with tan-

talum standard Ta-13. Sixteen spectra of this standard were put on one plate, and the coefficient of variation was calculated in the usual manner (12). Table Ill gives the results for the 12 elements.

101~-

100-

8 i l I I / I I I I i I I I I / I I I I I

1 o .~o . g o l oo J ~0o

O P T I C A L DENSITY U N I T S

FIG. 1. T Y P I C A L A N A L Y T I C A L C U R V E S

A P P L I E D SPECTROSCOPY

It was found that reproducibility was improved by using a pointed counter electrode rather than a flat, ~ in. graphite rod. In using the standards over a long period of time, the individual points of the analytical curves tend to stray unless the standards vials are shaken vigorously before each use.

Accuracy Because there are no tantalum oxide standards or stan-

dard analytical procedures available, there is no inde- pendent check on the reliability of the spectrographic procedure proposed in this paper. As has been done before in such cases, the impurity elements were added to the tantalum oxide base and then analyzed as a sample. Com- parison is made in Table IV of the recoveries of these elements along with the amount actually there. The re- sults show the method to be valid,

T A B L E I V . A C C U R A C Y D A T A OF A N A L Y S I S

Conch. Concn. Error, Element Added, ppm Found, ppm %

A1 87 82 6 Cr 82 90 10 Cu 22 25 13 Fe 92 100 9 Mg 209 210 I Mn 70 82 17 Ni 680 670 2 Pb 690 700 2 Sl 170 180 6 Sn 72 70 3 Ti 87 95 9 V 77 84 9

Conclusions This spectrographic procedure for the analysis of im-

purities in tantalum oxide is rapid and rehable. The use of a 100 mg sample charge and elimination of the addition of a small amount of internal standard reduces the chance for errors in preparing the sample for analysis. Substantial savings in time is possible by eliminating ratio calculations and background corrections. The high level of precision without the use of an internal standard makes this method desirable for quality control in production facilities. Other elements of interest could be added so that this method would be adapted with little change to other situations.

Literature Cited (1) R. Cypress, BULL. CENTRE PHYS. NUCL. UNIV.

LIBRE BRUXELLES 40, 19 (1953) (2) D. Andrychuk and J. Massengale, The Spectro-

graphic Analysis of Tantalum Metal Powder by the Carrier Distillation Method, Tenth Annual Symposium on Spectroscopy, Chicago, Ill., May 1959

(3) D. E. Fornwalt and M. J. Healy, APPLIED SPEC- TROSCOPY 13, 39 (1959)

(4) E. I. Zakharev, L. V. Lipis, and K. I. Petrov, ZI-I. A N A L I T . K H I M . 14, 1 3 5 ( 1 9 5 9 ) ; C H E M . ABSTR.

53, 9893c (1959) (5) L. S. Lomonosova, ZAVODSK. LAB. 2 1 , 1080 (1955) ;

CHEM. ABSTR. 50, 9220C (1956) (6) L. H. Ahrens, Spectrochemical Analysis, Addison-

Wesley Press, Cambridge, 1954 (7) G. R. Harrison, M. I. T. Wavelength Tables, John

Wiley & Sons, New York, 1939 (8) R. D. Laib, Carrwr Distillation Techniques, 39th

General Motors Spectrographic Committee Confer- ence, Publ. S-349, 1960

VoL. 17, No. 6, 1963 163

(9) C. E. Harvey, Spectrochemical Procedures, Applied Research Laboratory, Glendale, Calif., 1950, p. 218

(10) H. A. Heller and R. W. Lewis, ANAL. CHEM. 25, 1038 (1953)

(11) N. H. Nachtrieb, Principles and Practice of Spec-

trochemical Analysis, McGraw-Hill Book Co., Inc., 1950, p. 256, fig. 12.8

(12) P. R. Ryder, Modern Statistical Methods, John Wi- ley & Sons, New York, 1948, p. 18

Submitted January 29, 1962

Y

The Use of a Combination of Atom and Ion Lines as Internal Standards in General Spectrochemical Analysis*

P. Tymchuk, D. S. Russell, and S. S. Berman

Division of Applied Chemistry, National Research Council Ottawa, Canada

Abstract This study showed that using the atom, Ion, and a combination

of atom and mn lines of only two elements as internal standards, ~t was possible to obtain very good results on a wade variety of elements m different matrices The samples were mixed into a rub~dmm chloride-graphite medmm and burned to completmn m a dc arc using a Stallwood jet. Analysis of the results, carried out on twelve elements m six matrices, showed that in 55 out of 70 cases an accuracy of better than 15~ was attained (29 of these were within 5%). Both major and minor components were determined on the same spectrogram. Elements included Na, Al, Ba, Ca, Fe, Mg, Mn, Pb, Si, Sn, Zn, and Zr with mdmm and lutecmm used as the internal standards.

Introduction Many investigations have been devoted to the improve-

ment of general or semiquantitative spectrochemical analy- sis, particularly in the examination of samples whose composition is completely unknown. Many investigators have advanced methods for the general treatment, which might apply to any material regardless of its nature or composition (1-6). Some time ago the ASTM Committee E-2 examined a large number of such methods and com- pared the results of six laboratories on standard samples (8). Although many improvements have been made in general methods, the answer to this important problem is still incomplete.

Most methods involve the dilution of the sample with a suitable base, which in effect reduces it to a collection of minor components in a common matrix. In addition, a separate internal standard is usually introduced (5, 6) to compensate for some of the uncontrolled factors that in practice seriously influence the llne intensity. However, each internal standard is suitable for only a limited num- ber of elements, while for the remainder it may be of limited use. The introduction of several internal standards on the other hand would be imnractical and tedious. The purpose of this investigation therefore is to extend the effectiveness of each internal standard over a larger range of elements.

Since a wide range of materials are examined in this laboratory, it is important that the buffer and the internal standard do not contain any of the elements that might be expected in the samples being examined. The use of indium as an internal standard has been recommended by several authors (6) while O'Neil and Suhr (7) recom- mended lutecium in the analysis of lignite ash. Both are

~Paper no. NRC 7676

considered practical since they are used in small quantities and may be obtained relatively easily in a pure form. In addition they are rare enough that they are not likely to interfere in general analysis.

Experimental Procedure

A mixture of rubidium chloride and graphite was chosen as being a satisfactory buffer for our purpose. It generally produces a uniform burn with negligible me- chanical loss regardless of the sample materials. Rubidium also produces relatively few lines in the ultraviolet region, which might cause interference.

The buffer which is used consists of a mixture of 60% graphite (UCP-2 Ultra Carbon Corp., Bay City, Mich.), 40% RbC1, 0.06% Lu203 (both Spex Industries, Inc., Metuchen, N. J.) and 0.03% In2Oa (Johnson-Matthey, London, Engl.) . The lutecium and the indium oxides are dissolved in a small quanti ty of hydrochloric acid and added to an aqueous solution of rubidium chloride. This solution is evaporated, the residue dried at 140°C, and mixed very thoroughly with the graphite powder.

The material to be examined is prepared in the form of a dried powder, usually as the oxide. A sample of 2.00 mg is mixed with 10.0 mg of the buffer mixture and transferred to the electrode. The general analytical conditions are shown in Table I along with a description of the equipment.

TABLE I. APPARATUS AND OPERATING CONDITIONS

Electrodes UCP-1 Ultra-Carbon Corp. Sample 3/16 in diam. with cavity 3 5 mm dlam,

4 mm deep. Counter ~ m. diam, pointed (15 o Cone)

Analytical gap, mm 6.0 Stallwood jet Spex Industrms, Inc. No. 9014

Air flow--5 0 l/rain Excitation current, amp 12 dc (220 V) Spectrograph Jarrell-Ash 3.4 m Ebert, 15,000 hne/m.

grating Wavelength range, A 2350-4350 first order Slit width, ~ 18 Falter 4 step rotating, transmission 1.1, 3.3, 10, and

30% Exposure, min 2 0 (completion) Photographic emulsion Kodak S A No 1 Developer Kodak D-19 Plate calibration Two-step method