1396
ANALYTICAL CHEMISTRY Brown, R . A , . ANAL. CHEM.,23, 430 (1951). Friedel, R. d.,and Sharkey, A. G., Jr., "Mess Spectra of Acetals and Analysis of Oxygenated Compounds," presented before 9th -4nnual Mass Spectrometer Group Meeting, Pasadena, Calif., hlay 1951. Hartough, H. D., "Thiophene and Its Derivatives," Sen. York. Interscience Publishers, 1952. King, Wm. J., and Nord, F. F., J.Org. Chem., 13, 635 (1948). Kinney, I. IV., Smith, J. R., and Ball, J. S.."Thiophenes i n Shale-Oil Naphtha," presented before Division of Petroleum Chemistry, 121st meeting of AMERICASCHEMICAI. SOCIETY, Milwaukee, IT%, 1952. lIcKittrick, D. S.,I , r ~ dEng. . Chem., 21, 585 (1929). Xlohler, F. L., TTilliamson, L.. Tyke, C. E., Wells. E. J., Dean, H. 11..and Bloom. E. G., J . Research ,Yafl. Rzcr. Standards, 44, 291 (1950). O'Xeal, 11.J.. Jr.. and Wier. T. P., J r . , . \ x ~ I , . C H E M .23, , 8:10
The application of these techniques to the identification of two unknown thiophenes is presented. ACKNOWLEDGMENT
This project was part of the Synthetic Fuels Program of the Bureau of Mines and was performed a t the Petroleum and OilShale Experiment Station under the general direction of H. P. Rue and H. M. Thorne. Suggestions and revieLy of the work were given by J. S.Ball, G. U. Dinneen, J. R. Smith, and C. M'. Bailey. The work was done under a cooperative agreement between the University of Wyoming and t,he Bureau of Mines, Department of Interior. Several of the thiophenes were made available through American Petroleum Institute Project 48 on Synthesis, Properties, and Identification of Sulfur Compounds in Petroleum. The authors wish to thank H. D. Hart,ough and the Socony-Vacuum I'aboratories for the samples of 2,3-, 2,4-, and 3,4-dimethglthiophoncs.
(1951).
Rock, S.X I . . I b i d . , 23, 261 (1951). Schmidt, O., Z . Elektrochrrii., 39, 969 (1933). Shriner, R. L.. and Fuson, R. C.. tematic Identification of Organic Compounds," p . 163, S ork. John Wiley 8- Sons, 1940.
LITER.1TURE CITED
( 1 ) .Inierican Petroleum Institute Research 1'1,oject44, Catalog of hlass Spectral Data. (2) Barnes, R. B., Gore, R. C., Stafford, R. JT-., aiid IfXliania, T, Z., AN.4L. C;(HEM., 2 0 , 4 0 2 (1948). (3) Blicke, F. F., and Sheets. 11, G.. J . d r r i . C'hem. .Sot.. 7 1 , 4010 (1949). (4) Bloom, E. (;.. Alohler, 1;. L., Lengel, J. H., arid &%e. C. E., .J. Research .\*atl. Biir. Stnndnrds. 41,129 (1948).
Steinkopf, IT,, a n d Killin Swarcs, >I.. .I. ( , ' h e m PI Ibid., 17, 431 11949;. Thompson, H. V.,,.I. Chr V I . Soc., 1948,328. Whitmore. E'.C.. C'hcm. Eng. A\-ctcs.26, ti68 (1948). Young, c'. \\-.. DuVall, R . R..and Wright. S . .-Ix.~L. THEM,,23, 709 (1951). RECEIVED for r t v i P w I'ehluary 8. 1952.
Accepted .June 23, 1952.
Ultraviolet Spectrophotometry in Detect ion of Food Product Subst it ut es New Applications R . J. RIORKIS, K. D. MACPHEEI,
iiw
E. L. K A N D Q L L
Lrnit$ersityof .Vevada, Reno, .Vet..
Rapid methods for the detection of substitution and adulteration in certain fatty food products are desirable, particularly for use by government agencies charged with protecting the public against these spurious practices. Research work was undertaken to improve existing methods for the determination of the purity of theseproducts. Ultraviolet spectrophotometric measurements showed that conjugated tetraenoic systems present in butterfats and olive oils were significantly absent in margarine fats and cottonseed oils. The previously- discovered fact
that trienoic systenis are considerablymore prevalent in horse fats than in pork or beef fats was confirmed through employing alkali conjugation and making ultraviolet absorption measurements. Ultraviolet spectrophotometrywas found to provide a successful basis for screening out substitutes among these fatty food products. Because of the rapidity with which substitutions or appreciable adulterations in these products can be deteeted by this procedure, the method will furnish a useful tool for chemists working in this field.
C
OSSIDERABLE attention has recently been dii ected toward the ultraviolet absorption characteristics of various fats and oils. Ultraviolet spectrophotometry now permits the quantitative measurement of conjugated dienoic, trienoic, and tetraenoic systems in fat and oil samples. Methods have been devised for catalytically inducing conjugation of double bonds with alkali, so that certain nonconjugated unsaturated components in fats and oils may also be estimated. .4n experimental program was decided upon at this laboratory, based upon these new techniques, and directed toward their application in distinguishing various food products from others. Ultraviolet spectrograms for a number of samples showed that by virtue of distinct differences in conjugated tetraenoic content, substitution of margarine for butter and cottonseed oil for olive
oil can quickly and routinely be detected. dlkaIi conjugation techniques, applied to pork, beef, and horse fat reaffirmed the fact that horse fat contains a much larger quantity of linolenic acid (tetraenoic system) than do pork or beef fats. Alkali conjugation coupled with spectrophotometry provides a rapid and routine method for distinguishing adulteration of ground beef or pork with horse meat. For this investigation samples of retail products were made available through the courtesy of the Weights and Measures Division of the State of Sevada. These samples were not only representative of products from this state but included foreign products as well as products from other parts of the United States.
Present address, Los Angeles County .4ir Pollution Control District, Los Angeles, Calif.
Fats were extracted from the butters and margarines quantitatively with ether a t room temperature. This procedure entailed
1
PREPARATION OF SAMPLES
1397
V O L U M E 24, NO. 9, S E P T E M B E R 1 9 5 2 stirring of a product with several successive portions of ethyl ether and collection of the decanted extracts. The extract vas t,hen subjected t o slow evaporation in a drying oven a t 50" C. for several hours, with subsequent application of vacuum to complete the removal of ether. I n several instances butterf:itr and margarine fats were isolated according to method of the Association of Official Agricultural Chemists ( I ) , in order t3 provide comparat,ive spectrophotometric information. This method involves heating hutter in a beaker over a flame to drive off moisture. Lops of n-eight of the sample indicates moisture content. This is follon-ed by extraction of the residue with cther t o take up t,he fats. F a t content is determined by evaporation of t,he ether solvent on a hot plate and 1%-eighing. I n some cases it is advantageous to test a butter for moisture, salt, and fat content as well as for spectroscopic characteristics. Because this procedure is generally employed for the first three of these tests, it should be convenient to carry out the spectrophotometric esamination on the fatty residue. Care must be esercisetl not to decompose the fat visibly during heating required by the met hod. Olive and cottonseed oils were not subjected t o any special preparation prior to ultraviolet analyses, as they n-ere soluble in t,he spectral solvent. Pork, beef, and horse fats were obtained from ground samples of -$he meats by extraction with petroleum ether 3,s describctl 11)Crowell ( 7 ) . S o at,tempt,was made t o secure lean samples of tlir meats.. since in practice adulteration of pork or beef ~vouldbe iusperted mainl)~in hamburger or ground pork products, which of nrccssity contain a high proportion of fat. The ground meat n'as shaken with petroleum ether and the mixture allon-ed to settle overnight a t room temperature. The ether Tal: then decanted through :I filter and evaporated, the fat, remaining as a repitiur.
Table I. Spectrophotometric Determination of Conjugated Systems i n Butterfats Tetraenoic Systeind
Biittcrf:it Ssrnpla Origin 1 Serada
-_______.
.,
,I
3
5
i'
6 7 8 9 10 11
12 13a 14a 15a
16 17 ' l
18 19
20 a 0
0.0044
0.0013
Sevadu
Sevada Nevada Kevada Illinois Ke vad a Kew Zealand Kew York Kew York Illinois Illinois Utah S e i ada Utah Minnesota California Utah T e \ ada Cdifornia
0.0029
0.0035
0,0022 0.0012 0.0022 0.0029 0.0022 0.0027 0.0040 0.0029
0.0010
0.0036 0.0036 0.0031 0.0043 0 0038 0.0018 0.0030
Trienoic Sys t e n i i yo by \\-eight 0.018
Dienoic
Sys t e nls
0,010 0,028 0.02i
1 00
0.011
O:9G
0.017 0.019 0.030 0.018 0.015 0.016 0.017 0.026 0.016 0,028 0.019 0.026 0.025 0.013 0.019 0.019
1:io 1.16 0.64 0.87 0.36 1.21 1 03
1:i3 0.74 0.99 I , 26 1.00
Ar. Si,ectrophotonietric tests performed on fats extractmi acoording tu ( f )
tetraenes,;. 274, 268. and 262 nip (conjugated trienes), and 233 mp (Conjugated dicnw 1 .
Tables I and 11 dewribe the results of the analyws. An average of the results showed that the butterfats cwntnincd :ipproximatel>- 1 ncight yo conjugated dienoic sy.*tc:mr, 0.02 xeiglit . w . i L y r r c A L RESULTS yo conjugated tricnoic qstcmp, and 0.003 \wight 7oconjugated A Beckniitn hloclel D E quartz spectrophotometer was enitetraenoic systems. ,ilthougli these values i w r c sni:ill, analyses )loved. Procedure.; and methods of calculation described in the !'Report of the Spectroscopy Committee" ( I S ) were f o l l o ~ ~ e d , in duplicate of a number of samples showed that 11ic.~- Tvi'rc reproducible. In contrast to the results ol)t:iincti with huttcrfats except that nitrogen blanketing during isomerization \vas found were those for margarine fats, ivhich \\-ere slio~wito rontaiii 0.6 unnecessary for this work. Butter and Margarine Fats. Fats which had been extracted weight yoconjugated dienoic sl-stems and 0.015 ivviyht % ' conjufrom a number of representative butters and margarines were gated trienoic systems. Of special interest was t h i s 1icur1y ('omitccurately weighed into a volumetric flask and diluted with nplete absence of conjugated tetrarnoic systems in tliv ixnrgarine heptane. (Spectroscopic grade n-heptane was purchased from the Westvaco Chemical Division, Food Machinery and Chemical fats. Fats prepared according to the A.O..i.C. nirthod ( I ) gave Corp., Xcwark, Calif.) Complete ultraviolet absorption spectrocomparable spectrophotometric analyscta in a11 instanws:. .is tlirrc gram3 were then obtained for each sample in the region from 220 were no significant differences in conjugated contenr using the to 330 mp. Particular attention was directed toward thc estwo very different sample preparation techniques, it i y :ipparent tinction coefficients (E:.&,) a t 322, 316, and 810 mp (c*onjug:itctl that oxidative processes did not occur t o any apprecialjlc ortcnt. Typical spectrograms for butterfats and niargarinii f a t :: :ire included in Figure 1. There is no ahsorption peak at 81(j nip, representative of conjugated tctraenoic systems, in thcl c:wc of the margarine fat. Background absorption varied foi, tlivse FIGURE I eoniplex mixtures. I n some instances, absorption C U I R E PRESENTATIVE SPE C T R O G Q 4 M i noted which exhibited inflections at characteristic .;pecific maxima but for which side bands or troughP xwrr o l w i w i d . Severtheless, calculations of thc pol:iinsnturatrs dit1 not :q)pcar -..- C O T T O N S E E D O I L No.7 to be signifit>nntlynffwtcd. _____ O L I V E OIL N O . 11.
.
MPRCARINE
, BUTTERFAT
FAT
NO.11
N O . 17.
Table 11.
Spectrophotometric Determination of Conjugated Systems in Margarine Fats Tetraenoic 'Trirnoic Sy+tcnis ___SJ-stenls 5; b y \l-eiglit
Margarine Tat Sample Origin 1" California California 2 3 Kansas 4a California California 5 California 6 7a California 8 S e w Jet sey 9 S e w Jersey Illinois 10 California 11 California 12Q 13" Texas Texas 14 15n California Illinois 16a
0.0000
0.0000 0.0000 0.0000 0.0001 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000
0.0000
260
280
300
320
W A V E L E N O T H , MILLIMICRONS
340
0,022 0.014 0.007 0.007 0.030 0.031 0.005
0:k
0 019 0.017 0.018 0.003 0.005 0.019 0.008 0.006 0 015
0.48
0.015
0 ' 60
..
0:i0 0.73 0.44 0.32 1.50 0.40
0:;; 0.44 0 88 a Spectrophotometric tests performed on fats e x t r a c t d according t o ( 1 ) . AV.
240
0.0000 0.0000 0.0000 0,0000 00000
IXr no ic Systenis
1398
ANALYTICAL CHEMISTRY
The absence of conjugated tetraenoic systems in margarine fats is not surprising in view of the preponderance of cottonseed oil now used in its manufacture, and because alkali refining, silicate absorption, deodorization (IO), and hydrogenation all act in a direction which should reduce ultraviolet absorption bands. Although the authors, as well as others (4),have found that ultraviolet absorption characteristics of butterfats vary somewhat between samples, the spectrophotometric determination of tetraenoic systems provides a firm basis for distinguishing margarine from butter. The test should find considerable applicability, as it is extremely rapid and relatively uninfluenced by the personal factor. Olive and Cottonseed Oils. -4number of commercial brands of olive and cottonseed oils were spectroscopically examined. No preparation of the products was necessary for these tests, the samples being diluted with n-heptane as required for accurate measurement of optical densities. Results obtained may be seen in Tables I11 and IV. The olive oils contained an average of about 0.2 weight % conjugated dienoic systems, 0.01 weight yo conjugated trienoic systems, and 0.001 weight % conjugated tetraenoic systems. Other investigators have shown that such small values are correct only in magnitude (6).
Table 111. Spectrophotometric Determination of Conjugated Systems in Refined Olive Oils Olive Oil _______ Sample la
2
3 4 5
6 7 8 0 IO
11 12 13 14
15 16 17
~~
Tetraenoic S s s t e nis
Origin
California California Italy California Italy Italy California Italy California California California Italy Italy Italy Californja California California
0.0005 0.0011 0.0015 0.0008 0.0010 0.0010 0 On17 0.0008
0.0006 0.0019 0.0006 0.0020
0.0010
0.0010 0.0006
0.0007 0.0009
.\v. 0.0010
Trienoic Systems by mright 0.001 0.011 0.010 0.005 0.009 0.008 0.012 0 004 0.005 0.018 0.001 0,013 0 008 0.006 0.015
.. ..
0 008
Dienoic
Systems 0.20 0.14 0.17 0.13 0. I4 0.21 0.21 0.15 0.22 0.32 0.27 0.18
..
0’lY
.. ..
0.20
Pressed olive oil
Duplicate analyses convinced the authors of the reproducibility of the results. It is interesting t o compare these values with 1 weight % conjugated dienoic systems and 0.04 weight % conjugated trienoic systems for cottonseed oils. Conjugated tetraenoic systems were essentially absent from cottonseed oils. Figure 1 contains typical absorption curves for olive and cottonseed oils. Occasional minor deviations in curve contour from these representative cases did not provide exceptions to the trend of analytical results. Careful scanning was necessary in the case of most olive oils in order to detect an inflection a t 230 mp, representative of conjugated dienoic systems, and occasionally the inflection was obscure. Mitchell and Kraybill ( I O ) and more recently Brice and Swain (6) have shown that low intensity bands characteristic of conjugated tetraenoic systems, which are frequently encountered in natural fats and oils, are the result of oxidation of linolenic acid (a tetraenoic system). Alkali conjugation convinced the authors that olive oil contains about 1 weight % linolenic acid. I t has been pointed out by other authors (fl,12) and confirmed in this laboratory that cottonseed oil contains no linolenic acid. The authors’ observation that the absorption band at 316 mp presents a reliable criterion for detecting substitutions of cottonseed for the much more expensive olive oil is in line with theories regarding the formation of conjugated tetraenoic system. As was the case for butter-margarine work, variation in the content of conjugated
tetraenoic systems between olive oil samples precludes the identification of admixture with cottonseed oils. Exception, however, would be provided when a particular olive oil of known composition is mixed with cottonseed oil.
Table IV. Spectrophotometric Determination of Conjugated Systems in Refined Cottonseed Oils Cottonseed Oils Sample Origin 1 California 2 California 3 California 4 California 5 Louisiana 6 Louisiana 7 Illinois 8 Illinois 9 California 10 California 11 Louisiana
Tetraenoic Systems
Trienoic Sys te nis % by weight
0.0000 0.0000 0.0000
n 0000
0 0000
0 0000 0 0000 0.0000 0 0000 0.0000 0 0002 Av. 0,0000
0.012
0.030 0,093 0.041
Dienoic
Systems 0 43 I 87 0 59 0 89
o.oi5
1.17 0 40 0 97 0 86
0.024 0 042
1:i5 0.95
0.003 0.059 0.072 0.052
Pork, Beef, and Horse Fats. The hexabromide method for determination of linolenic acid has been widely used for the detection of adulteration of pork or beef with horse meat (7). As linolenic acid can be determined spectroscopically, it was decided to examine several pork, beef, and horse fats after alkali conjugation. Spectroscopic techniques should save valuable man-hours, since the fatty acid extraction and 24-hour bromination, required in the hexabromide method, would be obviated. I n their pioneering work Beadle et al. used alkali conjugation to determine adulteration of lard with hydrogenated vegetable oils (S), and Kass and coworkers originated the method for measuring nonconjugated systems after alkali treatment (8,9). Two different samples of horse fat gave 14.2 and 19.1 weight % linolenic acid, respectively, using alkali conjugation and ultraviolet analyses, while beef fat showed only 0.2 and pork fat 0.4 weight %linolenic acid. This is to be compared to an average of 1.04 weight % linolenic acid for horse fat, 0.04% for beef, and 0.14% linolenic acid for pork fat previously reported for the hexabromide method ( 7 ) . Many tests by the hexabromide procedure a t this laboratory have failed to shorn any outstanding deviations from these reported average results The statement that the hexabromide method inherently gives low results ( 2 ) was confirmed by analyses. In employing alkali conjugation for detecting the substitution of horse meat for pork or beef, it 15 ould be important t o recognize the increased order of magnitude of the results over those obtained by bromination. CONCLUSIONS
Margarine can be rapidly distinguished from butter by testing the fat for conjugated tetraenoic systems using an ultraviolet spectrophotometer. Butterfats contain about 0.001 to 0.004% conjugated tetraenoic systems whereas margarine fats contain essentially none. Cottonseed oil can be distinguished from olive oil by a spectrophotometric determination of conjugated tetraenoic systems. It was noted that olive oils may contain from 0.0005 to 0.002 weight % of these compounds, whereas cottonseed oil contains none. A method for measuring adulteration of pork or beef with horse meat has previously been described which involves a determination of the tetraenoic content of the fat by bromination, Spectrophotometric measurements are more rapid and bear out the great difference in tetraenoic content between horse fat and the fats of pork and beef. In addition, the magnitude of the results are greater than by bromination. LITERATURE CITED
(1) rlssoc. Offic. -4gr. Chemists, “Official and Tentative Methods of Analysis,” jthed., Method 2 2 , 112, 1940. ( 2 ) Bailey, A. E., “Industrial Oil and Fat Products,” New York,
Interscience Publishers, 1945.
V O L U M E 2 4 , N O . 9, S E P T E M B E R 1 9 5 2 (3) Beadle, B. W., Kraybill, H. R., and Stricker, L.A., Oil urd Soup, 2 2 , 5 0 (1945).
(4)Booth, R. G., Kon, S. K., Dann, K.J., and Moore, T., Bzochcin. J.. 29.133-7 (1935). . . ( 5 ) Brice, A., and Swain, 11. L., J . Am. Oil Chemists’
u.
SOC.,26, 272-7 (1949). (6) Brice, B. A., Swain, Margaret, Schaeffer, €3. B., andilult, W.C., Ibid., 22,219-24 (1945). 17) CroNelI, G. K., J . Assoc. Ofic.A g r . Chemists, 27, 448-50 (1944). \ R ) Kass, J. P., Miller, E. S., Hendrickson, RI., and Burr, G. O., Abstracts of Papers, 99th Meeting, Awamc.w CHEMICAL SoCIETY,
Cincinnati, Ohio, April 1940.
1399 (9) Mattirllo. .J. J., “Protective and Decorative Coatings,” Vol. IT, S e w Tc~i,k. ,Juhn TF-iley Br Sons, 1944. (10) Mitchcll. .I. l l . , Jr., and Kraybill, H. R., J . Am. C h m . S O C . , 64, 9 9 s - 9 4 11942). (11) ;\litc.hell. J. H., Jr., Kraybill, €1. R., and Zscheile, F. P., ISD. Esc;. (’HEM,, ASAL. ED.,15, 1-3 (1943). (12) O’Connor, R. T., Heinselman, D. C., and Dollear, F. C., Oil a n d SOUP,22,257-63 (1945). (13) Stillman, R. C., J . Am. Oil Chemists’ SOC.,26, 399-401 (1949).
RECEIVED for review October 3, 1949. Bccelited J u n e 21, 1952.
Quantitative Spectrochemical Analysis of Rare Earth Mixtures ,I. i.\OHHIS 4biD c. E:. 1’E:l’PE:K S t a b l e Isotope Research a n d I’rorlitrtiou Diri\iori. O a k Ridge \ationul I m b o r u t o r ? . 3 - 1 2 Irru. O d i Ridge, Tenri. \ porous cup spectrochemical procedure for analysis of rare earth mixtures has been developed to assist in the preparation of pure rare earth compounds for use in the electromagnetic enrichment of the isotopes of these elements. Precision of this method. *lO% at a 99.59’0 confidence level, is such that small differences can be detected, thus permitting better evaluation of the chemical separation processes. Analyses of 10 samples for 5 rare earths can be completed in approximately 1 man-day. This procedure permits the quantitative determination of individual rare earths in complex rare earth mixtures and is limited only to the elements available for preparation of standards. Use of the solution technique eliminates possible inhomogeneities in sampling. No difficulties with interelement effect, outside of direct line interference, have been noted. Data obtained give strong evidence of the importance of proper application of the spectroscopic principle of homologous lines in the selection of the internal standard element.
T
HE work of the Stable Isotope Research and Production Division has led to a study of new methods of preparation
of pure rare earth materials for use in the electromagnetic separators (calutrons). Thus far, both chemical and isotope separations have been made on samarium and neodymium, which are more readily prepared in the pure state than the other rare earths. Chemical separation methods tried have included the well-established fractional crystallization of double salts, selective precipitation with organic materials, and liquid-liquid extraction processes. The feasibility of any such purification method is evaluated principally by the concentration coefficient obtained per stage for the particular process under investigation. To establish this coefficient] a method of analysis is required that can detect differences between samples with high precision. The preparation of pure gadolinium compounds from rare earth concentrates initiated spectrochemical studies on the determination of yttrium, lanthanum, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, and ytterbium in mixtures of from 1 to 1OOc7,. Analytical values obtained by this method are limited only by the availability of pure rare earth compounds for standards ; hoxr ever, where the absolute value of concentration is not necessary, all detectable rare earths can be determined in terms of concentration ratios using one particular sample as the reference.
HISTORICAL
I n the past tht. majorit!- of rare earth analyses have been made by spectrophotometric methods. Scandium, yttrium, and Ianthanum, which frequently occur with the rare earths, and terbium and lutetium are not readily determined by this means. oeller and Brantley ( $ 3 )contains a comprehensive survey and considerable new experimental data on rare earth absorption spectra. Sumerous methods havr been reported on the emission spectrographic determination of the rare earths. Table I summarizes the salient points of niost of these techniques. O f the methods given, that of Fassel ( 6 ) is the only one applicable to the determination of major constituents in rare earth mixturefi. A method for the analysis of rare Parth fractions by molecular spectra is given by Piccardi (%j. EXPERI.MMENTAL METHOD
As the ran: earth oxides, except ceric oxide, are readily soluble in hydrochloric acid, the porous cup method (9) was considered to be easily adaptable for this analysis. Initially, a concentration of 4 mg. of rare earth oxide per ml. x-as used when the sample was exposed in the first order of a Jarrell--4sh 21-foot spectrograph; platinum was used as the internal standard. Recent work has been standardized a t a concentration of 10 mg. per mi., using the second order of the same spectrograph n i t h drontium as the internal st,andard element. The detection of 100 micrograms per nil. of impurity in a solution concentration of 10 mg. per ml. necessitated use of the spark source a t a high power level (approximately 5 to 7 radiofrequency amperes a t 12,000volts as compared to 3 radiofrequency amperes a t the sitme voltage for normal porous cup work in this laboratory). The dissipation of twice as much power resulted in undesirable boiling of the solution, an effect soon eliminated by a specia,l vater-cooled tantalum metal electrode holder (Figure 1). Tantalum metal was selected because it was the most acid-resistant material readily available. It-ith a water flow of 300 to 350 ml. per minute, t h c electrodes arc’ kept cool a n d the solutions s h o r no tendency to boil over. I’RE1’AR.ATION O F STANDARDS
Standards were made from the purest oxides commercially available (95 to 99.5%). The samarium and neodymium oxides were obtained locally (prepared by Boyd Keaver and K. A. .411en, Oak Ridge Sational Laboratory), \\-lwreas t h c gadolinium, europium, praseodymium, ytterbium, arid yttrium oxides yere obtained from Research Chenlicals, Inr., Burbank, Calif., and used as received n-ith corrections for any impurities found. I t is anticipated that subsequent ?tandai,cls \ d l be prepared from t,he purified fractions as they become available. The individual rare earth oxides \yere di;isolved in dilute hydrochloric acid to a mncentration of 20 mg. per i d . for dysprosium, gadolinium, Ianthmum, neodymium, prtneo$ymium, samarium. and yttrium: for the lees rommoa europium and
