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JOURNAL OF RESEARCH of the National Bureau of Standards- A. Physics and Vol. 68A, No. 1, January- February 1964 X-Ray Spectrometric Analysis of Noble Dental Alloys B. W. Mulligan/ H. J. Caul, S. D. Rasberry, and B. F. Scribner (Octobcr 9, 1963) The analysis of noble metal dental a lloys for the va riou ~ co nstituent clements is a difficult and t edious task by chemical or fi re a say proced u res. X-ray spec tro 'copy o ffered the possibility of increased speed, especially if solid m etal samph's were employed. Thi ~ technique was investigated pa rticularly with respect to the analysis of dental alloys h aving the nominal composition in percent, of go ld 72, sil ver 12, J 0, 2, 2, and zi nc 2. Alloys were prepared by melting the compon('nt elpments in a high frequency furnace and casting the m etal into di sk form . Compositioll s of the cast in gs I," ere deter­ mined by chcmical analysis. Optimum procedurcs for castin g the sam pic and for X-ray analysis were establish ed , and analytical curves \\" (' re devcloped relating co ncentrations to m easured intensities of the X-ray lines Au Lp, Ag K a , Cu 1\"., Pt L., Pel 1\". , a nd Zn K •. The obser ved typical coe ffi cients of variatio n for the method \I'ere Au 0.34 p ercent, Ag 0.44 perc('nt, Cu 2.2 percent, Pt 1.6 perc('nt, Pel 1.2 percent, a nd Zn 0.72 perc(' nt. The r esults indicate that the method is suffici ently accurate a nd has ma rked ach"antages of speed and simpli city compa red to clll'Jllical a na l .vs i ~.

1. Introduction con and Popoff [1] described quantitn,tive X-rfLY fluorcsccncc analyses of' glass 1'01' ] 5 or more elements The analysis of noblc mctal alloys by traditional including platinum and . Othcr rererences of methods o[ solution chemistry [3] 2 is one of the more in terest include the dctcrminaLion of platinum in a complex and time-consuming tasks conrronting the petroleum rcforming catalyst [4], and Lhe detcl'lnina­ analytical . Three 1,0 fOUl' man-weeks are tion 01' sil ver in plwtogl'iLphic films [.5]. usually required for the analysis or thrcc samples in duplicate. The methods involve painsLaking sep­ 2. Experimental Procedure arations with numerous prccipitaLions and fi lLrations. Fire assay techniques also arc uscd but chi efly ror tlle determination of . If' p latinum and p alla­ In ordcr to invcstigfLte the di rect X-ray spcctro­ dium are present in the sfLmpl e bein g assayed, Lh e mctri c Im ;tl.,·sis or solid alloys, sa.mplcs werc syn­ alloy has a much higher nnd thcre are Lhesizccl initially b.,' meILin g togethcr in a high more losses due to volaLilihation [2]. In addiLion , freqlwn c ~ ' rUl'n ace solid mctnJ pieces of thtl desired further chemical separations arc ncccssary to rcmovc componcn L eleme n ts. Foul' samples wcre prcpa red the platinum and palladium bel' orc gravimetrically in this manncl'. Thc moltcn ;L11ol's of these elcments were cast in determining the gold. In the ass~,y Leelmique the the shape 0 1' disks 'approximn,tcly H-in . thi ck with a base m etals are either absorbed inLo the cupel 01' volatilized and h ence are not determined . d iamcter or 1% in . to fit Lhc s;unplc holders o( the In conjunction with the problem or analysis of' X-nL.'· s pcctrometcr to be cmplo.,·ecl. EllCIl di sk so gold alloys [or the Research of the American cast weighed ;lbOllt :35 g. The SUrf;LCCS of thc disks Dental Association, the development and evaluation were then precision JlHlchined 1'0), the X -l' fLY measurc­ ments. The compositions o( the casLin gs werc detcr­ 01' an instrumental method of analysis was considercd. X-ray spectrometry with its high precision offered mined by wct chemic;,l an;tlys is nccording to the the best possibility for a rapid method, especially il' method 01' G ilchrist [3], ;wet t he results of the analysis wct chemical processes could be eliminated in pre­ are shown ill Lllble 1. paring the samples. A typical composition 1'01' the nlIoys to be analyzed was, in percent, gold 72, silvcr T A RLE 1. Results oj chemical analysis 12, copper 10, platinum 2, palladium 2, and zinc 2. There are only a Jew references in the liLerature Alloy No. co ncerning the X-ray spectrometric determination E1ell1e nt o( the noble . MacN evin and Hakkila, [6] 4 made determinations of palladium, p latinum, rho­ ------1------Percent Percent Percent Perrent dium, and in a liquid sample that was applied Au ...... 68.27 72.01 78.08 7Q. 08 to heavy chromatographic paper, dried, and com­ Ag ...... 11. 63 10. 86 8. 13 13.88 C u ...... 11.88 9.95 7.86 6.01 pared with a previously calibrated color chart. Ba- P L ...... 6.34 3.76 2. 17 .00 Pel ...... 00 1. 97 2.84 1.04 Zn_ ...... 1. 87 1. 44 .94 .00

1 Present address Army Materiel Command, Harry Diamond L a boratol'i e~, Tota ls...... 99.99 99. 99 100. 02 100.02 W ashington, D .C .. 20438. , Figures in brackets indicate the literature references at the end of this paper. 5 The optimum conditions for X-ray fluorescence TABLE 2. Operating parameters 1 for noble metal alloy analysis 'intensity measurements were determined experi­ mentally and are given in table 2. Using these X-ray D egrees, Counts per p rtl·itmeters, measurements of intensities at the X-ray Element line 20 LiF Wavelength X-ray tube determi­ crystal nation wavelengths listed were mitde on the disks 1'01' all six constituent elements. The resultant analytical A kv ma curves were found generally to be linear itnd the pre­ Au _ ...... L ~ 31.19 l. 083 50 30 256,000 Ag_. __ __ ...... _ K. 15.95 0.559 Cu ______50 45 256, 000 cision of repeated determinations appeared favorable K. 44.96 1. 540 50 45 256.000 PL ______.. __ .. _____ La 38.05 1.313 50 45 128,000 for an itnalyticitl method. Before citrrying the de­ Pel ______K. 16. 70 .585 50 45 128, 000 velopment of the method further , it was considered Zn .. _____ .. __ .. _.. __ K. 41. 74 1. 435 50 45 128,000 ad visable to estitblish it prepamtion technique requiring it snm ller quantity of per , Norelco Inverted-Sample Three-Position Spec trogra with molybdenum target X:-ray tube, lithium Hu oridc crystal, and scintillation co unter detector sitmple. The technique generally applied to prepare ( Philips Electronic Instruments, ,\11 . Vernon, :-.r.Y.). castings in dentistry, known as investment casting or the "lost WitX" process [8 , 9], appeared promising. This technique was itpplied, using it small ce n trifugal casting ma.chine, to obtain disks of the gold alloys placed around the disk and cone and the cylinder is filled with an investment material (consisting of 50 % varying in thickness from } ~ to %4 in. The various steps of this process are illustrated in figures 1 and 2. crystobolite and 50% gypsum). The investment Disks are cut from wax sheets of the desired t hi ckness hardens in a few minutes and the filled cyEnder is and mounted vertically on edge on the t ip of a brass placed in a furnace and heated to approximatelY cone as s hown in fi gure 1. A metal c.'·linder is then 700° 0. During the heating the wax volatili zes completely leaving a mold cavity having the dimen­ sions of the wax disk. The hertted cylinder is removed from the furnace, cooled to about 500 °0, and mounted on the cen ­ trifugal casting machine shown in figure 2. In the machine the mold is shown adjacent to the snMll refractory melting cup at the center. The gold alloy is melted in the cup by means of a torch and is cast immeeli rttely by releas ing the spring loaded arm of the macJline. The molten metal is forced in to the mold cavity where it solidifies in disk form. The furn ace in the background served [or preparing the mold. 1"'1'''\,''\,''1'''1'''1'''1'''\\ After cooling, the mold is brolmn apart and the I) , .. e " l " 1 metal disk lmel sprue are extracted. The sprue is I. ', .1 \',.IL' ., cut from the disk by means of a jeweler's saw, and the metal disk sample is mounted in me thyl metha­ F I G URE 1. Preparation of sample-Cleft) lVax disk mounted on metal cone; (center) metal cylinder (inverted) containing crylate resin for h andling during surface preparation mold cavity of wax disk; (j'ight) noble metal allo y disk before and analysis. The exposed sUl'face of the resiD­ removal oj' sprue. mounted sample is prepared for analysis by wet grinding to It flat surflwe, finishing with a flOO-grit abrasive. X -ra? intensity measurements for the selected wavelengths were made for all of the disks, and no differences could be detected within the precision of the determinations it1110ng the disks of the same alloy having different thicknesses. Subsequently, elisks of 7~2 -in . thickness were selected as the best compromise between rigidit.'· of sample and minimum ll,mount of rtlloy per sample. The disk and sprue together weighed about 20 g with the sprue account­ ing for sligh tly over half of this weight. Samples of the four previousl.'T rtnalyzecl gold alloys were prepared in duplicate by this technique and X-ray fluorescence meaSUl'ements were made in t he spectrometer in a sequence designed to minimize effe cts of instrumental drift [7] . The overall time required for this procedure was approxima,tely 5 hI', which included llbout 3 hI' for sample preparation and about 2 hI' for X-ray measurements. The effect FIG lInE 2. Pre paration of sample-fltrn 'l ce and centrifugal of melting and casting on the apparent composition casting machine with mounted mold cylinder. of the alloys will be discussed later in the text. 6 3. Results and Discussion number of counts per measurement. In the case of gold, sil ver, and copper, where 256,000 counts The ftnfLlytical curves obtained from these runs were me fLsured per determination , this would be for gold, s il ver, copper, platinum, palladium, a.nd ltbout 0.20 percent. For platinul1l , pltlladiullJ , lmd zinc fLre shown in figures 3 through 8. Each pomt. zi nc, where 128,000 counts were meftSllrecl , it would on f1 curve is the mean of four individual determina­ be about 0.28 percent. This precision was ap­ t ions. The precision of the method, calculated as proached in the case or gold Hnd silver, but not in the coefficient of variation for an individual deter­ t he other CftSeS , poss i b ]~r because or Lhe greater mination in a group of four, is given in ti1ble 3. influence of bftCkground variation (whi ch could DoL In the case of platinum, sample number two re­ be measured simulLa,neo Ll sl." ) when Lil e lille-Lo­ sulted in a higher count compared to the linear bfwkground rfttio is 101V. relationship of the other three samples for reasons as yet unknown. A least squares treatment of the 38X I02 r-~------'------'------'------~ data indicated that the point was in error by 0.14 percent. The theoretical precision that should be obtained solely on the basis of the counting statistics is equal to the reciprocal of the squfLre root of the total

TARLE 3. Reproducibility of single determinations '" 32 "-u

Au cOlleen- cV 1 A I! concen- C I' Cli con ee n- C I' " i ra tion Lrntion tra Lion "

Pt cOlleen- C I' Pd COll COIl- e l ' Zn cOll ce n- C V Lration tra tion traLi on ------% % % 6. 34 1. 15 0. 00 2. 24 1. 87 O. i3 3. 76 1. (i3 1. 97 1. 32 1. 44 .81 2. 17 2. 64 2.84 1.18 0. 94 . (i2 2 0 L-~ ______~ ______~ ______~ ______~ 2 O. 00 1. 3G 1. 04 1.19 . 00 2. 15 8 10 12 14 16 1 C V = Obsc rvcd coeffi cient o f va riation for an individual dctcflllina tion in a CO NCENTRAT ION OF SILVER, % group of four FIG VHE 4. AnCLlyticCLI Ctirve fOl' silver . J( ,£c-C)" C V ~ lO O 64X 10 2 r-,------r------,------,-----, C -y 11-1 2 Coc lTicicnt of variation for 7,e ro con ce ntra tion is tile valli e (or the background rad iation.

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66 70 74 78 82 6 8 10 12 CO NCENTRATION OF GOLD , % CONCENTRATION OF COPPER.·.4 li'I GURE 3. Analytical curve for gold. F I GUHE 5. Analytical curve for co pper. 7 15XI02 ,------.------.------,---~----, IBXI02,------,------,------,

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o L-______~ ______~ ______~ ______~ o

o L-______~ ______~ ______~ o 2 4 6 CO NCENTRATI ON OF PLATINUM. % o I 2 CON CENTRAT ION OF ZIN C. % FIGU RE 6. Analytical curve for platinum. FIGURE 8. Analytical cur've for zinc. 12 XIO' ,------,------.------,------. channel instruments are available which would decrease the meaSlll'ement time by a substantial 10 factor. The resul ts indicate that the method is sufficiently precise and has distinct advantages of simplicity -s 8 of sample preparation and speed of measurement o " compared to chemical analysis. The melting and casting procedure employed in this work would appear to offer advantages as a general method for preparing samples and standards especially for X -ray spectrometry. The casting procedlll'e prob­ ably could be speeded up by use of a permanent split mold insteacl of the investment mold. The extension of the technique to the analysis of silver alloys is currently in progress. 2 The authors are indebted to Cbarles L. Gordon of CONC ENTRATION OF PALLADIUM. '%. the National Bureau of Standards for the chemical FIG URE 7. Analytical cW've for palladiu m. analysis of the noble metal alloys and to Dennis A possible drawback to this procedure is the loss Worthen and Laszlo cle Simon of the Walter Reed of more volatile constituents in the melting and Army Medical Center for their assistance in specimen casting operation. To investigate this possibility, preparation. samples were repeatedly melted and Cftst in an 4. References automatic machine which CflSt the metal when a [1] Bacon, J . F ., Popoff, V. , Pittsburgh Conference on selected temperature WftS reached. The samples Ana lytical Chemistry and Applied Spect roscopy, were melted, then heated to 1080 °C and cooled to Pittsburgh, Pa., March, 1955 (an abstract). 870°C six times before casting. The castings were [2] Bugbee, E. E., Textbook of Fire Assaying, 3d cd., 1940, p. 103 (John Wiley & Sons, New York, N . Y.) . then analyzed by the X-ray procedure described [3] G ilchrist, R. , J . Res. NBS 20, 745 (1938). and compftred with samples of the same material [41 Gunn, E. L., Anal. Chem. 28, 1433 (1956). melted once at 1080 °0 and cast immediately. [5] Lofstrom, J . G., Smith, T. D., Norelco R ept r. 3, 9 (1956). Within the precision of the method no differences [6] l\IacNevin, W. M ., Hakkila, E. A., Anal. Chem. 29, 1019 (1957). co uld be detected between the samples prepared by [7] National Bureau of Standards Circ. 600 (1959) , pp. 11 the two cflsting procedures, indicating negligible and 12. Superintendent of Documents, U.S. Govern­ loss on mel ting. m ent Printing Office, Washington, D.C., 20402. The spectrometer used (table 2) is a research­ [8] P eyton, F. A., et a I. , Restorative D ental M a terials, p. 306 (C. V. Mosby Co., St. Louis, Mo., 1960). type instrument in which individual determinations [9] S kinner, E. W. , Phillips, R. W . The Science of Dental must be made sequentially (since it has only one Materials, 5t h cd., p . 412 (W. B. Saunders Co., Phila., crystal and detector) and wavelength settings must P a. , 1960) . be changed manually. More automatic multi- (Paper 68Al- 25l) 8