Fluorometric Determination of Gold in Rocks with Rhodamine B John Marinenko and Irving M a y United States Geological Survey Washington, D.C. 20242
GEOCHEMICAL STUDIES of the gold content of rocks require accurate and reasonably rapid determinations of gold at the parts-per-billion level. Neutron activation methods are sufficiently sensitive but require expensive instrumentation. Spectrographic procedures require preconcentration to attain the requisite sensitivity and to ensure representative sampling. Spectrophotometric methods are fast but are too insensitive for determining submicrogram amounts of gold. Beamish ( I ) recommended the bromaurate, stannous, and (sic.) rhodanine reagents. Chow and Beamish ( 2 ) recently described their modifications of the bromaurate and the methyl violet methods. The red-violet salt of gold(II1) with rhodamine B was used by McNulty and Woollard (3) for the spectrophotometric determination of gold. They extracted the rhodamine B chloraurate into isopropyl ether. Onishi ( 4 ) later used benzene for extracting this salt. Atomic absorption is useful for gold concentrations down to about 50 ppb using 15-gram samples of rocks. I t is best used in conjunction with a fire assay or with an extractive separation of gold, or with both (5). The fluorescence of rhodamine B chloraurate has not been studied in detail as a quantitative method for gold. Goto (6) qualitatively estimated gold by measuring the disappearance of the fluorescence of rhodamine B in aqueous solution after adding trivalent gold. Onishi (7) reported a benzeneextractable orange-yellow fluorescent interference of gold with the fluorometric determination of gallium as rhodamine B chlorgallate. We have investigated the gold-rhodamine B fluorescence for determining submicrogram amounts of gold. EXPERIMENTAL Reagents, RHODAMINE B (0.05 %). Mechanically shake 0.13 gram of rhodamine B in 200 ml of water, dilute t o 250 ml, and filter. Prepare sulfurous acid fresh by saturating water with sulfur dioxide. TELLURIUM (0.2%). Dissolve 1 gram of 99.99% tellurium powder in 4 ml of H N O p 4 ml of HCI, evaporate to dryness with several 4-ml portions of HCl and dissolve the residue in 50 ml of HCI and dilute to 500 ml with water. SODIUMCYANIDE SOLUTION should be 2.5% N a C N and 0.002% NaOH. AMMONIUM CHLORIDE SOLUTIONshould be saturated and filtered through a Millipore filter. HYDRAZINE HYDROCHLORIDE SOLUTION. Prepare a 10 solution and filter through a Millipore filter. Standards. Dissolve pure gold in aqua regia. Evaporate to near dryness with four portions of 6N HCI and prepare dilute standards in 6N HCI. Gold solutions of less than 10 pglml should be stored away from light and kept no longer than one week.
+
Apparatus. A Baird Atomic Fluorispec fluorescence spectrophotometer with a xenon arc source was used for all fluorescence measurements. Fluorescence curves were obtained with an auxiliary Varian x-y recorder. All measurements were made in I-cm rectangular quartz cuvettes. Procedure. Roast a 10- to 30-gram rock sample in a porcelain dish for 2 hours at 600 "C to decompose any sulfides and tellurides. Cool and transfer to a 250-ml Erlenmeyer flask. Add 60 ml of the cyanide reagent and heat on a steam bath for 30 minutes while bubbling oxygen through the suspension. Cool and filter through a Millipore filter (0.45-micron porosity). Add 13 ml of HCl t o the filtrate and heat near boiling for 1 hour. Then add 10 ml of bromine water and heat on the steam bath for 0.5 hour, adding bromine water as necessary so that free bromine remains in solution. Cool and filter as above. The volume should now be about 70 ml. Heat to expel bromine and then add 2 mg of tellurium. Heat to near boiling, add 15 ml of the SOn solution and 10 ml of the hydrazine hydrochloride solution. Boil for a few minutes and add 20 ml of the SO1 solution. Cover the beaker and heat on a steam bath for 2 hours. Add 10 ml more of SOzsolution and heat uncovered on a steam bath to expel SOz. Cool, filter through a medium-porosity filter paper and wash five times with 1N hydrochloric acid. Ignite in an uncovered 25-ml porcelain crucible for 2 hours at 500 "C, starting with a cold furnace. Cool, add 3 ml of aqua regia, cover, and heat on a steam bath for 1 hour. Add 1 ml of 0.1 sodium chloride and evaporate. Expel nitrates by evaporating three times with 3-ml portions of 6N HCI. During the last evaporation with HCI, add 0.5 ml of bromine water to reoxidize any reduced gold. Avoid prolonged heating of the residue, else some of the gold may be rendered insoluble. When the gold content is less than 0.5 pg and no dilution is required, add 1 ml of saturated ammonium chloride and evaporate to dryness. In this case, do not add ammonium chloride in the fluorometric finish, but add 0.8 ml of water. Add 1.5 ml of 6N HCI, 15 ml of water, 1 ml of ammonium chloride solution, and 5 ml of rhodamine B solution. The final solution has a volume of 22.5 ml which is 0.35M in "&I, 0.4M in HCl, and 2.5 X 10-4M in rhodamine B. Extract the rhodamine B chloraurate into 10 ml of isopropyl ether. Measure the fluorescence of the extract at 575 mp using excitation at 550 mp. When decomposing with aqua regia, roast, add 30 ml of aqua regia and cover. Digest on a steam bath for 2 hours, stirring occasionally. Dilute to 100 ml and filter. Transfer the filtrate to a 150-ml borosilicate glass evaporating dish, add 2 ml of 0.1 % NaCl and evaporate. Evaporate to dryness three times with 20 ml of 8 M HCI. Dissolve soluble salts by warming with 10 ml of concentrated HCI, 60 ml of H 2 0 , and 10 ml of bromine water. Filter and then precipitate gold using tellurium as carrier. Finish as described above. RESULTS AND DISCUSSION
(1) F. E. Beamish, ANAL.CHEM.,33, 1059 (1961). (2) A. Chow and F. E. Bearnisli, Talanta, 10, 883 (1963). (3) B. J. MacNulty and L. D. Woollard, Anal. Clzim. Acta, 13, 154 (1955). (4) H. Onishi, Mikrochim. Acta, 1, 9 (1959). (5) C . Huffman, J. Mensik, and L. Riley, U. S. Geol. Sziicey Circular 544, (1967). ( 6 ) H. Goto, Sci. Rept. Tohokir Unic., First Ser., 29,204 (1940). (7) H. Onishi, ANAL.CHEM.,27, 832 (1955).
Fluorescence Studies. The strongest excitation occurs at 550 m p with three weaker peaks about 1/25 as intense, at 300, 350, and 425 mp. Excitation a t 550 mp was selected because of its high sensitivity and because long wavelength excitation is less likely to excite interfering fluorescence. The proximity of excitation and fluorescence wavelengths requires reasonably narrow slits to avoid interference from scattered exciting light. VOL. 40,
NO. 7,JUNE 1968
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Table I. Comparison of Fluorometric Gold Determinations with Those by Other Methods Sample
Decomposition and separation
Fluorometric (ppm)
Meru mite
Aqua regia and Te
0.9
Silver ore (500 ppm Sb)
Aqua regia and Te
7.6
Quartz-pyrite (equal parts)
Aqua regia and Te
3.6,2.9
Gold-bearing quartz from the Maryland Mine Iron oxide with silica (500 ppm Sb) Altered zone
Cyanide and Te
2 . 2 i 0.1
FA and Te
1.2,O.9
Cyanide and Te
0.12
Mud from the Red Sea
Cyanide and Te Aqua regia and Te Cyanide and Te
0.090
Cyanide and Te
0.0020.0.0014 0.0015
Blended rock USGS Std. Rock W-1
c
0.085
0.022
Other methods (ppm) 0 . 3 FA-AA" (F. Simon, USGS) 7 FA-AAa (F. Simon) 3.3, 4.0 FA-AAa (F. Brown, USGS) 2 . 5 FA-AAa 1.0 FA-AAa (C. Huffman, USGS) 0.13 FA-NAb (F. Simon) 0.13 NAc (F. Brown and J. Rowe) 0.019 NAc (F. Brown) 0.0048, 0.0049 N& (Baedecker et a / . ) (8) 0.0034 i O.OOO6 NA" (H. Millard, USGS)
FA-AA. Combined fire assay-atomic absorption (5). FA-NA. Fire assay-neutron activation (9). NA. Neutron activation (10).
There is a sharp rise in the net fluorescence with increase in rhodamine B concentration, the rise becoming more gradual beyond a mole ratio of 1000 for rhodamine B to Au. Although optimum fluorescence is obtained in the absence of ammonium chloride, its final concentration is kept at 0.35M because ammonium chloride aids in the removal of nitrates. Optimum range of HC1 concentration is 0.35 to 0.45M. Fluorescein solution is a convenient fluorescence standard because the same excitation and fluorescence wavelengths can be used as for the rhodamine B chloraurate. Extracting an aqueous solution containing 2 ppb gold gives a fluorescence intensity equivalent to 140 ppm fluorescein in 6 X 10-3M NaOH. The aeration-basic cyanide attack is satisfactory for most low-grade ore samples with the advantage that few sample constituents are dissolved with the gold. High concentrations of cyanide-complexing elements may require additional cyanide. Coprecipitation of Gold on Tellurium. Reduction of tellurium with sulfurous acid plus hydrazine hydrochloride gives lower blanks than does stannous chloride. Samples high in iron require more reducing reagent and the precipitation is much slower. To avoid losing gold by adsorption on the crucible walls, a milligram of sodium chloride is added during the evaporation. Nitrates are removed by two evaporations with 6N HC1 and a third evaporation with a bromine-HCI mixture. In determining low gold levels where no subsequent aliquoting is required, a n additional evaporation on the steam bath with one ml of saturated NHICl reduces the blank caused by traces of nitrate. Tests of the above tellurium separation procedure with submicrogram amounts of gold-198 gave recoveries averaging 97%. (8) E'. A. Baedecker and W. D. Ehmann, Geochim. Cosmochim. Acta, 29, 329 (1965). (9) F. Simon and H. T. Millard, ANAL.CHEM.,40, 1150 (1968). (10) H. T. Millard, J. J. Rowe, and F. W. Brown, U. S . Geological Survey, Washington, D. C., unpublished data.
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ANALYTICAL CHEMISTRY
Tracer tests using gold-198 established that there is no loss of gold by adsorption on the insoluble residue in the cyanide solution procedure, nor in the subsequent filtration and extraction steps. Using gold-198t racer, the distribution ratio Auether/ A U E ~was O found to be 4.1. Attempts were made to improve the extractability of the salt by purifying the isopropyl ether. Neither passing the ether through a column of activated alumina nor extracting the ether with ceric sulfate or with titanium(1V) sulfate solutions increased the extractability. Interferences. The serious interferences in rhodamine B methods for gold are antimony(V), thallium(III), indium, and gallium. Indium is not a strong interference in rhodamine B fluorescence methods. One milligram of trivalent indium gives an apparent gold value of 0.3 pg. The precipitation of gold with tellurium separates it not only from gallium, thallium, and antimony ( 4 ) , but also from indium. In tracer studies, only 6 pg of indium was found in the tellurium precipitate when 1 ,ug of gold was precipitated with tellurium in the presence of 2 mg of indium. Comparative Analytical Results. Table I shows a comparison of results of gold determinations by different methods on representative samples. The first sample, merumite, is a chromium mineral which presents problems in fire-assay decomposition. The higher fluorometric value is probably more reliable. The fluorometric value for the gold-bearing quartz from the Maryland mine is the average of seven determinations. To determine the precision of the method, a blended rock sample was prepared by mixing a gold-bearing rock with a gold-free rock. Determinations on 10-gram sample splits ranged from 0.023 to 0.091 ppm. Ascribing part of the lack of precision to sample inhomogeneity, a 60-gram sample was taken through the cyanide solution of gold. The sample was filtered and the filtrate was divided into six equal parts. The gold in each solution split was determined after coprecipitation with tellurium. Gold values obtained ranged between 0.018 and 0.027 ppm. The average was 0.022 ppm with a
standard deviation of 0.003. The gold content determined by neutron activation was 0.019 ppm.
times more sensitive than atomic absorption procedures. When used with appropriate separations, the method is reasonably accurate and rapid. I t can serve as a routine method particularly valuable for geochemical investigations.
CONCLUSION
The rhodamine B fluorometric method has the requisite sensitivity to determine gold a t the low parts per billion level in rocks or in solution. This method is approximately 25
RECEIVED for review on January 19, 1968. Accepted March 28, 1968. Publication authorized by the Director, U. S. Geological Survey.
Correlation between Molecular Structure of Sterols and Retention Time in Gas Chromatography Nobuo Ikekawa and Reiko Watanuki Rikagaku Kenkyusho (The Institute of Physical and Chemical Research), Yamatomachi, Saitama-ken, Japan
Kyosuke Tsuda Institute of Applied Microbiology, University of Tokyo, Bunkyo-ku, Tokyo
Kiyoshi Sakai Central Research Laboratories, Sankyo Co., Ltd., Shinagawa-ku, Tokyo
NATURAL STEROLS with structures closely resembling each other can only be separated by gas chromatography; thus gas chromatography is a n essential technique for the study of sterols. I n our previous paper on the gas chromatographic behavior of sterols, we briefly reported the correlation between the structure of sterol and relative retention times in gas chromatography using SE-30 ( I ) . This report describes similar correlations in detail on various liquid phases. In 1962, Clayton reported the behavior of sterol methyl ethers on gas chromatography with diethylene glycol succinate (DGS) and discussed the correlation between the molecular structure and retention data (2). Knights reported a method for the characterization of sterol double bonds involving hydroboration of double bonds followed by gas chromatographic analysis (3). Relative retention times of several free sterols (4-9), acetates (5, 8, IO, 11) and trimethylsilyl ether (9,12,13)using different kinds of packing have been reported. But the relationship between the separation factor for double bond isomers and liquid phase has not been in-
(1) K. Tsuda, K. Sakai, and N. Ikekawa, Chem. Pharm. Buff. (Tokyo), 9,835 (1961). (2) R. B. Clayton, Nature, 190, 1071 (1961); 192, 524 (1961); Biochem., 1, 357 (1962). (3) B. A. Knights, J. Gas Cllromatog., 2, 160 (1964). (4) M. J. Thompson, W. E. Robbins, and G. L. Baker, Steroids, 2, 505 (1963). (5) W. E. Robbins, M. J. Thompson, J. N. Kaplanis, and T. J. Shortino, Steroids, 4, 635 (1964). (6) R. Fumagalli, P. Capella, and W. J. A. VandenHeuvel, Anal. Biochem., 10, 377 (1965). (7) E. Fedeli, A. Lanzani, P. Capella, and G. Jacini, J. Am. Oil Chemisrs’ Soc., 43, 254 (1966). (8) B. A. Knights, J . Gas Chromatog., 2, 338 (1964). (9) A. Rozanski, ANAL.CHEM.,38, 36 (1966). (10) J. W. Copius-Peereboom, J . Gas Chromatog., 3, 325 (1965). (11) G. Osske and K. Schreiber, Tetrahedron, 21, 1559 (1965). (12) W . W. Wells and M. Makita, Anal. Biochem., 4, 204 (1962). (13) P. Eneroth, K. Hellstrom, and R. Ryhage, Steroids, 6, 707 (1965); J . Lipid Res. 5, 245 (1964).
vestigated. In this report, retention times of 30 sterols as free alcohols determined o n polar and also nonpolar liquid phases are compared with separation factors. EXPERIMENTAL The samples used in this study were Cn,-sterols, C2*sterols, and Cn9-sterolsas listed in Table I. A Shimadzu Seisakusho Model GC-1C gas chromatograph equipped with a hydrogen flame ionization detector was used. The column consisted of U-type glass tube, 180-cm X 4-mm i.d. Column packings were prepared according t o Horning, Vanden Heuvel, and Creech (14) using Shimalite W (Shimadzu Co.), 80-100 mesh, as the support after washing with hydrochloric acid and silanization with dimethyldichlorosilane in toluene. Liquid phases used in this study were 1.5% SE-30 (methyl silicone gum, General Electric Co.), 1.5% SE-52 (methyl of phenyl groups, G.E.), phenyl silicone containing 5 mole 1.0% XE-61 (phenyl methyl silicone containing 35 mole of phenyl groups, G.E.), 1.5% QF-I (fluorinated alkyl silicone, D o w Chemical Co.), 1.5 XE-60 (nitrile silicone, G.E.), and 2.0 % N G S (neopentyl glycol succinate, Applied Science Lab.).
z
z
RESULTS AND DISCUSSION
The relative retention times of free sterols to cholestane on six kinds of packing are listed in Table I. The separation factors between double bond isomers in various positions obtained from Table I are shown in Table 11. Separation factors o n SE-52 and QF-1 are not shown in Table 11, because their values are comparable with that of SE-30 and XE-60, respectively. G a s chromatographic behavior of free sterol isomers on polar and nonpolar phases and also the kinds of phase which must be used for the separation of any pair of sterol isomers are shown in Tables I and 11. As can be seen
(14) E. C. Horning, W. J. A. VandenHeuvel, and B. G. Creech, “Methods of Biochemical Analysis” Vol. XI, D. Glick, ed., p 69, Interscience, New York, 1963. VOL. 40, NO. 7, JUNE 1968
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