Separation of Cadmium from Silver and Other Elements by Anion Exchange Chromatography in Hydrobromic Acid and Preparation of Carrier-Free Cadmium-109 from Cyclotron Targets F. W. E. Strelow. W. J. Louw, and C. H.-S. W. Weinhert National Chetnical Research Laboratory, Pretoria, South Africa Silver is quantitatively separated from cadmium by elution with 6.7M HBr from a column of AGLX8 anion exchange resin in the bromide form. Zn, In, Cu(ll), Co(ll), Ni(ll), Mn(ll), Fe(lll), Ga, and U(VI) can be eluted with O.2M HNOj containing 0.05M HBr. Cd i s retained by the column and can be eluted with 3M HNOa or with 1M NHIOH containing 0.lM ",NOa and 0.02M NH,Br. Separations are sharp and quantitative for Cd:Ag ratios from 1OOO:l to 1:30000 and above. The method has been applied to the separation of carrier-free Io9Cd from several grams of cyclotron irradiated silver. Tables of distribution coefficients for Cd and Ag in HBr, NH,Br, HI, and KI are presented together with elution curves and results of quantitative separations of synthetic mixtures.
NUMEROUS METHODS have been described for the ion exchange separation of Cd from other elements. Most prominent are those using cation exchange (1-4) or anion exchange (5-8) in halide solutions. Separations are carried out in dilute solutions and therefore cannot be used for the separation of Cd from large amounts of Ag because of the insolubility of silver halides. At high halide concentrations, silver halides become soluble because of complex formation, solubilities increasing from chloride t o iodide. Anion exchange distribution coefficients for both C d and Ag are known only for HCI solutions ( 5 ) , while coefficients for C d in HBr ( 6 ) also are known. No information about coefficients in iodide solution seems t o be available. According t o the coefficients published by Kraus and Nelson (9,separation of C d from Ag should be possible by anion exchange in HCl between 5 M a n d 10M. For the preparation of carrier-free 'OQCdby the lo9Ag(d, 2n) 1 W d reaction in a cyclotron, a separation of submicrogram quantities of C d from several grams of Ag is required. The radiochemical methods for separating Cd developed by Devoe and Meinke (9) cannot be applied in this case. Grachev et a/. (10)have described a method in which the bulk of the Ag and C u from the target is precipitated by the addition of hydroiodic acid t o a solution of the nitrates. Last traces then are removed by anion exchange in HCI. I n this method a considerable percentage of the produced l09Cd is lost by inclusion and adsorption o n the large iodide precipitate. Better recovery can be obtained by omitting the precipitation step and (I) (2) (3) (4) (5)
Y. Yoshino and M. Kojinia, Jczpati Atialyst, 5 , 311 (1955). F. W. E. Strelow, ANAL.CHEM.. 32, 363 (1960). J. S . Fritz and B. B. Garralda, ibid., 34, 102 (1962). S. Kallmann. H. Oberthin, and R. Liu, ibid.,32, 58 (1960). K. A. Kraus and F. Nelson, Proc. Itirerri. CoizJ Peacefrrl Uses
At. Energy, Getieca 1955, 7, 113 (1956). (6) T. Anderson and A. B. Knutsen, Acta Chem. Scand., 16, 849
(1962). (7) E. R. Baggott and R. G. W. Willcocks, Annlysf, 80, 53 (1954). (8) S. Kallmann, H. Oberthin, and R. Liu, ANAL. CHEM.,30, 1846 (1958). (9) J. R. Devoe and W. W. Meinke, ANAL.CHEM., 31, 1428 (1959). (10) S. A. Grachev, V. N. Melnikov, Yu A. Ryukhin, and M. A. Toropova, Radiokhimiya, 3, 116 (1961); C . A . 57, 305711 (1962).
using only an anion exchange procedure. Optimum conditions are found in about 9MHC1 with a distribution coefficient of about 50 for Cd and a solubility of about 2.5 mg Ag per ml (11). Lower concentrations of HCI decrease the solubility of AgCl while higher concentrations decrease the coefficients of Cd. The method has been used successfully for several years in this laboratory with lo9Cdrecoveries of 99% and better. Its disadvantage is the limited solubility of AgCl in HCI. About 1200 ml are required to keep 3 grams of target material in solution. As a consequence and because of the fairly low distribution coefficient of Cd, a resin column of about 100-ml volume has to be used. In bromide or iodide solutions, Ag is considerably more soluble than in chloride solutions ( I / ) . Thus useful separations of C d from Ag could become possible in these media, provided the respective distribution coefficients are favorable. A systematic investigation of the coefficients of Cd and Ag in HBr, HI, NH4Br, and KI therefore was undertaken, and as a result a considerably improved anion exchange method, using hydrobromic acid for the separation of C d from Ag, has been developed and applied to the separation of lo9Cd from cyclotron targets. Furthermore, by using a mixed nitric-hydrobromic acid eluent, improved separations of C d from In, Zn, and other elements could be obtained. EXPERIMENTAL
Reagents and Apparatus. Chemicals of AR grade purity were used throughout. Borosilicate glass tubes of 15, 20, or 25 mm (only for 100-ml columns) diameter, fitted with B19 ground glass joints at the top and fused-in No. 2 porosity sinterplates and taps at the bottom were used as columns. The resin was the AGl-X8 anion exchanger of 100 to 200 mesh particle size supplied by the Bio-Rad Laboratories of Richmond, Calif. A Perkin-Elmer 303 atomic absorption spectrometer was used for the determination of small amounts of silver, cadmium, and other elements. Irradiations were carried out in the conventional 110-cm cyclotron of the National Physical Research Laboratory in Pretoria. Distribution Coefficients. One millimole of C d o r Ag in 250 ml of halide solution of the desired concentration was equilibrated with 2.500 grams of AGl-X8 resin in the chloride form by shaking mechanically for 24 hours a t 25 "C. The resin had been dried a t 60 " C in a vacuum pistol with P , 0 5 as drying agent. After the resin had been separated by filtration, the amounts of Cd and Ag in the resin and the aqueous phase were determined by appropriate analytical methods. Equilibrium distribution coefficients
D =
moles in resin moles in solutions
ml solution gram resin
were calculated from the analytical results and are presented in Tables I and 11. (1 1) A. Seidell, "Solubilities of Inorganic and Metal Organic
Compounds," Volume I, Fourth Ed., Van Nostrand, New York, 1958, pp 9, 60, and 93. VOL. 40, NO. 13, NOVEMBER 1968
2021
Table I. Anion Exchange Distribution Coefficients of Cadmium Eluent HBr NH4Br HI KI
0. IN
0.2N
0.5N
1 .ON
2.ON
4.ON
1410 720
2720 1250
7300 3400
13000 7500
>io4
>io4
>io4
>io4
>io4
>io4
>io4
>io4
9800 4900 1190 >io4
3450 1450 120 1830
5.ON 2150 886 63
I
.
1500
1140
...
NQNOJ
+ 0
02M
NH48r,
'.i
J
0 , l O M HBr
... ... ...
45 2
I
Cd
.
Elution Curves. From the above results and from practical considerations, it was concluded that between 6.5 and 7.OM HBr provided the best eluting agent for the separation of Ag and Cd. When small amounts of nitrate also are present, the acid seriously attacks the resin at concentrations above 7 M HBr. Because nitric acid is the most favorable agent for the dissolution of silver, the presence of nitrate has to be accepted. Figure 1 shows an experimental elution curve for a mixture of 2 mmoles of Ag and 1 mmole of Cd. The mixture in 6.7M HBr was percolated through a column of 12 ml AGl-X8 resin of 200 to 400 mesh particle size. The resin column was 6.0 cm in length and 1.5 cm in diameter. It had been equilibrated with 6.7M HBr. Elution at a flow rate of 2.0 f 0.3 ml per minute (1.0 ml per minute per cm2) was carried out as indicated on the graph. Fractions (25 ml) were taken from the beginning of the absorption step using an automatic fractionator. After the excess acid had been removed by evaporation, the silver bromide was dissolved in sodium thiosulfate, and the concentrations of Ag and Cd in the fractions were determined by atomic absorptjon spectrometry using an acetylene-air flame and the 3281 A and 2288 A absorption lines, respectively. According to Anderson and Knutsen (6), In and Zn can be separated from Cd by anion exchange chromatography using 0.1M HBr as eluting agent. Quantitative work carried out in this laboratory showed very strong tailing which led to incomplete recovery when eluting In with 0.1M HBr even when using resin of 200 to 400 mesh particle size. Zn could be separated from Cd on a column of 12 ml AGl-X8 resin ABSORPTION
7.ON
... 40 ...
I.OOM NH40H t 0 I O M
Table 11. Anion Exchange Distribution Coefficients of Silver Eluent 2.ON 3.ON 4.ON 5.ON 6.ON ?.ON HBr ... ... 67 24.8 13.7 9.0 NH4Br .., . .. 21.2 10.1 ... ... HI 520 162 50 18.3 7.2 ... KI 420 180 81 ... ...
6.ON
b
100
200
300
400
500
600
700
800
ml Eluole
Figure 2. Separation of Zn-Cd by elution with 0.1M HBr Column of 12 ml AGl-XS resin, 200 to 400 mesh; length 6 cm; diameter 1.5 cm. Flow rate 2.0 f0.3 ml/min
by elution with 0.1M HBr as shown in Figure 2 but also exhibited some tailing. 0.20M HNOP containing 0.05M NH4Br proved to be a considerably more effective eluting agent for the separation of In and Zn from Cd. Figure 3 shows an elution curve with a 12-ml column as described, eluting In with the above eluting agent and Cd with 1.00M ",OH containing 0.10M NH4N03 and 0.02M NH4Br. A flow rate of 2.0 =t 0.3 ml per minute was maintained throughout. Zn, Cu(II), Co(II), Ni(II), Mn(II), Fe(III), Ga, and U(V1) are eluted together with or ahead of In. All these elements show considerably less tailing than In. Quantitative Separations. From the foregoing, a method for the quantitative separation has been elaborated and applied to synthetic mixtures of Cd with one other element. Amounts of standard solutions of the elements were measured out, mixed, and percolated through a column of 12 ml (5 grams) AG1-X8 resin of 200 to 400 mesh particle size. Solutions containing Ag were made up in 6.7M HBr, others in 0.1M HBr. The resin column had been equilibrated with
0 IM HBr IW M \
NH40H t 0 IOM N q N 4 C 0 . 0 2 M N%bBr
-
J
7
3 0
0 2 0 M HNO
tO05MHBr
10
8
%
6
p E
0 I
Cd
In
P 0
11
4
2
100
ml Eluate
200
300
400 ! ml Eluate
600
700
BOO
4-
Figure 1. Separation of Ag-Cd by elution with 6.7M HBr
Figure 3. Separation of In-Cd by elution with 0.2M " 0 2 0.05MHBr
Column of 12 ml AG1-XI resin, 200 to 400 mesh; length 6 cm; diameter 1.5 cm. Flow rate 2.0 f 0.3 ml/min
Column of 12 ml AGl-XS resin, 200 to 400 mesh; length 6 cm; diameter 1.5 cm. Flow rate 2.0 4~ 0.3 ml/min
2022
ANALYTICAL CHEMISTRY
u
250
M O
500
1.000
1,500
2,000
ml Elualr
Figure 5. Elution curve for 3-gram silver target with 9M HCI
750
rnl Eluols
Figure 4. Elution curve for 3-gram silver target with 6.8MHBr Column 23 ml AG1-XS resin, 100 to 200 mesh; length 7.5 cm; diameter 2.0 cm. Flow rate 3.0 & 0.3 ml/rnin
50 ml HBr of the same concentration. Ag was eluted with 200 ml of 6.7M HBr. I n the case of In, 400 ml of 0.20M HNOd containing 0.05M HBr were used for elution, while only 200 ml were used for Zn, Cu(II), and the other elements. The eluate was taken from the beginning of the absorption step. Subsequently, C d was eluted with 250 ml of 1.00M N H 4 0 H containing 0.10M N H 4 N 0 3 and 0.02M NHIBr. A flow rate of 2.0 i 0.3 ml per minute was maintained throughout. The amounts of the elements in the eluates were determined by conventional analytical methods after the excess eluting agent had been removed by evaporation. Silver was separated from bromide by dissolution in concentrated ammonia solution and precipitation of silver sulfide which was separated and dissolved in nitric acid. Small amounts of Ag, Cd, and Z n were determined by atomic absorptioa spectrometry using !n acetylene-air flame and the 3281 A , 2288 A, and 2138 A lines, respectively. Small amounts of silver bromide were dissolved in sodium thiosulfate and measured against standards containing the same concentration of this reagent. The results of the quantitative separations are presented in Table 111. Preparation of Carrier-free logCd. The irradiated target material was weighed, dissolved in a minimum of 3M H N 0 3 , and the solutions were evaporated to incipient dryness. The salts were dissolved in a small amount of water and the solution was added with vigorous stirring to 9M HBr using 75 ml of HBr per gram of silver. Enough water was added to give a final concentration of 6.7 to 6.8M HBr. The solution was passed through a column of 20-mm diameter containing 23 ml (10 grams) AGl-XS resin of 100 to 200 mesh particle size which had been equilibrated with 6.7M HBr. The silver then was washed onto the resin and eluted with 250 ml of 6.7M HBr, followed by 150 ml of 0.3N HNOj which eluted accidental impurities such as "Zn which can
Column 100 rnl AG1-XI resin, 100 to 200 mesh; length 22 cm; diameter 2.5 cm. Flow rate 3.5 i 0.3 ml/min
originate from a (d,2n) reaction of 65Cufrom the target backing. Finally Cd was eluted with 200 ml of 3M H N 0 3 . A flow rate of 3.0 i 0.2 ml per minute was maintained throughout. Figure 4 shows an experimental elution curve for the above separation using 3 grams of silver target material. Twenty-five-milliliter fractions were taken with an automatic fractionator and the elution of silver and cadmium was followed by counting the ?-radiation emitted by llomAg and of lo9Cd in equilibrium with its 40-second daughter lo9mAg. An elution curve with the same amount of target material but using 9M HCl as eluent for Ag is shown in Figure 5 for comparison. A resin volume of 100 ml had to be used in this case t o prevent a premature breakthrough of Cd. RESULTS AND DISCUSSION
The described method provides a useful means for the quantitative separation of C d from silver. Separations are sharp and quantitative for Cd-Ag ratios from 1000: 1 t o 1 :30000 and above. The method has been applied successfully to the separation of carrier-free loRCdfrom cyclotron irradiated silver targets. Recoveries of 99% and better were obtained on synthetic mixtures containing known amounts of activities. F o r carrier-free amounts of Cd, a sharp elution peak can be obtained with 3M HN03, but for large amounts 1M N H 4 0 H containing O.lMNH4N03 and 0.02M NH4Br was found to be a superior eluting agent. 0.20M " O B containing 0.05M HBr is a n excellent eluting agent for the quantitative separation of In, Zn, and other elements from C d by anion exchange chromatography. The strong tailing which prevents the quantitative elution of In with O.1MHBr is suppressed and the tailing of Zn disappears. Cu(II), Co(II), Ni(II), Mn(II), Fe(III), Ga, and U(V1) also can be separated quantitatively from Cd in 1 mmole amounts on a
Table 111. Quantitative Separations Taken, mg Cd
111.8 223.6 0.112 111.8
223.6 0.112 111.8 111.8 111.8 111.8 111.8 111.8 111.8 111.8
Ag Ag Ag
Zn Zn Zn
In Cu(I1) Co(I1) Ni(I1)
Mn(I1) Ga U(W Fe(II1)
Found, mg Other element 107.6 0.108
3000 65.74 0.131 657.4 114.0 63.81 58.14 58.64 54.82 69.06 237.4 55.51
Cd
111.8 zt 0.2 223.7 i 0.3 0.111 i. 0.002 111.7 i 0.2 223.5 i 0 . 3 0.112 i 0.002 111.8 i 0 . 3 111.7 =t 0 . 2 111.7 + 0 . 2 111.9 + 0.2 111.8 i 0 . 2 111.7 i 0.3 111.8 =t 0.2 111.7 + 0.2
Other element 107.5 =k 0.2 0.107 i 0.002 Not determined 65.71 i 0.06 0.130 i 0.002 657.2 i 0 . 6 113.9 + 0 . 2 63.80 i 0.06 58.16 i 0.07 58.61 -Ir 0.07 54.81 =t 0.05 69.02 i 0.09 237.4 i. 0 . 2 55.49 i 0.06
VOL. 40, NO. 13, NOVEMBER 1968
2023
12 m l ( 5 grams) column of AGl-X8 resin. Elements such as the alkali metals, alkaline earths, rare earths, Ti(IV), Zr, Hf, Th, and AI, which in aqueous solution have no or only very slight tendencies to the formation of bromide complexes, have not been investigated, but it seems to be reasonable t o assume that they also can be separated quantitatively from C d by the above procedure. Au(III), Tl(III), Hg(II), Pd(II), Pt(IV), and Bi(II1) accompany Cd quantitatively and Pb(I1) does so partially. Pb(I1) can be eluted with 0.30M H N 0 3 containing 0.025M HBr, while Cd is retained by the column, but a larger column (23 ml) is required and only limited amounts of C d can
be separated (12). Ge(IV), Sn(IV), and Sb(II1) can be eluted with 0.1 M HBr according to literature information (6). With 0.20M HNOI containing 0.05M HBr, separation should be even better. However, as n o quantitative separations seem t o have been carried out with 0.1M HBr as eluent, the quantitative aspects of these separations still will have t o be investigated. RECEIVED for review May 14, 1968. Accepted July 15, 1968. (12) F. W. E. Strelow and F. von S. Toerien, ANAL.CHEM.,38, 545 (1966).
Luminescence Characteristics of Aflatoxins B1 and G, B. L. V a n Duuren, Tze-Lock Chan, and F. M. I r a n i Laboratory of Organic Chemistry and Carcinogenesis, Institute of Enuironmental Medicine, New York University Medical Center, New York, N . Y . 10016
A study on the fluorescence and phosphorescence characteristics of aflatoxins B, and GIwas carried out. A self-correcting luminescence spectrophotometer was used and it was found that characteristic fluorescence spectra of these toxins could be recorded for submicrogram quantities to pg/ml). The sensitivities for the detection of these substances by fluorescence and thin-layer chromatography were compared. The corrected fluorescence emission and excitation spectra of both aflatoxins were measured at room temperature and at 77 O K and in potassium bromide. In their fluorescence, they exhibit a blue shift when measured at 77 O K compared to their room temperature solution spectra. Phosphorescence spectra and decay curves were also recorded.
THEAFLATOXINS are metabolites produced by the mould Aspergillus frcrvus ( I ) . These compounds have been implicated as toxic contaminants in human and animal foodstuffs (2-4), in milk (3, and possibly in tobacco leaf and smoke (6). There is, therefore, a wide interest in sensitive analytical procedures for the qualitative detection and quantitative analysis of these materials in a variety of products. Several brief reports have described their fluorescence emission maxima ( 7 , 81, but a detailed study of their luminescence characteristics and limits of detection by spectroscopic methods has not been reported. Although the materials can be detected by thin-layer chromatography, it is desirable to have such identifications, which depend solely on RF value, confirmed beyond doubt by a spectroscopic method. Of these, luminescence is by far the most sensitive. The present report describes the following luminescence characteristics for aflatoxins B1 and GI: corrected fluorescence (1) K. Sargent, R. B. A. Carnaghan, and R. Allcroft, Chem. Ind. (London), 1963, 50. ( 2 ) K . Sargent and R. B. A. Carnaghan, Brit. Vet. J., 119, 178 (1963). (3) B. H. Armbrecht, F. A. Hodges, H.R. Smith, and A. A. Nelson, J . Assoc. Ofic.Agr. Chemists, 46, 805 (1963). (4) . . N. D. Davis, U. L. Diener, and D. W. Eldridge, - . Apul. . - Microbiol., 14, 378 (1966). (.5 )_H. de Ioneh. R. 0. Vles. and J. W. van Pelt. Nature. 202. 466 (1964). (6) T. C. Tso and T. Sorokin, Beitrzge zur Tubakforschung, 4, 18 (1967). ( 7 ) R. B. A. Carnaghan, R. D. Hartley, and J. O'Kelly, Nature, 200, 1101 (1963). (8) J. A. Robertson, W. S. Pons, and L. A. Goldblatt, J . Agr. Food Chem., 15, 799 (1967). I
2024
,
ANALYTICAL CHEMISTRY
I
excitation and emission spectra, corrected phosphorescence spectra, quantum efficiency of fluorescence, phosphorescence decay times, and limits of detectability. EXPERIMENTAL
Aflatoxins BI and GI. Chromatographically-pure substances used in this investigation were supplied by Professor G. N. Wogan of Massachusetts Institute of Technology (9). Each compound showed a single spot by thin-layer chromatography o n silica gel G (Merck) using chloroformmethyl alcohol (97:3) as solvent. The RF values were: 0.52 for aflatoxin B, and 0.46 for aflatoxin G1. Solvents. Luminescence spectra were recorded by using freshly prepared solutions in methyl alcohol and in frozen ethyl alcohol-methyl alcohol (4 :1). Fluorometric grade methyl alcohol (Hartman-Leddon Co., Philadelphia, Pa.) was used without further purification. Ethyl alcohol was repeatedly distilled until the fluorescence background was minimal. Potassium Bromide. Infrared-quality potassium bromide (Harshaw Co., Cleveland, Ohio) was used for the preparation of pellets. The previously described procedure ( I O ) was followed. Quinine Bisulfate. Commercial grade material (K & K Laboratories, Plainview, N. Y . )was purified by three crystallizations from water. Purity was established by the absorption spectrum and thin-layer chromatography. Thin-Layer Chromatography Adsorbent. Merck silica gel G was continuously extracted with ethyl acetate-methyl alcohol (1 :1) for two days and dried overnight at 110 "C prior to use. Absorption Instrumentation. Absorption spectra were obtained with a Cary Model-14 ultraviolet-visible spectrophotometer using 1-cm path-length cuvettes. Luminescence Instrumentation. A multipurpose self-correcting luminescence spectrophotometer (Farrand Optical CO., New York, N . Y . ) was used for all fluorescence and phosphorescence measurements in this work. The details of its design and performance are described elsewhere (11, 12). This instrument is equipped with a high-intensity 150-watt dc xenon arc (Hanovia Lamp Division, Newark, N. J.),
,
(9) T. Asoa, G. Buchi, M. M. Abdel-Kader, S . B. Chang, E. L. Wick, and G. N. Wogan, J . Amer. Chem. SOC.,87, 882 (1965). (10) B. L. Van Duuren and C. E. Bardi, ANAL.CHEM.,35, 2198 (1963). (11) S . Cravitt and B. L. Van Duuren, Chem. Instrum. 1, 71 (1968). (12) S. Cravitt and B. L. Van Duuren, Abstracts, Pittsburgh Conf. on Anal. Chem. and Applied Spectroscopy, 1968, p 90.
