Anal. Chem. 1980, 52, 2420-2422
2420
(31) Littlewood, A. B.; Phillips, C. S.G.; Price, D. T. J. Chem. Sac. 1955, 1480. (32) Hoffrnann, D.: Bondirell, W. E.; Wynder, E. L. Science 1974, 783,215. (33) Newman, M. S. “Carcinogensis-A Comprehensive Survey, VOI. I , Polynuclear Aromatic Hydrocarbons, Chemistry, Metabolism and Carcinogenesis”; Freudenthal, R. I., Jones, P. W., Eds.; Raven Press: New York, 1976; p 203. (34) Coombs, M. M.; Dixon, C.; Kissonerghis, A. M. Cancer Res. 1976, 36,
4525.
RECEIVED for review June 9, 1980. Accepted September 25, 1980. This research was sponsored by the National Cancer Institute under Contract No. N01-CO-75380 with Litton Bionetics, Inc.
CORRESPONDENCE Quantitative Separation of Samarium from Neodymium by Anion Exchange Chromatography in Dilute Nitric Acid-Methanol Sir: The lanthanides are adsorbed by anion exchange resins from aqueous nitric acid solutions only to a small (light lanthanides) or a negligible extent (heavy lanthanides) (1-3). Differences in the values for the distribution coefficients of the light lanthanides in fairly concentrated nitric acid are much too small to allow efficient separations. The situation is improved considerably when a water-soluble nonaqueous component is added to the aqueous phase. Distribution coefficients for the light lanthanides increase very dramatically and the separation factors are also enhanced considerably. Edge seems to have been the first t o use nitric acid-ethanol mixtures for the separation of elements of the lanthanide group from each other (4). Yet he did not attempt separation of adjacent members of the group in his first publication nor does the publication contain any quantitative results (4). In later work (5) describing a group separation of neodymium plus the lighter lanthanides from samarium plus the heavier lanthanides, 35% of 3.5 M nitric acid plus 65% methanol was used as the best eluting agent. Separations were quantitative for mixtures containing 200 pg of neodymium and 50 pg of samarium. Several percent of the neodymium appeared in the samarium fraction when 400 kg of neodymium was present. As pointed out by Edge already ( 5 ) ,a better separation can be expected by using a resin of finer particle size and lower cross-linkage and, if necessary, also by using lower flow rates. Distribution data for a resin of 4% cross-linkage were published by Faris et al. (6), but their work was mainly aimed a t the preparation of pure lanthanide oxide fractions and does not contain data on quantitative separations. Some other work either is mainly concerned with distribution data (7-9) or is aimed a t separating trace amounts of lanthanides into groups mainly for neutron activation analysis (10-12). In addition, some recent work which was developed for the separation of neodymium prior to its determination by isotope dilution mass spectrometry to establish the burn up rates of nuclear fuel also makes use of anion exchange in nitric acidmethanol mixtures (13-15). In some cases HPLC methods were used (16-18). All these methods were developed for the partial separation of micro- or nanogram amounts of neodymium and are not aimed a t nor do they provide a quantitative separation of larger amounts of neodymium and samarium. Considerable overlapping of bands of adjacent lanthanides occurs even a t very low concentrations of lanthanides (14,151. Another more recent publication (19) presents additional information about distribution coefficients of lanthanides in nitric acid-methanol and some elution curves but makes no attempt to obtain quantitative results for a separation. Furthermore, the interesting region of low nitric acid and high 0003-2700/80/0352-2420$01 .OO/O
methanol concentration is not covered. Apparently there has not been any successful attempt t o separate larger amounts of neodymium and samarium and thus separate the lanthanides quantitatively into two groups by using anion exchange in nitric acid-organic solvent mixtures. Papers describing work with common microporous resins show a tendency to use relatively high concentrations of nitric acid for the separation (5-11, 12, 16). Yet the distribution data of Faris et al. (6) indicate a t careful study that separation factors between adjacent lanthanides increase with increasing methanol concentration. Because distribution coefficients also increase with increasing nitric acid concentration and a high distribution coefficient for the eluted element would be impractical, the tendency to use high concentration of nitric acid seems to be surprising. In fact one should aim to make the methanol concentration as high as possible and the nitric acid concentration as low as possible while aiming a t a distribution coefficient of about 10 & 5 for the eluted element. This paper describes a method which was developed not only for the separation of trace amounts of lanthanides into two groups but for the quantitative separation of neodymium and samarium in amounts up to 100 mg and more. For this purpose it was tried to optimize experimental parameters such as resin particle size, resin cross-linkage, nitric acid and methanol concentration, and flow rate, as far as was possible and practical under the circumstances.
EXPERIMENTAL SECTION Reagents. The resins used were the AGl-X4 and AG1-X8 strongly basic anion exchangers on polystyrene bases marketed by BIO.RAD Laboratories of Richmond, CA. Resin of 200-400 mesh particle size was used for column work. Water was distilled and then passed through an Elgastat deionizer. Neodymium, samarium, and other lanthanide oxides of 299.9% purity were obtained from Fluka AG, Buchs, Switzerland. Neodymium and samarium oxide were further purified by using anion exchange chromatography in nitric acid-methanol. The final products contained less than 5 ppm of the “other” oxide respectively. Apparatus, Borosilicate glass tubes of 21 mm bore and 42 cm length, fitted with a B19 ground-glassjoint at the top and a no. 2 glass sinter plate and a tap at the bottom, were used as columns. Alternatively tubes of 12 mm bore and 30 cm length, fitted with a B12 ground-glass joint at the top were used for smaller amounts of the elements. A Carl Zeiss ultraviolet-visible spectrophotometer was used for determinations by molecular absorption spectrometry, and Varian-MAT SMIBF mass spectrograph modified as indicated previously (20) was used for mass spectrometric determinations. Elution Curves. A column containing 58 mL, (20 g dry weight) of AG1-X4 resin of 20C&400 mesh particle size was converted to 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980 5,O
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Figure 2. Elution curve Srn-Nd with 0.08 M HNO, in 8 5 % methanol: 58 mL of AGl-X4 resin, 200-400 mesh, NO,' form [I85 X 2.0 cm]; flow rate 0.9 & 0.1 mL/rnin.
nitrate form and then equilibrated by passing through about 100 mL of 0.5 M nitric acid containing 90% methanol. The original resin column in water was 18.5 cm long and 2.0 cm in diameter. I t shrank about 2 cm in length on conversion and equilibration. A solution containing 0.5 mmol of both neodymium and samarium in 20 mL of 0.5 M nitric acid containing 90% methanol was passed onto the column and washed onto the resin with a solution containing the same concentrations of nitric acid and methanol. Samarium followed by neodymium was then eluted with 1.00 M nitric acid containing 70% methanol. A flow rate of 1.0 f 0.1 mL/min was used throughout. Fractions of 25 mL volume were taken with an automatic fractionater. The eluting agent was removed by evaporation and after dissolution in a small amount of dilute hydrochloric acid the amounts of samarium and neodymium were determined gravimetrically as oxides after precipitation with excess ammonia. About 0.05 mg of the rare earths produced a visible precipitate in 3-4 mL of solution after some time on the water bath for coagulation. The experimental elution curve is presented in Figure 1. Figure 2 shows an elution curve with the same amounts of samarium and neodymium and the same column and experimental conditions as described for Figure 1 but 0.06 M nitric acid containing 85% methanol as eluting agent and Figure 3 a curve with the same column and conditions but with 0.10 M nitric acid in 85% methanol as eluting agent. Also included in Figure 2 is the experimental elution peak for neodymium as obtained when this element is eluted with 0.1 M nitric acid after 500 mL of 0.06 M nitric acid containing 85% methanol has been passed through the column for the elution of samarium. Obviously considerably smaller columns can be used for smaller amounts of the two lanthanides. Figure 4 shows an elution curve for 3.80 mg of samarium and 3.47 mg of neodymium. A column only 1.20 cm wide was used in this case. It contained 14.5 mL (5 g dry weight) of AGl-X4 resin of 200-400 mesh particle size. Adsorption took place from 10 mL of 0.5 M nitric acid containing 90% methanol. Nitric acid (0.06 M) containing 85% methanol was used for elution. The flow rate was 0.30 f 0.05 mL/min. Determinations of the lanthanides were carried out spectrophotometrically by measuring the optical density of the complex
Figure 4. Elution curve Sm-Nd with 0.06 M HNO, in 85 % methanol: 14.5 mL of AGl-X4 resin, 200-400 mesh, NO3- form [ 12.8 X 1.2 cm]; 0.05 mL/min. flow rate 0.3
*
Table I. Results of Quantitative SeparationsU taken rngSm m g N d
found
Ing N d
rng Srn
75.94 69.45 75.91 i 0.04 69.46 i 0.03 15.19 138.9 15.18 t 0.03 138.5 i 0.1 0.152 69.45 0.151 i 0.001 69.45 ? 0.03 75.94 0.139 75.92 * 0.03 0.140 i 0.002 75.94 13.89 75.93 t 0.04 13.90 * 0.03 Averages of triplicate results. ~-
formed by samarium and neodymium with chlorophosphonazo I11 at pH 1 (0.1 M nitric acid) at 675 nm in the presence of DPTA (diethylenetriaminepentaacetic acid) as complexing agent. Quantitative Separations. Appropriate volumes of standard solutions of samarium and neodymium in 1 M nitric acid were measured out in triplicate, mixed, and evaporated to dryness on the water bath. About 20 mL of 0.5 M nitric acid containing 90% methanol was added, and after the salts had completely dissolved, the solutions were passed through columns containing 58 mL (20 g dry weight) of AGl-X4 anion exchange resin of 2OC-400 mesh particle size and in the nitrate form as described above. The lanthanides were washed onto the resin with 0.5 M nitric acid containing 90% methanol. Samarium was eluted with 500 mL of 0.06 M nitric acid containing 85% methanol, and the walls of the column were rinsed carefully with about 10.mL portions at the beginning to transfer the lanthanides from the top into the column. The flow rate for adsorption and the elution of samarium was kept at 1.0 f 0.1 mL/min. Finally neodymium was eluted with 200 mL of 0.10 M nitric acid at a flow rate of 3.0 f 0.3 mL/min. The eluates were evaporated to dryness on the water bath carrying out the final evaporation in small beakers, and samarium and neodymium were determined by suitable analytical methods. For large amounts (>lo mg) the lanthanides were precipitated as oxalates with oxalic acid from 0.1 M hydrochloric acid using small beakers, separated by filtration, ignited to the oxides at 900' C, and weighed in small platinum crucibles (10 mL volume) on a semimicroscale. Small amounts of the lanthanides were determined spectrophotometrically a t 675 nm as the complexes with chlorophosphonazo I11 at pH 1 in the presence of DTPA as
2422
Anal. Chem. 1980, 52, 2422-2425
complexing agent for suppressing interferences. Three standards, which were measured out together with the samples, were analyzed as controls. Furthermore, two blank runs on reagents were taken through the procedure and the results corrected when necessary. The results are presented in Table I. Cross Contamination. Because the exchange of the nitrate complexes of the lanthanides is very slow and some tailing of samarium and heading of neodymium is observed even at the slow flow rates employed, the amount of cross contamination in both eluate fractions as used for the quantitative separations was determined. About 5 mg of the lanthanide oxides resulting from the determinations (first line of Table I) were thoroughly mixed with 0.2 g of specpure graphite, and the graphite was pressed into electrodes which were analyzed by spark source mass spectrography as described before (20).
DISCUSSION T h e described method provides a very satisfactory means for the separation of relatively large amounts of samarium and neodymium and is excellently suited for the separation of larger amounts of lanthanides (up to a few hundred milligrams) into two groups, the heavier ones accompanying samarium and the lighter ones neodymium. Because of the relatively very slow exchange rates of the lanthanide nitrate complexes in organic solvents, the superior kinetics of a 4% cross-linked resin is a n essential requirement when larger amounts of the lanthanides have to be separated. Peaks of samarium and neodymium overlap to a considerable extend when the previously described methods (5-12, 1 4 , 15) are applied to the separation of amounts of samarium and neodymium in the milligram range. An important aspect of the described separation is the use of a low concentration of nitric acid, while other more recent work ( 9 , 11, 12, 19) seems to indicate a tendency to use relatively high concentrations when working with common microporous resins. This is somewhat surprising because the distribution coefficient curves published Faris et al. (6) seem to indicate that the separation factor between samarium and neodymium increases slightly with increasing methanol concentration. T o elute samarium relatively easily a t optimum conditions, one should carry out the separation at a nitric acid concentration as low as practically possible, the lower limit being set by the fact that a t very low nitric acid concentrations severe peak broadening of samarium occurs when larger amounts are present. This is due to the fact that the local nitrate concentration around the adsorption region increases excessively during desorption and prevents further desorption until the nitrate has been transported downward. Our experiments showed that on elution of 0.5 mmol of samarium with 0.01 M nitric acid containing 90% methanol the samarium peak was substantially lower and wider than that on elution with 0.06 M nitric acid containing 85% methanol, though the separation factor was even larger. Figures 1 and 2 demonstrate the increase in the separation factor (from about 3.5 to 5 ) and in the quality of separation, when decreasing the nitric acid concentration from l to 0.06 M with the appropriate concentration of methanol to obtain a distribution coefficient of about 12. Figure 3 shows that by using a higher distribution coefficient for samarium (about 15-20)
a very wide gap between the samarium and neodymium peaks can be obtained. This can be useful when small amounts of samarium have to be separated from very large amounts of neodymium and the lighter lanthanides. Because the samarium peak on Figure 3 is wider and samarium requires more reagent for elution, the eluting agent of Figure 2 was preferred for the average analytical work. Figure 4 demonstrates that a considerably smaller column can be used for the separation of smaller amounts of the lanthanides. Even smaller columns can be used for trace amounts. Because of the slow exchange kinetics the flow rate used is an important aspect of this separation, and comparative linear flow rates must be used. Faster flow rates lead to peak broadening and increase in tailing and heading. An increase in flow rate from 1.0 to 1.5 mL/min for the large columns has quite an appreciable effect. Separations are quite satisfactory and quantitative under optimum conditions. When 75.94 mg of samarium was separated from 69.45 mg of neodymium (Table I), 8 f 4 pg of samarium or 0.01% was found with the neodymium and 0.4 f 0.1 wg of neodymium or 0.0005% with the samarium using spark source mass spectrometry. Whenever a separation of the lanthanides into two groups is required to obtain a better accuracy and sensitivity of results as in neutron activation or X-ray fluorescence work for the analysis of geological samples, this method should be useful. It is as good as or better than the best separation using organic complexing agents such as hydroxyisobutyric, lactic, or citric acid buffers or EDTA for this purpose and has the advantage that the eluting agent can be easily removed by evaporation.
LITERATURE CITED (1) Danon, J. J. Inorg. Nucl. Chem. 1958, 5 , 237. (2) Ichikawa. F.; Uruno, Sh.; Irnai, H. Bull. Chem. Sm. Jpn. 1981, 3 4 , 952. (3) Faris, J. P.; Buchanan, R. F. Anal. Chem. 1984, 36, 1157. (4) Edge, R. A. J. Chromatogr. 1981, 5, 526. (5) Edge, R. A. Anal. Chim. Acta 1963, 29,321. (6) Faris, J. P.; Warton, J. W. Anal. Chem. 1982, 3 4 , 1077. (7) Korkisch, J.; Tera, F. Anal. Chem. 1981, 33, 1264. (8) Korkisch, J.; Hagan, I.; Arrhenius, G. Talanta 1963, 10, 865. (9) Roelandts, I.; Duyckaerts, G.; Brunfelt, A. 0. Anal. Chim. Acta 1974, 73, 141. (IO) Desai, H. B.; Krishnamoorthy Iyer, R.; Sanakar Das, M. Talanta 1984, 1 1 , 1249. (11) Alstad, J.; Brunfelt, A. 0. Anal. Chim. Acta 1987, 38, 185. (12) Brunfelt, A. 0.; Steiness, E. Analyst (London) 1989, 94,979. (13) Savage, D. J.; Drummond, J. L. TRG Report 1496D; United Kingdom Atomic Energy Authority: April 1967; AERE 20. (14) Marsh, S. F.; Abernathey, R. M.; Rein, J. E. Paper LA-5568; Los Alamos, NM, 1974. (15) Ramakumar, K. U.; Raman, V. A,; Khodade, P. S.; Jain, H. C. India, A. E. C., Bhabha At. Res. Cent. [ R e p ] I 9 7 9 * B.A.R.C. 1006. (16) Larsen, N. R.; Pedersen, W. 6. J. Radioanal. Chem. 1978, 45, 135. (17) Larsen, N. R. J. Radioanal. Chem. 1979, 52,85. (18) Raaphorst, J. G.; Haremaker, H. J. Radioanal. Chem. 1979, 53, 71. (Taipei) 1972, 79,93. (19) Chen, Y. M.; Lias, S. C. J . Chin. Chem. SOC. (20) Strelow, F. W. E.; Jackson, P. F. S. Anal. Chem. 1974 46, 1481.
F. W. E. Strelow National Chemical Research laboratory P.O. Box 395 Pretoria 0001, Republic of South Africa RECE~VED for review April 24,1980. Accepted August 25,1980.
Nonflame, Source-Induced Sulfur Fluorescence Detector for Sulfur-Containing Compounds Sir: The flame photometric detector (FPD) is currently the most commonly used physiochemical transducer for the determination of gaseous sulfur-containing compounds, both 0003-2700/80/0352-2422$01.00/0
as an atmospheric continuous monitor and as a selective gas chromatographic detector (1-4). The response of the F P D in the sulfur detection mode arises from the chemilumines1980 American Chemical Society
