Optimization of Experimental Conditions for Spectrofluorimetric Determination of Europium, Samarium, and Terbium as Their Hexafluoroacetylacetone-Trioctylphosphine Oxide Complexes R. P. Fisher and J. D. Winefordner Department of Chemistry, Uniaersity of Florida, Gainesville, Fla. 32601 THE USE OF /3-DIKETONE chelating agents in the spectrofluorimetric determination of trace amounts of rare earth ions has been described ( I , 2), and the basis of the enhancement of the fluorescence by synergic agents has been discussed elsewhere (3). Fluorescence analysis of the rare earths in solution by this method is potentially quite useful because the intense, line-like spectra obtained are characteristic of the metal ions themselves and are relatively independent of interferences. The purpose of this study was to investigate the effect of pH o n the fluorescence of the hexafluoroacetylacetoneltri-n-octylphosphine oxide complexes of the rare earths with the ultimate aim of optimizing experimental conditions so as to provide a useful analytical method. EXPERIMENTAL
The oxides of cerium, praseodymium, neodymium, samarium, lanthanum, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium (Rare Earth Division, American Potash and Chemical Corp., West Chicago, Ill.) were obtained in purities of 99.975 or better, except for Dy and H o which were of 99% minimum purity. Hexafluoroacetylacetone (HFA) was J. T. Baker practical grade: initial experiments were performed with HFA purified according to the extraction method of Rydberg (4, 5 ) ; this later proved unnecessary, and the practical grade H F A was thenceforth used without further purification. Tri-n-octylphosphine oxide (TOPO) was practical grade (obtained from Pfaltz and Bauer, Inc., Flushing, N. Y.).Several grades of methylcyclohexane were tried, but it was found that Eastman practical grade could be used without further purification. Stock solutions of the rare earth ions were prepared by dissolving the rare earth oxide in concentrated HCI or H N O a and diluting with sodium hydroxide solution to obtain 500 ml of 0.01M rare earth ion at approximately pH 5 . Serial dilutions of the stock solutions were made with acetic acid/sodium acetate buffer solutions at the p H of interest in the given experiment. Fluorescence spectra and analytical curve data were taken using an Aminco-Bowman spectrofluorometer (American Instrument Co., Inc., Silver Spring, Md.) equipped with a high pressure xenon arc source (powered by a Harrison 6268A DC Power Supply, Hewlett-Packard, Orlando, Fla.), a n Aminco Ellipsoidal Condensing System, and an R136 (red sensitive) multiplier phototube. Phototube power and readout was provided by an Aminco Photomultiplier Microphotometer, and spectra were recorded on an Aminco X-Yrecorder. p H measurements were made with a combination p H electrode (Model 4858 L60, A. H. Thomas Co., Philadelphia, Pa.) and a p H meter (Model LS, E. H. Sargent and Co., Chicago, Ill.) (1) R. Belcher, R. Perry, and W. I. Stephen, Analyst, 94, 26 (1969). (2) T. Shigematsu, M. Matsui, and R. Wake, Anal. Chim.Acta, 46, 101 (1969). (3) F. Halvorsen, J. S. Brinen, and J. R. Leto, J . Chem. Pliys., 41, 157 ( I 964). (4) J. Rydberg, Sc. Kern. Tidskr., 62, 179 (1950). (5) J. Stary, “The Solvent Extraction of Metal Chelates,” The Macmillan Company, New York, N. Y., 1964, p 51.
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The procedure for the preparation of the chelates was as follows: to a 15-ml glass-stoppered centrifuge tube were added 3 ml of rare-earth ion solution, 3 ml of 6 X 10-4M H F A in methylcyclohexane, and 3 ml of 0.01M TOPO in methylcyclohexane. The tube was stoppered and shaken for approximately 15 seconds, the layers were allowed to stratify (about 1 minute), and approximately 3 ml of the upper layer was poured into a 1 cm X 1 cm quartz cell. Apparent fluorescence excitation and emission spectra were obtained at room temperature, and optimum excitation and emission wavelengths were selected from these spectra. In the preparation of the analytical curves, all fluorescence intensity readings were referred to a standard chelate solution whose fluorescence intensity was measured often, so as to compensate for long-term source variation. All intensity readings were corrected for a blank, prepared by extracting 3 ml of the acetate buffer used in the given experiment with 6 ml of the HFAiTOPO solution. OPTIMUM EXPERIMENTAL CONDITIONS
As expected, only europium, samarium, and terbium yielded analytically useful fluorescent chelates. m o t e that Stanley et al. (6) have suggested that by using an instrument utilizing a monochromator and transducer capable of providing resolution and detection in the near infrared, one should be able to determine the majority of the rare earth ions by the @-diketone fluorescence method.] Several factors should be noted concerning the extraction method used in the present study: Preparation of a solid sample for analysis did not involve the evaporation of quantities of concentrated acids. The reagents could be used as obtained commercially; no further purification was necessary. Multiple extractions, although they would undoubtedly increase the efficiency of the method, were not necessary to obtain linear analytical curves and low limits of detection. The extraction time was not critical; apparently equilibrium is attained quite rapidly. Only one stock solution of chelating/synergic agent was necessary to cover the entire concentration range of the method. A p H of 3 was found to be most suitable for analytical measurements of the three rare earths. It is to be expected that there would be a pH which would yield optimum extraction of the rare earth ions by H F A ; at low pH values, hydronium ion competition with the metal ion for a n enolate ion should reduce extraction efficiency, while a t high pH, the metal ion should form hydrated hydroxides rather than chelate complexes. An experiment in which fluorescence intensity of a europium chelate was measured as a function of initial (pre-extraction) europium solution p H showed results in agreement with those of Shigematsu et al. (2)-namely, that optimum extraction (6) E. Stanley, B. Kinneberg, and L. Varga, ANAL.CHEM.,38, 1362 ( 1966).
Table I. Analytical Determination of Samarium, Europium, and Terbium. Range of analytical curve linearity, moles rare earth Xex,b xem,c ion/liter in solution Ion nm nm to be extracted 350 565 I x IO-‘ to 5 x 10-7 Samarium 360 615 1 x 10-4 to 1 x 10-9 Europium 350 550 1 x IO-‘ to 1 x 10-7 Terbium a All analytical curves were prepared using the extraction method described in text; [HFA] = 6 X 10-‘M, [TOPO] = 0.01M, pH = 3 (acetate buffer). * Approximate wavelength of excitation. c Approximate wavelength of observation of emission, chosen to facilitate the analysis of mixtures of the three ions. occurs near p H 6. However, analytical curves obtained from europium solutions at p H 6 (other conditions same as described above) showed anomalous “humps,” limiting the useful range of the method to a lower limit of about lO-7M europium. When, however, the p H was reduced to 3, the analytical curve was useful to europium ion concentrations of 10-9 moles per liter. A p H of 3 was also found to be useful for samarium and terbium.
I n Table I, the results obtained for the three rare earths which exhibited appreciable fluorescence by this method are given. The lower limits represent the lowest concentrations for which reproducible results could be obtained, consistent with the remainder of the analytical curve. Note that for all three rare earths, concentrations one to three orders of magnitude below the lower limits listed could be detected above the blank, but the intensity values were not consistent or reproducible. The precision and selectivity of measurement were similar to those found by Belcher et a f . ( I ) and Shigematsu et a/. (2). N o recovery data will be given here; however, students in a n undergraduate instrumental analysis course (during three different terms) have obtained essentially complete recovery of europium (with no apparent difficulties) in synthetic solutions with this extraction-measurement procedure.
RECEIVED for review August 12, 1970. Accepted November 16, 1970. Research was carried out as a part of a study o n the phosphorimetric analysis of drugs in blood and urine, supported by a U. s. Public Health Service Grant (GM-11373-07).
Kinetic Study of the Beckmann Rearrangement of 6 ~~-Methyl-17~~-Acetoxyprogesterone-3-0xime by Cathode Ray Polarography Arvin P. Shroff and Charles J. Shaw Analytical Research Group, Dicision of Organic Chemistry, Ortho Research Foundation, Raritan, N .J . 08869
THE BECKMANN REARRANGEMENT has offered the organic chemist a convenient method by which to introduce a nitrogen into the steroid ring system. These heterocyclic steroids have been prepared using a variety of solvents and such catalysts as tosyl chloride (Z-6), thionyl chloride (7, 8 ) , phosphorus pentachloride (1, 2), p-acetylaminobenzenesulfonyl chloride ( 4 ) , and p-aminobenzenesulfonyl chloride (9, 10). The yield of the lactam obtained by these methods has been variable because it is believed ( I 1, 12) that only the syn isomer participates in the rearrangement of the a,p-unsaturated ketoxime. The kinetics of this transformation as followed by cathode ray polarography is the subject of this paper. Previous methods of analysis in the study of rates of rearrangement of cyclic and acyclic ketoximes, involved re(1) (2) (3) (4) (5) (6) (7) (8) (9)
s. Hara, Pharm. Bull (Japan), 3, 209 (1955).
S. Hara, Yakirgaku Zasshi, 78, 1027 (1958). Ibid, p 1030. S. Kaufrnann, J . Amer. Cliem. Soc., 73, 1779 (1951). R. H. Mazur, ibid., 81, 1454 (1959). K . Tsuda and R. Hayatsu, ibid., 78, 4107 (1956). B. M. Regan and F. N. Hayes, ibid., p 639. C . W. Shoppee and J. C . Sly,J. Cltem. Soc., 1958, 3458. H. Heusser, J. Wohlfahrt, M. Muller, and R. Anliker, Helc. Chim. Acta., 38, 1399 (1955). (IO) R. Anliker, M. Muller, J. Wohlfahrt, and H. Heusser, ibid., p 1404. (11) C . W. Shoppee, M. I. Akthar, and R . E. Lack, J. Chem. Soc., 1962, 1050. (12) C . W. Shoppee, R. E. Lack, R. N. Mirrington, and L. R. Smith, ibid., 1965, 5868.
fractive index (13), gravimetric analysis (14, 15), colorimetric assay (16) and ultraviolet methods (17, 18). All of these methods, though appropriate for the particular problems, lack sensitivity or specificity or both. A literature survey indicated that only a few electrochemical reduction studies have been made with C=N in contrast to C=O compounds which have received a substantial amount of attention from the polarographers. They have indicated (19, 20) that amines are the products of reduction in protic solvents such as ethanol. The present investigation was undertaken to evaluate the applicability of cathode ray polarography in studying the kinetic transformation of 6a-methyl-17a-acetoxyprogesterone 3-oxime (MAPO) to its corresponding lactam (Scheme 1). It would offer sensitivity with concurrent selectivity when used with the appropriate supporting electrolyte. The data
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(13) D. M. Dimitrijevic and 0. K. Stejanovic, G/ns. Hem. Drirst., Beograd, 28, 353 (1963); C.A., 63, 2868(1965). (14) D. E. Pearson, J. F. Baxter, and J. C . Martin,J. OrK. Cliem.. 17, 1511 (1952). (15) D. E. Pearson and J. D. Bruton, ihid.. 19,957 (19543. (16) P. T. Scott, D. E. Pearson. and L. J. Bircher, ihid.. p 1815. (17) P. T. McNulty and D. E. Pearson, J . Amer. Cliem. Soc., 81 612 (1959). (18) N. G. Zarakhani, V. V. Budylina, and M. I. Vinnik, 2%. Fiz. Khim.,39, 1561 (1965). (19) H. Lund, Acta, Cltim. S c a d . . 13, 249 (1959). (20) P. Zuman and 0. Exner, Collect. Czech. Cltem. Commitri., 30, 1832 (1965). ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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