toxic levels are detectable. The limit of detectability is generally assumed to be three times the standard deviation of the background. Using our results, one can infer the detectability limits as a function of atomic number, and these results are shown in Figure 7. By increasing the bombarding time by a factor of four, the detectability limit will be improved by a factor of two. An important facet of any measuring technique is the time needed for analysis. The cleaning and weighing of the beakers (twice) takes about 1 man-hour for 30 samples. The ashing time is about 72 hours; but although we worked with 30 samples a t a time, t h e asher would easily handle 100 samples, and larger ashers are available. Further. the asher loading time was only a few minutes. The Formvar backings could be produced quite rapidly. One man-hour per 60 foils is easily achieved. The production of targets from the ash takes about 2 minutes apiece, not counting the drying time of about 3 hours. The largest expenditure of time per sample was in the actual beam irradiation. We used bombardment times of 500 seconds. Although the beam cannot be increased without endangering the samples, decreasing the accumulation time by a factor of two would increase the relative standard deviations for Fe. Zn, and Rb to about 7, 7, and 2770, respectively. The precision of the Fe and Zn measurements are limited by target inhomogeneities and beam profile variations; the Rb results are primarily determined by counting statistics. It seems, then, that one person could perform a t least 40 assays per %hour day under production conditions.
CONCLUSIONS This work shows that proton-induced X-ray emission analysis of whole blood samples is capable of precision and accuracy. The spectrum is not particularly rich because of the dominance of the Fe in the spectrum and the necessity of its strong attenuation which also attenuates other heavier elements but not as severely. It is reasonable to assume that developmental efforts similar to this one for measurements on other specific samples should meet with a t least equal success.
ACKNOWLEDGMENT We are deeply indebted to a number of people whose enthusiastic cooperation made this work possible. We particularly thank Robert Keepin for his continued interest and support of this work. We thank Evan Campbell for several stimulating discussions and Pat Stein for the comparison measurements using AAS. Jean Lindsey cheerfully supplied many gallons of redistilled water. We thank Marty Holland and Jerry London for providing the mouse samples, and Joe Tafoya for providing the human blood samples. We also tender our appreciation to Colleen Burns for sample and foil preparation and to Ed Adams for his efficient Van de Graaff operation. One of us (R.C.B.) wishes to thank the staff of the Los Alamos Scientific Laboratory for their generous hospitality. Received for review August 10, 1973. Accepted November 15, 1973. Work performed under the auspices of the U.S. Atomic Energy Commission.
Substoichiometric Extraction of Cations with Mixtures of Hexafluoroacetylacetone and Tri-n-Octylphosphine Oxide in Cyclohexane J. W . Mitchell Bell Laboratories, Murray H i / / . N . J . 07974
Roland Ganges Stanford University. Stanford. Calif.
A new method has been developed for substoichiornetric extraction of cations by adduct-reactions in the presence of excess hexafluoroacetylacetone (H HFA) and of substoichiometric quantities of the neutral donor, tri-n-octylphosphine oxide (TOPO), in cyclohexane. The equilibria of adduct-extraction systems are discussed to show advantages over the conventional approach of reacting cations with substoichiometric amounts of chelating ligands. Distribution curves for the substoichiornetric extraction of Coz', C u z - , F e z + , M n " , Z n 2 - , F e 3 + , E u 3 r , and Lu3+, have been measured, and experimental results with relative standard deviations of 0.92, 1.8, and 3.1% tor the substoichiometric isolation of Z n z + , Cu", and Eu3+ are reported. The general utility of this extraction system for substoichiometric separation of cations following neutron activation is discussed.
Recently the utility of methods of separation in nuclear analyses has been enhanced by the development of sub-
st'oichiometric methods, which extend the application of radioisotope dilution to the determination of trace elements and also make it possible to perform quantitative neutron activation analyses without measurements of radiochemical yield. Such methods have been developed rapidly and applied frequently since Ruzicka and Stary first summarized their techniques in 1968 (1).In a second review, these authors discussed methods published during the period 1968 to 1970 ( 2 ) . Since this review, several additional solvent extraction methods have been reported (3-16). A general discussion of substoichiometric methods ( 1 ) J Ruzicka and J. Stary, "Substoichlometry in Radiochemical Analysis." Pergamon Press, New York, N . Y . , 1968. (2) J. Stary and J. Ruzicka, Talanta. 18, 1 (1971). (3) F. Kukula and M. Simkova, J. Radioanal. Chem., 4, 271 (1970). (4) G . N. Btlimovich and N. N Churkina, J. Radioanal Chem.. 8, 53 (1971). (5) U V. Yakovlev and R. V . Stepanets. Zh. Anal. Khim.. 25, 578 (1970). (6) R. A . Nadkarni and B. C. Haldar. Anal. Chem., 44, 1504 (1972) (7) B M . Tejam and B. C Haldar, Radiochem. Radioanai. Lett.. 9 , 19 ( 1972). A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 4, A P R I L 1974
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for multielemental separation by metal chelate extractions (17),a report on the selectivity of substoichiometric extraction methods (18), and a general review of the substoichiometric approach in analysis have also been published (19). Most previous investigators have used substoichiometric quantities of chelating (1, 2) or ion-associating agents (20) to reproducibly isolate and then determine cations or anions by neutron activation (N.A.), or by radioisotope dilution (I.D.). Although numerous reagents for extraction are available (21, 22), applications for substoichiometric separations have been limited either by insufficient stability of the corresponding metal complexes or by lack of stability of the extractant a t low concentrations. The present authors report a new method, based on extraction of cations in the presence of an excess amount of a chelating ligand, hexafluoroacetylacetone (HHFA) and a substoichiometric quantity of a neutral donor, tri-n-octylphosphine oxide (TOPO). Particular advantages of this system are described for applications in activation analysis.
EXPERIMENTAL Tri-n-octylphosphine oxide was obtained from Eastman Kodak and used without further purification. Hexafluoroacetylacetone from Peninsular Chemresearch, Inc. was freshly distilled before use. Reagent-grade cyclohexane and buffer solutions, prepared by mixing appropriate amounts of reagent-grade sodium acetate and acetic acid, were used. Except for 64Cu which was prepared by irradiation of nonactive carrier with thermal neutrons a t the Industrial Reactor Laboratory in Plainsboro, N.J., t h e radioisotopes, 59Fe, 'j0Co, 65Zn, 54Mn, 51Cr, 152$154Eu, and 177Lu were purchased from commercial suppliers, Stock solutions of the corresponding nonactive cations were prepared by dissolving either pure salts or accurately weighed amounts of the highly pure metals. Solutions prepared from salts were standardized subsequently by EDTA titrations. P r e p a r a t i o n of Aqueous a n d O r g a n i c Phases. Method I. One hundred milliters of sodium acetate-acetic acid buffer (initial p H 6.37) was equilibrated in a separatory funnel with an equal volume of 0.07M HHFA in cyclohexane. The solution of HHFA was prepared by pipetting 1.0 ml of freshly distilled reagent (pre-equilibrated at 25 f 2") into a 100-ml volumetric flask containing solvent and then diluting t o volume. After the mixture was shaken for five minutes, the aqueous ( p H 4.75) and organic phases were separated and used as described below. In experiments for measuring the substoichiometric distribution of cations u s . pH, a series of aqueous phases were prepared by mixing aliquots of stock cation solutions a n d appropriate amounts of corresponding radiosotopes and diluting t o near volume (9.5 ml) in 10-ml volumetric flasks with portions of buffer solution. The p H of each solution was then adjusted to t h e desired value by dropwise addi(8) R. A . Nadkarni and B. C. Halder, Radiochem. Radioanai. Left., 11, 237 ( 1 9 7 2 ) . (9) W . J . Zmijewska, J. Radioanai. Chem., 10, 187 ( 1 9 7 2 ) . ( l o ) B. M . Tejam and B. C Haldar, Radiochem. Radioanai Lett.. 9, 7 7 (1972). ( 1 1 ) R. A . Nadkarni and B. C. Haldar. J. Radioana/. Chem.. IO, 181 (19 7 2 ) . ( 1 2 ) R . A . Nadkarni and E. C. Haldar. Radiochem. Radioanal. Lett.. 9, 205 ( 1 9 7 2 ) . ( 1 3 ) Z. K . Doctor and B. C. Haldar. J . Radioana/. Chem.. 9, 19 ( 1 9 7 1 ) . ( 1 4 ) E. M . Tejam and B. C. Haldar. Radiochem. Radioanal. Lett.. 9. 189 (1972). ( 1 5 ) R . A . Nadkarni and E. C Haldar, Radiochem. Radioanai. Lett.. 8, 341 ( 1 9 7 1 ) . ( 1 6 ) R . A . Nadkarni and B. C. Haldar, J. Radioanal. Chem.. 8, 4 5 (1971). (17) A . Elek. J. Bogances and E Szabo, J. Radioanai. Chem.. 4. 281 (1 9 7 0 ) . (18) G A . Perexhogin, Zh. Anal. Khim.. 25, 1245 (1970) (19) N. K . Baishya. R. B. Heslop. and J. R.DeVoe. Crit. Rev A n a / Chem.. 2, 345 (1971) 1201 I . P Alimarin and G . A . Perezhoain. Talanta. 1 4 , 109 (1967) i21) G . H . Morrison and H . FreisG, "Solvent Extraction in Analytical Chemistry,"John Wiley and Sons, lnc.. New Yolk N . Y , 1957. ( 2 2 ) J . Stary. "The Solvent Extraction of Metal Chelates," pergamon Press Inc . New York. N . Y . , 1964
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tion of 8.OM HCI or 1.OM N H 4 0 H . T h e resulting solution was diluted to volume by adding 0.2 to 0.4 ml of deionized HzO. In these experiments, the amount of cation present initially in t h e aqueous phase was selected to approximate t h e concentration expected in typical activation analyses, where at least 1- to 10-mg of nonactive carrier are added during the dissolution of samples following neutron irradiation. The organic phase was prepared by pipetting appropriate volumes of 0.1M TOPO-cyclohexane from a stock solution into a 50-ml volumetric flask and by diluting to volume with a portion of the organic phase which had previously been pre-equilibrated with the acetate buffer as described earlier. The initial concentration of T O P O in the organic phase was selected to he 25 t o 50% of the amount necessary for complete extraction of carrier from the aqueous phase. Method I l . As an alternative method, organic phases were prepared as follows: 0.5 ml of HHFA, pre-equilibrated a t 25', and aliquots of 0.10M stock solution of T O P O were pipetted into a 50-ml volumetric flask and diluted t o volume with cyclohexane. Corresponding aqueous phases were prepared by adding carrier and tracer and diluting t o volume with a series of acetate buffers covering the desired p H region. In this case no previous equilibration of buffers with 0.07M HHFA in cyclohexane was made. Five-ml volumes of aqueous and organic phases were transferred to 15-ml screw-cap centrifuge tubes, sealed with a polyethylene cover and capped securely. Following 30 t o 60 minutes of equilibration on a wrist-action shaker, each tube was centrifuged to facilitate separation of phases. After 2- to 4-ml aliquots of the organic and aqueous phase were pippetted into polypropylene and polystyrene vials. respectively. gamma-ray activities from each phase were measured by counting in a 3-in. X 3-in. well-type NaI detector connected to a 1024 multichanneled analyzer. The equilibrium p H of the aqueous phase was then measured with a combination-electrode and meter. R e a g e n t Stability. Five-ml aliquots from a set of aqueous and organic phases prepared by Methods I and I1 were pipetted from stock solutions, transferred into centrifuge tubes, equilibrated until equilibrium was attained, and centrifuged. After this the y-ray activity of each phase was measured. At regular intervals. the entire procedure was repeated until the original solutions had aged for several hours. Substoichiometric Extraction of 64Cu from SiOz. A set of 0.5-gram samples of ultrapure SiOz (triplicate analyses by N . A. showed 0.037, 0.059. and 0.012 ppm Cu) were irradiated independently for 20 minutes in a flux of thermal neutrons at loL3n/cmz sec along with standard solutions of copper (0.535 X 10- gram Cu/mlj. Following the irradiation, 1 ml of the 64Cu solution was added to two platinum crucibles in which 1 mg of Cu carrier, 10 ml of H F , 1 ml of 1:l "03, and 1 ml of HClO4 were added. After quantitatively transferring the irradiated sample of Si02 to one crucible. an equal quantity of nonirradiated Si02 was added to the other. Both mixtures were then heated on a hot plate to dissolve Si02 and fumed until complete evaporation occurred. The resulting residue was dissolved in 3 ml of acetate buffer that was pre-equilibrated with "FA-cyclohexane and the solution was transferred quantitatively t o a 60-ml separator!. funnel by rinsing the crucible with additional portions of the buffer solution. The p H of this aqueous phase with a total volume approximately equal to 10 ml was adjusted to a value greater t h a n 4.8 by appropriate addition of 'HC1 or N H 4 0 H . A 10-ml portion of 0.001M T O P O in cyclohexane was added and copper substoichiometrically extracted by shaking for 15 minutes on a wristaction shaker. In some experiments. the mixture of acids and nonirradiated SiOz was not added. The solutions containing only Cu carrier and standard 64Cu were then mixed directly with buffer and extracted with the substoichiometric reagent. In similar experiments. 0.5-pg amounts of Zn. Co, Mn, and Eu tagged with the corresponding isotopes, e5Zn, 60C0, 54Mn. and lS4Eu, were added along with 10 mg of Cu carrier. In this case, a preliminary extraction with 4 ml of 0.005M TOPO-cyclohexane was performed prior to the substoichiometric extraction of 64Cu with 5 ml of 0.01M TOPO-cyclohexane.
RESULTS AND DISCUSSION The paramount importance of the stability of the chelate co&plex in determining the applicability of a reagent for substoichiometric separations by solvent extraction has been discussed previously ( 1 ) . Although extraction
methods that form highly stable chelates are very satisfactory, the number of usable chelate systems are somewhat limited. However, it is well known that few cations other than Hg2+, Cd2+, Cu2+, and ZnZ+ (with dithizone), and Fe3+ (with cupferron) form primary chelates that are more stable than a ternary or adduct-complex consisting of the cation, a suitable chelating agent and a neutral donor-molecule. For example, a comparison of extraction constants of thenoyltrifluoroacetonates (M(TTA)n) with corresponding adduct-complexes of TOPO (M(TTA)n(TOP0)m) shows orders of magnitude higher constants for the ternary complexes (23-28). While available data on extraction- or formation-constants for other ternary complexes of cations with acidic, fluorinated, beta-diketones and TOPO are scarce, metal chelates (MAn) formed by complexation with ligands more acidic than HTTA, particularly hexafluoroacetylacetonates, are expected to react very strongly with TOPO (29). Chelates of hexafluoroacetylacetone, M( HFA)n, form more stable adducts with tri-n-butylphosphate (TBP) than corresponding M(TTA)n chelates as indicated by adduct-constants (&) for Tm(HFA)3(TBP)2 and Tm(TTA)3(TBP)2in cyclohexane; log PZ = 10.8 (30) and 8.2 (28), respectively. The constant for Lu(HFA)3(TOP0)z in benzene is reported as log /32 = 12.50 (31). Adduct-reactions in cyclohexane are stronger by a t least two orders of magnitude indicating a conservative estimate of log dz = 14.5 in this solvent (32).Inasmuch as a variety of cations are capable of forming ternary complexes that are significantly more stable than binary-chelate species, the present investigation was conducted to develop methods for substoichiometric extractions with donor molecules. Theory of Substoichiometric Extractions in Ternary, Adduct o r Synergic Systems. The extraction of a cation of charge n, (Mn-), by a substoichiometric amount of a chelating ligand (HA),represented by the general reaction
+
+
M.'+ n H A , , , e MA,, nH+ (1) where the subscript, (o), indicates organic-phase species, has been treated by Ruzicka and Stary ( 1 ) . These investigators demonstrated that the threshold pH a t which the same quantity of metal can always be extracted in such systems can be calculated by substituting values for the concentration of [MAn],, [HA],, [ M n + ]and K" a t equilibrium into the expression for the extraction constant for reaction 1. ,(I
KO
=
[MA ,,I [ H+l [Mii+][HA],;
dissolves and dissociates readily in the aqueous phase to complex the cation via the stepwise reactions Mn+ + A- 1MA(n-1' (3a) MA"-11 + A- p MA;"-? (3b)
+
MA,,l-l,n-(n-l
A-
===
MA,
i3c)
If ligand, HA, is a very poor extractant, the concentration of chelate (MAn) in the organic phase a t equilibrium will be small even though excess HA is used-ie. [MA,],
