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isolated, 30 pm diameter particle of NH4NOBis ca. 0.024 pg, roughly 1% of the mass of a 2.2-pg crust. Such small, isolated particles might be more easily removed from the film surface by the shock wave following the explosion than the thicker crust studied here. It thus appears that while electrically vaporized thin film spectrometry is a promising method for direct analysis of solid powders, the problem of vaporizing crusty residues from aqueous solutions is a major one for the analysis of high-salt solutions. There are three possible remedies for the loading effect problem. One is the addition of reagents, possibly 2-propanol, to lower the surface tension of water droplets and allow them to spread more widely, leaving thinner crusts which can be more effectively ablated by the plasma. The second, use of an internal reference, has been used successfully (6). The third is preconcentration/separation of trace metals using an anion exchanger, an approach whose utility in a sodium chloride matrix also has been demonstrated (3).
LITERATURE CITED (1) Clark, E. M.; Sacks, R. D. Specfrochim. Acfa, Part 8 1080, 358, 471-488. (2) Goldberg, J.; Sacks, R. Anal. Chem. 1082, 54, 2179-2186. (3) Brewer, S. W.; Sacks, R. D. Anal. Chem. 1085, 57, 724-729. (4) Duchane, D. V.; Sacks, R. D. Anal. Chem. 1078, 50. 1752-1757. (5) Holcornbe, J. A,; Brinkrnan, D. W.; Sacks, R. D. Anal. Chem. 1075, 4 7 , 441-446. (6) Duchane, D. V.; Sacks, R. D. Anal. Chem. 1078, 5 0 , 1765-1769.
Stephen W. Brewer* Chemistry Department Eastern Michigan University Ypsilanti, Michigan 48197
Philip T. Fischer Richard D. Sacks Department of Chemistry University of Michigan Ann Arbor, Michigan 48109
ACKNOWLEDGMENT Photomicrographs were taken by L. Marcotty, W. Wagner, and B. Winter of the Electron Microbeam Analysis Laboratory a t the University of Michigan-Ann Arbor. Registry No. In, 7440-74-6; Mo, 7439-98-7; Cd, 7440-43-9; Al, 7429-90-5; Ag, 7440-22-4; Au, 7440-57-5; NHINOS, 6484-52-2; Li,S04, 10377-48-7.
RECEIVED for review January 14,1985. Resubmitted May 13, 1985. Accepted May 30,1985. This work was supported by the National Science Foundation through Grants CHE 761164646A01and CHE 78-25542 and by the Graduate School of Eastern Michigan University through the Research and Sabbatical Leaves Program.
Separation of Platinum Group Metal Ions by Donnan Dialysis Sir: The Donnan dialysis of metal ions across cation-exchange membranes has been used to accomplish preconcentration, matrix normalization, speciation, and separation (1-7). In terms of practical application of this method, the most interesting characteristic is that the composition of the receiver electrolyte strongly influences the chemistry at the Sample/membrane interface. This is probably the result of incursion of ions from the relatively concentrated receiver electrolyte into the ion-exchange membrane. It is therefore possible to perform dialyses from a wide variety of samples in a controlled manner without using any chemical modification of their matrices. An extension of the above observation is that separations of metal ions on the basis of Donnan dialysis across anionexchange membranes should be possible if the receiver electrolyte composition favors the formation of selected anionic complexes of the sample metal ions. Moreover, such a separation has the possibility of being better suited for some applications than batch or column experiments with anionexchange resins. For example, if a member of the test set is strongly attached to anion-exchange sites, the fiial distribution can involve three phases: ions remaining in the sample, ions attached to the membrane, and ions Donnan dialyzed into the receiver electrolyte. In the present paper the above hypotheses are tested on the platinum-group metal ions, Pt(IV), Rh(III), Pd(II), Ir(III), and Ir(1V). Separation of these ions as a set from other species as well as from each other is typically performed by ion exchange on the basis of their tendency to form anionic halo complexes (8). A complexing agent can be added to vary their affinities for the anion-exchange resin (9). The separation of a set of these ions is considered one of the more difficult problems in inorganic separation chemistry (8). The distribution coefficients of these species between anion-exchange resins and solutions containing high concentrations of chloride or bromide indicate that a chromato-
graphic procedure should be simple (10, 11);however, for several reasons, this does not turn out to be the case. Chemical equilibria involving these species are very complicated (12), stable compounds seem to be formed on the anion exchanger (10, II), and elution of some of the platinum-group metal complexes is highly asymmetric (11). Rh(II1) apparently retains some cationic character even in concentrated chloride media (12). These problems complicate the usual ion-exchange separation procedures; however, they make the system a useful one for a test of the Donnan dialysis separation model.
EXPERIMENTAL SECTION The anion-exchange membranes were type P-1035 (RAI Research Corp., New York). They were pretreated in 1M HC1 prior to initial use. Before each experiment they were soaked in a solution of the same composition as the receiver electrolyte for 1 h. The dialysis cells were cylindrical Teflon tubes. Comparable to a cell we reported elsewhere (13),the membrane (2.5 cm diameter) was held in place across one end by a threaded Teflon sleeve. The receiver electrolyte (5 mL) was pipetted into the dialysis cell. To initiate the experiment, the membrane face of the cell was placed in contact with a magnetically stirred, 50-mL sample. The dialysis time was 1h. After dialysis, the receiver chamber contents were transferred to a 10-mL volumetric flask, and the residual sample was transferred to a 100-mL volumetric flask. Both were diluted to volume with water. The concentrations of the platinum metals were determined by atomic absorption spectrometry (Beckman Model 1272 with a graphite furnace atomizer, Pye Unicam GRM Model 1268). The sample concentrations were the M; following: Pt, 6.2 X low5M; Pd, 1.1 X lo-* M; Rh, 1.4 X M. and Ir, 5.4 X All chemicals used were Reagent Grade. All solutions were prepared by using doubly distilled water. The solutions of Rh, Pt, Pd, and Ir were prepared from the chlorides of these metals (Koch Light Laboratories). The Ir and Pt salts were hexachlorides of the metals, and the Rh and Pd solutions were prepared in the presence of HCI (9);hence, in the samples, the metals were all
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Table I. Effect of the Receiver Electrolyte Anion on the Donnan Dialysis of Platinum-Group Metal Ions
receiver"
Rh(II1)
NaC10, MgSO4 MgCh KCl NHdCl
0.06 0.3 0.5 0.6 0.5
enrichment factorbVc Ir(II1) Ir(1V) Pd(I1) 0.2 0.7 1.6 2.3 3.6
0.3 0.3 0.9 1.0 1.4
0.07 0.05 0.6 0.05 0.4
dialysis time, min
Pd(I1)
Rh(II1)
Ir(II1)
Pt(1V)
0.00 0.08 0.2 0.05 0.1
40 60 80 120 180
88 79 66 50 24
94 87 81 77 74
81 65 51 40 18
78 60 48 34 17
Table 11. Partitioning of the Platinum-Group Metal Ions onto the Ion-Exchange Membrane Phase with Different Receiver Electrolytes % of the metal on the membrane after
NaClO, MtZSO4 MgClz KCl
NHdCl
Rh(II1) 1.6 2.1 0.8 4.2 4.0
Donnan dialvsisbvc Ir(II1) Pd(I1) 13 9.2 7.3 1.0 0.0
16 12 19 12 14
% of the stated metal retained
Pt(1V)
All receivers, 0.5 M. Enrichment factor is defined as the concentration of the metal in the diluted receiver after dialysis divided by the initial samde concentration. cDialvsistime. 1 h.
receiverD
Table 111. Retention of the Platinum-Group Metal Ions in the Sample during Donnan Dialysisa
Pt(IV) 27 17 29 24 38
Concentration, 0.5 M. bDeterminedby difference between the initial concentration and the final solution-phase concentration. cDialysistime, 1 h. to an extent in the form of anionic chloro complexes. When Ir(1II) was used, hydroxylamine was included to prevent oxidation; it did not alter the Donnan dialysis behavior of any of the metals studied.
RESULTS AND DISCUSSION Even though the test metals were primarily in an anionic form in the sample, the presence of chloride in the receiver electrolyte should influence the Donnan dialysis across an anion-exchange membrane if the earlier-mentioned hypothesis is correct. This point is demonstrated by the data in Table I. For example, with MgClz rather than MgS04 as the receiver, the Donnan dialysis of all species was increased although relatively small amounts of Rh(III), Pd(II), and Pt(1V) entered the receiver electrolyte compared to Ir(II1) and Ir(1V). With any of the chloride-containing receiver electrolytes, it is apparent that Ir can be selectively preconcentrated, particularly if it is first reduced to Ir(II1). It should be noted that varying the receiver concentrations in the range 0.1-1.0 M had an insignificant effect on the relative results so the remainder of the study employed 0.5 M receivers. The second part of the separation procedure is to consider the amounts of the various metals that are partitioned onto the membrane phase. As shown in Table 11, all of the metals are retained to some degree on the membrane, but Pd(I1) and Pt(1V) are especially partitioned onto that phase. In addition, Ir(1V) is significantly retained on the membrane; for example, with MgC1, and NH&l receivers, 22% and l l % , respectively, of the total quantity are on the membrane after a 1h dialysis. This supports the earlier suggestion that a separation scheme should include reduction of all Ir species to Ir(II1). From the data in Tables I and I1 it is apparent that certain separations can be performed with, for example, the 0.5 M NH4Cl receiver electrolyte. The preconcentration ratios of Ir(II1) into that solution relative to Rh(III), Pt(IV), and Pd(I1) are 7.2, 36, and 9, respectively. Hence, that metal can be separated quite well from the others unless they are in large excess in the sample. Likewise, either Rh(II1) or Pt(1V) can be separated to some extent from Pd(I1).
Receiver electrolyte, 0.5 M NH4C1. Table IV. Separation of Mixtures by Donnan Dialysis"
sample component Pt(1V) Rh(II1) Ir(II1) Pd(I1)
results with Pd(I1) absent (present) % retained on EF the membrane 0.1 (0) 0.5 (0.5) 3.0 (1.9) (0.4)
39 (30) 3 (2) 0 (0) (13)
"Receiver, 0.5 M NHaCl at pH 2; dialysis time, 1 h. By recovering the metals from the membrane after the dialysis, Pt can be enriched relative to the other metals. Assuming equal amounts of the metals in the sample, the relative enrichments of Pt vs. Rh and Pd are by factors of 9.5 and 2.7, respectively. No Ir(II1) was found on the membrane phase. In this regard it is important to note that Rh(III), Pd(II), and Ir(II1) can be recovered from the anion-exchange membrane after the dialyses by soaking it in 1M HC1 for 1 h, Pt(1V) is recovered by soaking the membrane in 0.5 M NaCl for 3 h, and Ir(1V) is recovered by soaking it in 0.5 M NaCl and 1 mM NHzOH for 3 h. Because Rh(II1) neither Donnan dialyzes to a great extent nor partitions onto the membrane extensively, one might conclude that it can be enriched relative to the other metals by simply recovering the residual sample. By use of the data in Tables I and 11, a mass balance calculation based on an assumption of equal initial concentrations does not support this inference. The ratios of Rh to the other metals when a 0.5 M NH4Cl receiver electrolyte is used are the following: Rh/Ir, 3.1; Rh/Pd, 1.1; and Rh/Pt, 1.4. When the dialysis is continued for a longer time, the selective retention of Rh(II1) in the sample does become apparent. Table I11 summarizes the quantities of the platinum-group metals retained in the sample chamber as a function of dialysis time. After 3 h, the residual sample concentration of Rh(II1) is at least %fold enhanced relative to the other metals. These data are consistent with the general observation that Rh(II1) has more cationic character in halide media than the other metals in this set. In the above experiments, each sample only contained a single metal. Table IV summarizes the results obtained when actual mixtures were dialyzed. When Pd(I1) is absent, the results are nearly the same as those found with single-component samples (Tables I and 11). With Pd(I1) included, about 20% less Pt is retained on the membrane, and the E F for Ir(II1) is decreased from 3.6 to 1.9. Nevertheless, the general predictions from Tables 1-111 are followed. A procedural difference in the Table IV experiments is that hydroxylamine was not added to suppress air oxidation of the Ir(II1). This may account in part for the lower EF observed for that metal. Although the separation factors obtained in this study are not dramatic, the data demonstrate the potential of Donnan dialysis as a means of partitioning a set of analytes among three phases. By variation of the sample matrix such as by
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Anal, Chem. 1985, 57, 2405-2407
adding another complexing agent and controlling the pH (9), better separations could undoubtedly be realized. Work on this approach is in progress. In addition, the data provide further evidence that the receiver electrolyte composition has a major influence on the chemistry at the sample-membrane interface during Donnan dialysis.
LITERATURE CITED cox, J. A.; DiNunzio, J. E. Anal. Chem. 1077, 4 9 , 1272-1275. cox, J. A.; Twardowski, Z. Anal. Chem. 1080, 5 2 , 1503-1505. Cox, J. A.; Twardowski, Z. Anal. Chlm. Acta 1080, 119, 39-45. Cox, J. A.; Olbrych, E.; BraJter, K. Anal. Chem. 1081, 53, 1308-1309. (5) Wilson, R. L.; DiNunzio, J. E. Anal. Chem. 1081, 53, 692-695. (6) DuNunzb, J. E.: Wllson, R. L.; Gatchell, F. P. Talanta 1083, 3 0 , 57-59. (7) Cox, J. A.; Slonawska, K.; Gatcheil, D. K.; Hiebert, A. G. Anal. Chem. 1084, 56, 650-653. (8) Minczewski, J.; Chwastowska, J.; Dybczynski, R. “Separation and Preconcentration Methods in Inorganic Trace Analysis”; Ellis Horwood Limited: West Sussex, England, 1982; p 466. (9) Brajter, K.; Slonawska, K. Talanta 1083, 3 0 , 471-474.
(1) (2) (3) (4)
(10) Berman, S. S.; McBryde, W. A. E. Can. J . Chem. 1058, 36, 835-844. (11) Dybczynski, R.; Malcszewkl, H. J . Redloanal. Chem. 1074, 2 7 , 229-245. (12) MacNevin, W. M.; McKay, E. S. Anal. Chem. 1057, 2 9 , 1220-1223. (13) Cox, J. A. Tanaka, N. Anal. Chem. 1085, 5 7 , 385-387.
Krystyna Brajter Krystyna Slonawska Department of Chemistry University of Warsaw Warsaw, Poland 02-093
James A. Cox* Department of Chemistry and Biochemistry Southern Illinois University Carbondale, Illinois 62901
RECEIVED for review April 1, 1985. Accepted June 7, 1985. This work was supported in part by the National Science Foundation under Grant CHE-8215371 to J.A.C.
Method for Determining the Association of Plastocyanin with Tris( 1,IO-phenanthroline)iron(I I) Using an Amberlite XAD-2 Resin Column Sir: The Hummel-Dreyer method ( I ) has been successfully applied to studying the association of macromolecules with low-molecular-weight molecules; we will hereafter call the former “host” and the latter “guest”. The method is based on the sieving action of a gel column; host molecules are excluded from the gel phase, while guest molecules are more or less free to enter the interior, and hence they are retarded. An eluent contains the guest of interest of a known concentration and a suitable electrolyte to overcome the Donnan effect. A portion of the host solution, which has been prepared in the eluent, is chromatographed on the gel column. If there is any association between the host and the guest, a pair of positive and negative peaks appears as a change in the background guest concentration. An essential requirement of the method is that host molecules must not be adsorbed onto the gel matrix. On the other hand, the method can be applied in such a case that the retardation is due to adsorption of guest molecules by the gel phase rather than by sieving (2, 3). The present study was undertaken to determine whether i t is possible to estimate association of host molecules with guest ones in such an extreme situation when the latter are too strongly adsorbed by a column packing to emerge from the column. Amberlite XAD-2 resin beads are used as the packing. The resin, which is made of styrene-divinylbenzene copolymer, exhibits a great tendency to adsorb hydrophobic substances on the surface. It has been demonstrated that the XAD resins act as reversed stationary supports in HPLC ( 4 ) and that enhanced retention is observed for various organic ions when suitable counterions such as tetraalkylammonium ions are present in the mobile phase (5). The mechanism, though not yet well elucidated, is explained on the basis of an electrical double layer, an ion exchange, or an ion-pair model (5, 6). The present XAD column procedure, in contrast to gel filtration, makes use of a solution as an eluent which contains the host of a known concentration and with which a sample containing the guest is prepared and carried into the column. Although any guest molecules present in the sample are not 0003~2700/85/0357-2405$0 1.50/0
eluted out of the column in the present technique, the term “eluent” is used for convenience. As a hydrophilic host, plastocyanin, a blue copper protein (mol wt. 10 500) with 99 amino acids, was used (abbreviated as PCu” or PCu’ according to its oxidation state). As a hydrophobic guest, redox inert tris(1,lO-phenanthroline)iron(II) complex was used. The Hummel-Dreyer method has also been applied to the same host-guest. It is of interest to compare the results from the two different techniques.
EXPERIMENTAL SECTION Plastocyanin was isolated from parsely leaves by a method described in the literature (7). The yield of the protein was ca. 50 mg from 10 kg of cut leaves. An absorption ratio at peak positions A27elA,97was 1.78, which is in a good agreement with the literature. The PCu” has t = 4.5 X lo3M-l cm-l at 597 nm, which was used to determine the protein concentrations. The PCu’ has a similar UV spectrum but no visible absorption. Protein solutions were dialyzed against required solutions for 24 h at ca. 0 “C (PCu’ solutions were prepared by adding a trace of ascorbic acid to PCu” to ensure complete reduction before dialysis). Tris(1,lO-phenanthroline)iron(II)was prepared as the chloride (8). A 0.01 M phosphate buffer solution, salt, [Fe(phen)3]C12.7H20 Na2HP04+ NaH2P04,pH 7.5, was used in preparing sample and eluent solutions and in column operations. Amberlite XAD-2 resin, 20-50 mesh (Rohm and Haas), was ground in an agate mortar, a 150-200 mesh portions was collected and washed successivelywith methanol by decantation until the supernatant became clear. The methanol-slurried XAD beads were packed in a tube made of Teflon (1 mm X 8 cm) up to ca. 3 cm. This was used as a column, the one end being attached to a rotary sample valve injector (Tokyo Rika, VMU-6 type) and the other to a line filter (Kyowa Seimitsu, KLF-U Type). The outlet of the column was connected through another tube made of Teflon (1 mm X 20 cm) to a flow cell of 10 mm optical path length incorporated into a JASCO type UVIDEC-100-111spectrophotometer. Before use, the column was throughly washed with pure water, subsequently M PCu” phosphate buffer with phosphate buffer. A 1.1X solution was eluted through the column at a flow rate of 0.632 mL min-l, and the absorbance at 597 nm was recorded by a 0 1985 American Chemical Society
