1314
Anal. Chem. 1985, 57, 1354-1358
easily because the column efficiency is drastically reduced in the experimental conditions of Figure 12A (flow rate 0.3 mL/min) which were selected to avoid any postcolumn split of the column effluent. On Figure 12B, the peaks of the triglycerides are barely detectable, practically lost in the large solvent peak which, on the other hand, is very small on Figure 12A. These results are in agreement with previous observations (6, 9, 20, 21) giving detection limits for triglycerides around ca. 5 pG with the RID.
CONCLUSION The combination of the uses of (i) the gradient elution technique, which is excellent for the separation of numerous components with a wide range of molecular weight, (ii) strong solvents suitable for NARP, and (iii) a sensitive and reliable detector which can analyze nonvolatile solutes in a mobile phase of rapidly changing composition without exhibiting the slightest degree of base-line drift permits new developments in the field of fat analysis. As it has been shown here, the heaviest components of oils and fats can be eluted easily. Phospholipids are also readily amenable to quantitative analysis by this method. Results with these compounds will be reported in a separate publication (23). The biosynthesis of triglycerides has now become easier to investigate. The determination of triglyceride profiles and of the distribution of known fatty acids among the different triglycerides also becomes simpler to investigate. Besides the development of quantitation and the obvious extension of this work to phospholipids, the comparison between the results of the quantitative determination of the fatty acid distribution in olive oil either obtained directly by gas chromatography or derived from the quantitative analysis of the triglycerides is under investigation. Results will be reported later. Since submission of this work, a report has been published on an independent investigation of the properties of a similar detector (24). These results are in substantial agreement with ours. ACKNOWLEDGMENT AS. thanks the Foundation SEA for a research grant. We are grateful to Guy Preau for his technical assistance in the
design and construction of the detector. We thank David Herman for fruitful discussions. We also acknowledge the gift of valuable samples of oils, fats, and authentic compounds by Andre Prevot (ITERG, Pessac, France) and the gift of the columns used in this work by M. Landriu (Laboratoires Merck-Clevenot, Nogent sur Marne, France). Registry No. Tricaprin, 621-71-6; tripalmitin, 555-44-2; triolein, 122-32-7;LaLaLa, 538-24-9; MMM, 56983-26-7;SSS, 555-43-1; EEE, 2752-99-0; BBB, 18641-57-1;PoPoPo, 20246-55-3; EiEiEi, 620-64-4;LLL, 537-40-6;LnLnLn, 14465-68-0.
LITERATURE CITED (1) James, A. T.; Martin, A. J. P. Blochem. J . 1952, 5 0 , 679-690. (2) Pelik, N. "Analysis of Lipids and Lipoproteins"; Perkins E. G., Ed.; Arnerican Oii Chemist Association: Washington, DC, 1975; Chapter 2. Traitler, H.; Prevot, A Rev. Fr. Corps Gras 1981, 28, 283-289. 1 Colin, H.: Krstulovic. A,: Excoffier. J. L.: Guiochon G. "Liouid Chromatography Literature"; Wiley: New York, 1984. Dong, M. W.; DiCesare, J. L. J . A m . Oil Chem. SOC. 1983, 60, 788-793. Lie Ken Jie, M. S. F. J . Chromatogr. 1980, 192,457-467. Perrin, J. L.; Naudet M. Rev. Fr. Corps Gras 1983, 8 ,279-287. Schulte, E. Fetfe Seifen Anstrichm. 1981, 83,289-298. Podlaha, 0.; Toregard, B. HRC CC., J . High Resolut Chromatogr. Chromatogr. Commun. 1982, 5, 553-558. Jensen, G. W. J . Chromatogr. 1981, 204, 407-416. Colin, H.; Diez-Masa, J. C.; Guiochon, G. Anal. Chem. 1981, 53, 146- 155. El-Harndy, A. H.; Perkins, E. G. J . Am. Oil Chem. SOC. 1981, 58, 867-879. Parrls, N. A. J . Chromatogr. 1978, 149,615-626. Parris, N. A. J . Chromatogr. Sci. 1979, 17, 541-545. Stoiyhwo, A.; Colin, H.; Guiochon, G. J . Chromatogr. 1983, 265, 1-18. Stolyhwo, A.; Colin, H.; Martin, M.; Guiochon, G. J . Chromatogr. 1984, 288, 253-275. Charlesworth, J. H. Anal. Chem. 1978, 5 0 , 1414-1420. McRae, R.; Dick, J. J . Chromatogr. 1981, 210, 138-145. Tchapia, A,; Colin, H.; Guiochon, G. Anal. Chem. 1984, 5 6 , 621-625. Goiffon, J. P.; Rerniniac, C.; Olle, M. Rev. Fr. Corps Gras 1981, 4 , 167- 178. Goiffon, J. P.; Rerniniac. C.; Furon, D. Rev. Fr. Corps Gras 1981, 5 , 199-208. Jandera, P.; Churacek, J.; Colin, H. J . Chromatogr. 1981, 214, 35-47. Stolyhwo, A,; Colin, H.; Guiochon, G., unpublished results. Mourey, T. H.; Oppenheimer, L. E. Anal. Chem. 1984, 5 6 , 2427-2434.
RECEIVED for review July 27, 1984. Resubmitted January 18, 1985. Accepted February 6, 1985.
Effect of Ternary Complex Formation on Chromatographic Selectivity Using in Situ Complexation Chromatography Kevin P. O'Riordan, Gerard Heneghan, and Gordon G. Wallace*
Chemistry Department, University College Cork, Cork, Ireland
I n sltu complexation chromatography is a relatlvely novel form of reversed-phase chromatography. In the course of thls work a method for controlling selectivity via ternary complex formatlon Is discussed. Thls method is potentially useful, not only for controlllng separations but also for enhanclng detectlon of the metal specles. Limltatlons of the method are dlscussed.
The separation of metal species using high-performance liquid chromatography (HPLC) is an area which has seen significant development in recent years (1-4). The contribution of workers to this field is evidenced by the fact that
many new analytical methods for determination of trace metals are based on HPLC (for example, see ref 1-13). This has resulted in methods of analysis which are capable of determining several metals simultaneously at detection levels which rival the more conventional method: atomic absorption spectrometry. Generally, chromatographic separation of metal species are achieved using either reversed-phase (5-9), normal-phase (10, I I ) , or ion-exchange (12,13)chromatography. Reversed-phase chromatography provides a highly efficient and reproducible method of separation for metal species, provided a suitable precolumn derivatizing agent is employed. Previously, it has been shown (5-9) that the dithiocarbamate (dtc) ligand may be used to form metal dithiocarbamate complexes which are
0003-2700/85/0357-1354$01.50/00 1985 American Chemical Society
amenable to separation using reversed-phase chromatography. Following chromatographic separation metal species are usually detected using either spectrophotometer or amperometric detection. T o ensure optimum performance of the detection processes, use of a suitable derivatizing agent, once again, proves most beneficial (1-13). The dithiocarbamate ligand forms metal complexes with relatively high extinction coefficients (14) and which are amenable to electrochemical oxidation (15);hence detection of the metal species is enhanced. Along with the properties mentioned above, the ability of the dithiocarbamate ligand to react rapidly with a range of metals has led to its use with in situ complexation chromatography. With this technique complex formation is achieved by including the ligand in the chromatographic solvent. Upon injection, the metal species reacts with the ligand in the following solvent (in a short reactor coil) prior to reaching the chromatographic column. The mechanism of separation then remains essentially the same as in reversed-phase chromatography. It situ complexation chromatography provides a convenient technique which often results in excellent separation of metal species (5-9). However, certain metal complexes (8) do have somewhat similar properties and hence are more difficult to resolve. T o overcome this problem more chromatographic selectivity must be provided. Dithiocarbamates and related compounds are known to form ternary complexes wherein the metal center is complexed by two different ligands (16-23). The different properties of ternary complexes can be utilized to increase the selectivity of analytical methods. This has been used to advantage in various areas of separation science: ion exchange liquid chromatography (24), liquid-liquid (solvent) extraction (25) gas chromatography (26), and normal- or reversed-phase liquid chromatography (27,281. T o date, however, no reports using ternary complex formation for the separation and determination of metal species with reversed-phase chromatography have appeared. With in situ complexation (reversed-phase) chromatography separation is achieved by forming and partitioning a complex between two phases. It should therefore, be possible to utilize the concept of ternary complex formation to achieve extra selectivity with this technique. In the course of this work the possibility of forming ternary dithiocarbamate complexes using in situ complexation chromatography was considered. The effects of ternary complex formation on the chromatography as well as on the spectrophotometric and electrochemical detection stages are discussed. Limitations of the method are also considered.
EXPERIMENTAL SECTION Reagents and Standard Solutions. All chemicals used were of A.R. grade purity unless otherwise stated. Ammonium pyrrolidinecarbodithioate (Ajax Chemicals) was purified by recrystallization from ethanol. Sodium morpholinecarbodithioate was prepared by adding morpholine to carbon disulfide in basic solution and recrystallizing the resulting white precipitate from a minimum amount of hot water. Solutions of morpholine dithiocarbamate in the chromatographic eluent were relatively unstable and were prepared, fresh, twice daily. Metal stock solutions were prepared by dissolving copper nitrate, mercuric nitrate, cobaltous nitrate, or nickel nitrate in distilled deionized water. Acetonitrile was HPLC grade obtrained from Rathburn Chemicals. Instrumentation. All chromatographic equipment was obtained from Waters Associates (Milford, MA). A Model 6000A solvent delivery system was employed in conjunction with a Model U6K Universal Injector. A reversedphase column, -C18 pBondapak (25 cm length, 3.8 mm i.d.), and
2 50
300
350
400
500
450
Wavelength ( n r n ) M [morph] in Figure 1. UV-visible absorption spectra of 7 X acetonitrile: (- - -) ligand solution only, (-) add 1.6 X loA5M copper; cell path length, 1 cm; reference, air. a Model 481 spectrophotometric detector were also employed. Electrochemicaldata were collected with a Princeton Applied Research (PAR) Model 174A polarographic analyzer. A threeelectrode system, consisting of a platinum auxiliary, a Ag/AgCl (3 M KCl) reference, and a glassy carbon working electrode, was employed. The glassy carbon electrode was obtained from Metrohm (Herisau, Switzerland). UV-visible spectrophotometric data were obtained by using a Shimadzu Model 260 spectrophotometer.
RESULTS AND DISCUSSION The following two dithiocarbamate ligands were investigated with respect to their ability to form ternary complexes, in situ: -
-
L
pyrrolidinecarbodithioate
[PYrrl
_1
morpholinecarbodithioate
[morph] [pyrr] or [morph] = dtc
Individually, both ligands react rapidly with a wide range of metals to form stable dithiocarbamate complexes. The metal-morpholine complexes exhibit significantly different chromatographic properties from their metal-pyrrolidine counterparts (see later). It was envisaged, therefore, that any ternary complex should exhibit intermediate properties and that chromatographic selectivity may be controlled in this way. Initial studies concentrated on the separation of copper and mercury species since previous work has indicated that the dithiocarbamate complexes of these two metals are relatively difficult to resolve (8). UV-Visible Spectrophotometry. A UV-visible spectrum for the [morph] ligand is shown in Figure 1. Similar results were obtained for the [pyrr] ligand (5-8). Addition of copper ions to the ligand solution results in rapid formation of the copper complex which absorbs in the visible as well as the UV region of the spectrum (Figure 1). Both Cu(pyrr)2 and Cu(morph), have a ,A, value of 423 nm. Addition of [pyrr], up to M, to the solution defined in Figure 1 had no effect on the,,,A value or the level of absorbance for the copper complex in the visible region. Hence, no indication of ternary complex formation could be obtained by using UV-visible spectrophotometry. Addition of mercury ions to either ligand solution results in rapid formation of a complex which absorbs only in the UV region of the spectrum. Once again, identification of a ternary complex was precluded due to the similar ,A, values (269 nm) obtained for both Hg(pyrr)2 and Hg(morph)2.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
Table I. Voltammetric Dataa
Table 111. Chromatographic Data
compound
EP(0db
morph +0.18, +1.10 i-0.18, +1.08 PYrr Cu(morph)z 4-0.60 Cu(PYrr)z +0.55 Hg(rnorph), +1.06 Hg(PYrr)z +LO4 "All data were obtained at a glassy carbon electrode in acetonitrile (0.1. M TEAP). M(dt& responses were obtained, in the presence of excess ligand. * Ep(m)was determined using differential pulse voltammetry: scan rate, 2 mV/s; duration between pulses, 0.5 s; pulse amplitude, 25 mV. Table 11. Variation in ED(.,=) for Oxidation of Copper Complex at Glassy Carbon in Acetonitrile (0.1 M Et4NC104) composition of solution 6X
M [morph] M [pyrr] + 6 X M [pyrr]
E*(or),omV
M [morph]
600 f 5 576 f 5 554 f 5
" Ep(or)was determined by using differential pulse voltammetry: scan rate, 2 mV/s; duration between pulses, 0.5 s; pulse amplitude, 25 mV.
ao
0.2
0.4
0.6
0.8
E (VOLTS) Figure 2. Differential pulse voltammogram of 5 X lo-' M [morph] in acetonitrile (0.1 M Et4NCI0,): scan rate, 5 mV/s; duration between pulses, 0.5 s; pulse height, 50 mV; (- - -) ligand solution only, (-) add M copper. Response 1 is due to oxidation of [morph]. 0.8 X Response 2 is due to oxidation of Cu(morph),.
Monitoring of all metal complex responses with time indicated stability over a period of a t least 24 h. Voltammetric Data. All voltammetric data are summarized in Table I. A response was observed a t +0.18 V vs. AgIAgC1, due to oxidation of the ligand while a t = +1.10 V a response due to oxidation of the thiuram disulfide dimer, a product of free ligand oxidation, was observed (5-8). Addition of copper ions to the ligand solution (Figure 2) results in a decrease of the free ligand oxidation and appearance of an additional response due to Cu(dtc)z oxidation. Values in Table I indicate that oxidation of the Cu(pyr& complex is slightly easier to achieve than oxidation of the Cu(morph)z complex. In order to determine if ternary complexes could be formed in situ, [pyrr] ligand was added to a solution which originally contained the Cu(morph)z complex in the presence of excess [morph]. Table I1 indicates that as the concentration of the [pyrr] ligand is increased the oxidation potential of the metal complex decreases until an E,(ox) value which corresponds to oxidation of Cu(pyrr), is obtained. Presumably, the intermediate values are due to an equilibrium situation which involves formation of a ternary complex. Cyclic voltammetric data indicated that while ternary complex formation altered the E,(ox) value for the system it
compound
V,,' mL
compound
Vr: mL
Cu(pyrr), Cu(morph),
9.9 4.2
Ni(pyrr), Ni(morph),
5.5 2.8
Hg(PYrr)z 9.9 Co(pyrr), 6.3 Hg(morph)z 4.3 Co(morph), 3.3 'Retention volumes were determined by using in situ complexation chromatography with 5 X M ligand dissolved in the chromatographic solvent. Table IV. Effect of Ternary Complex Formation on Chromatography" concn (morph)*in chromatographic solvent 0 1 x 10-5 M
4 x 10-5 M 7 x 10-5 M 10-4 M
VI,' mL Hg(dtc)z
VI,h mL Cu(dtc),
selectivity
9.9 9.9
9.9 9.9 7.4 6.7 5.2
1.0 1.0 1.19 1.25 1.19
8.8
8.4 6.2
coeffr (a)
resolutiond (R,) 0.0 0.0
2.0 2.4
1.8
'Injection of 20 gL of aqueous solution containing metal ions. The chromatographic solvent was initially 60:40 acetonitri1e:aceM [pyrr]. Other tate buffer, pH 5.5 (0.02 M), 0.001 M NaNO,, chromatographic conditions were as described in the experimental section. c a = V:/V,b. d R , = (V," - V,b)/(l/2)(Wa+ wb);W = peak volume. had no marked effect on the kinetics of the electrode process. That is, the peak-peak separation remained constant as the [pyrr] ligand was added and presumably a one-electron transfer step resulting in the copper center being oxidized does, as previously described (549, still occur. Addition of mercury ions to the solution containing [morph] ligand again resulted in decrease of the ligand oxidation response and appearance of a response due to oxidation of Hg(morph)2. Unfortunately the response due to oxidation of the metal complex is not well resolved from oxidation of the thiuram disulfide dimer. During ternary complex formation studies, as carried out for copper, the metal complex was therefore swamped by the dimer oxidation process upon addition of [ pyrr] . Chromatography. Chromatograms obtained by using in situ complexation chromatography are shown in Figure 3. By use of either the [pyrr] (Figure 3A) or the [morph] (Figure 3B) ligand alone, it is obvious that separation of the copper and mercury complexes is not obtained. For all metal complexes considered the morpholine complex eluted prior to the pyrrolidine complex (Table 111). The large difference in elution volume between metal morpholine and the corresponding pyrrolidine complex implies that ternary complexes, with intermediate properties, should prove must useful in controlling chromatographic behavior. Table IV indicates the effect on the selectivity coefficient ( a )as the concentration of [morph] is increased in a chromatographic solvent which originally contained only the [pyrr] ligand. (Under the chromatographic conditions employed only one peak was detected for each metal, either copper or mercury, injected into the system.) As the concentration of the morpholine ligand is increased, there is obviously an optimum ratio of the two ligands when, presumably due to ternary complex formation, maximum selectivity is achieved. This occurs when the [morph] and [pyrr] ligands are employed in the ratio 7-31. Similar results were obtained by using greater total concentrations of ligand provided the ratio of [morph] to [pyrr] was kept constant.
ANALYTICAL CHEMISTRY, VOL. 57, NO, 7, JUNE 1985
1357
aR
0
2
4
6
8
Vr (m L) Flgure 4. Chromatographic conditions as in Figure 3C; injection, 20 pL of aqueous solution containing 1.1 ng of nickel (9.5 X lo-' M). Response 1 was due to ligand being reduced in chromatographic solvent; response 2 was due to nickel complex species.
0
2
4
6
8
1
0
V,(mL) Flgure 3. I n situ complexation chromatography of copper and mercury. Chromatographic conditions are given in the Experimental Section. Injection was 20 pL of aqueous solution containing 1.2 ng of copper (9.5 X M) and 4 ng of mercury (lo-' M). Chromatographic solvent was 60:40 acetonitri1e:acetate (pH 5.5) (0.02M) (A) Containing M [morph], or (C) M [pyrr], (B) containing 7 X containing 7 X M [morph] M [pyrr]. Response 1 dip was due to concentration of ligand in chromatographic solvent being reduced during complex formation, response 2 was due to copper complex, and response 3 was due to mercury complex.
+
The equilibria which are established during ternary complex formation for the copper and mercury species may be described by M 2L1 M(LJ2 (1)
+ M + 2L2
--
M(L2),
M(L1)2 + M(L2)z + 2MLiLz
(2)
(3)
where M is metal, L1 is [morph], and L2 is [pyrr]). Previous reports (5-9) indicate that metals considered in this work (copper, mercury, nickel, and cobalt) react rapidly with dithiocarbamate ligands to form stable complexes (eq 1-2). However it is the establishment of an equilibrium which results in a rapid formation of a ternary complex, eq 3, which is of prime importance in determining the success, or otherwise, of the method under investigation. Since only one well-defined response is observed upon injection of copper or mercury ions it would appear that chromatographic conditions are such that the above criteria (rapid formation of a ternary complex) are met. This behavior is understandable due to the labile nature of the copper and mercury complexes (18-22). Although M(LJ2 and M(Lz), may initially be present in the reactor coil, they would be separated from M(Ll)(Lz)upon the sample enering the chromatographic column. In the presence of excess L, and L2,which are in the chromatographic solvent, the binary complexes would then rapidly react to form the respective ternary complexes. The rapid interconversion and subsequent formation of the ternary complex ensure optimum chromatographic responses. The benefits of this method of obtaining extra selectivity are apparent when one considers that while a has increased from 1to 1.25 (using the optimum ligand mixture) the volume of eluent needed to effect separation decreases. Subsequent to chromatographic separation both spectrophotometric and amperomeric detection were considered. Since ternary complex formation has no effect on the extinction coefficient, or the mechanism of the electrode process, detection levels were similar to those reported previously (5-8). That is 0.2 ng/20 pL for injection of either metal with electrochemical, or 1.0 ng/pL for injection of either metal with spectrophotometric, detection could be detected. With either detection mode calibration curves were linear at least to levels of 20 ng/20 pL injected. Limitations of the Method. Preliminary investigations into the separation and subsequent detection of other metals, namely, nickel and cobalt, emphasized the limitations of this method. The chromatogram obtained upon injection of nickel ions into the system, which has been optimized for separation of copper and mercury, is shown in Figure 4. As for previous metals investigated, the equilibria established upon injection of nickel ions into the system may be described by eq 1-3. The elution volume recorded for injection of nickel ions is intermediate to values obtained for Ni(pyrr)zand Ni(morph), suggesting that ternary complex formation does occur. The response observed, however, is extremely broad and is not analytically useful. This, presumably, is due to the relatively
1358
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
0
2
4
6
8
is relatively slow, as for nickel, or very slow, as for cobalt, the (analytical) usefulness of the method is limited. However, these problems may be alleviated by thermostating the chromatographic column or by using more suitable ligands or solvents, and such approaches are under investigation in our own laboratory a t present. Current work with mixtures of dithiocarbamate and other ligands (e.g., see ref 20) capable of forming ternary complexes or mixtures of dithiocarbamates and other compounds capable of adduct formation (29, 30) appears most promising and will be presented in a future publication. As well as effecting chromatographic selectivity, the above concepts may be useful in controlling the selectivity of the amperometric or spectrophotometric detection of the metal species by altering electrode processes and extinction coefficients, respectively. Registry No. Cu(morph),, 14024-15-8;Cu(pyrr),, 23301-60-2; Hg(morph),, 14024-75-0; Hg(pyrr),, 41060-60-0; Ni(pyrr),, 30117-29-4;Ni(morph),, 14024-77-2;Co(pyrr),, 24412-38-2;Co(morph),, 27796-33-4;NH4[pyrr],5108-96-3;Na[morph],873-58-5; Cu, 7440-50-8; Hg, 7439-97-6; Co, 7440-48-4; Ni, 7440-02-0.
V,(mL) Figure 5. Chromatographic conditions as in Figure 3C: injection, 20 pL of aqueous solution containing 1.2 ng of cobalt (10" M). Response 1 was due to ligand being reduced in the chromatographic solvent, response 2 was due to Co(morph),, response 3 was due to Co(morph),(pyrr) and Co(pyrr),(morph), and response 4 was due to Co(PYW3.
slow kinetics involved in formation of the nickel ternary complex (18-21). The injection of cobalt ions into the system (Figure 5) highlights some further problems. Cobalt is known to form a tris(dithi0carbamate) complex (5-9), and hence, quaternary complex formation, according to CO(LJ3 + CO(L2)3 6 CO(L,),(L,) + CO(L,),(L,) (4) is possible. Even in the ideal case, where kinetics are such that rapid formation of mixed complexes occurs, two peaks due to Co(LJ2(L2)and Co(L2)2(Ll)should be observed. However, under the chromatographic conditions employed in this study, injection of cobalt ions results in three peaks. It appears that the peaks due to Co(LJ2L2and Co(LJ2L1 are unresolved (V, = 4.6 mL). The other two peaks are due to Co(LJ3 and Co(L2)3, respectively (Table 111). By use of this method the analysis of cobalt ions is limited by the very slow kinetics involved in the conversion of the Co(LJ3 and Co(L2)3species to the mixed ligand complexes (eq 4). Other workers (15) have shown that cobalt tris(dithiocarbamate) complexes are relatively inert and do not readily undergo exchange reactions. Conclusion and Future Development. It has been shown that the concept of ternary complex formation can be employed using in situ complexation chromatography. Under optimum conditions enhanced chromatographic selectivity may be obtained. The method is limited to determination of species that form labile dithiocarbamate complees, which consequently results in rapid formation of the ternary complex. In other cases where the formation of the ternary complex
LITERATURE CITED Veening, H.; Wilieford, B. R. Rev. Znorg. Chem. 1979, 1, 281. Schwedt, G. Chromatographla 1979, 12,613. Cassidy, R. M. "Trace Analysis"; Lawrence, J. F.. Ed.; Academic Press: New York, 1981; Voi. 1, pp 47-120. Wlileford, 8. R.; Veening, H. J . Chromatogr. (Chromatogr. Rev.) 1982, 257,61. Bond, A. M.; Wallace, G. G. Anal. Chem. 1981, 53, 1209. Bond, A. M.; Wallace, G. G. Anal. Chem. 1982, 54, 1206. Bond, A. M.; Wallace, G. G. Anal. Chem. 1983, 55, 718. Bond, A. M.; Wallace, G. G. Anal. Chem. 1984, 56, 2085. Smith, R. M.; Yankey, L. E. Analyst (London) 1982, 107,744. Gaetini, E.; Laureri, C. F.; Mangia, A. Ann. Chlm. (Rome) 1979, 69, 181. Edward-Inatirni, E. B. J. Chromatogr. 1983, 256,253. Girard, J. E. Anal. Chem. 1979, 51, 836. Cassidy, R. M.; Elchuck, S.; McHugh, J. 0. Anal. Chem. 1982, 54, 727. Jorgenson, C. K. Znorg. Chlm. Acta Rev. 1968, 2 ,65. Coucouvanis, D. Prog. Znorg. Chem. 1979, 26,301. Pignolet, L. H.; Que, L.; Edgar, B. L.; Duffy, D. J.; Piazzotto, M. C. J. Am. Chem. SOC. 1973, 95, 4537. Kheiri, F.; Tsipis, C. A.; Tsiarnis, C. L.; Manoussakis, G. E. Can. J. Chem. 1979, 57, 767. Moriyasu, M.; Hashirnoto, Y. Bull. Chem. SOC.Jpn. 1980, 53,3590. Moriyasu, M.; Hashimoto, Y.; Endo, M. Bull. Chem. SOC.Jpn. 1981, 54, 3369. Moriyasu, M.; Hashimoto, Y. Bull. Chem. SOC.Jpn. 1981, 54, 3374. Lehotay, J.; Liska, 0.; Brandsteterova, E. J . Chromatogr. 1979, 172, 379. Lehotay, J.; Liska, 0.; Koiek, E.; Garaj, J. J. Chromatogr. 1983, 258, 233. Soundararajan, G.; Subbiyan, M. J. Zndlan Chem. SOC. 1983, 60, 1182. Muller, D.; Jozefonvicz, J.; Petit, M. A. J. Znorg. Nucl. Chem. 1980, 42, 1083. West, T. S. Analyst (London) 1988, 91, 69. OBrien, T. P.; O'Laughlin, J. W. Talanta 1976, 23,805. Low, G. K. C.; Haddad, P. R.; Duffield, A. M. J. Llq. Chromatogr. 1983, 6, 311. Low, G. K. C.; Haddad, P. R.; Duffield, A. M. Chromatographia 1983, 17, 16. Mohapatra, B. 6.; Pujari, S. K.; Chiranjeevi, A. J . Zndlan Chem. SOC. 1981, 58, 714. Mohapatra, B. 6.; Mohapatra, B. K.; Gurv, S. J . Znorg. Nucl. Chem. 1977, 39, 1577.
RECEIVED for review October 26, 1984. Accepted February 6, 1985.
