The Separation of Copper, Iron, and Cobalt Tetramethylene Dithiocarbamates by HPLC Stephen Lehame School of Social and Environmental Education. Thames Polytechnic, Bexley Road, London, SE9 2PQ, England Metal ion chelation and subseauent organic solvent extraction is an important and well &ed a n a k i c a l technique in inorganic chemistrv. Generallv the method has been used as a prelude to metal analysis bieither solution spectrophotometry or atomic absorption spectroscopy. However, the methodology has also found a place in inorganic chromatographic analysis. Separations by TLC1 and GC2 have been . . extensively reviewed, and more recently workers have begun to investigate the use of HI'LC in the separation and quantification of metal chelate*? Much of the work reoorted in the literature has been concerned with separation; of synthetic mixtures of metal chelates. However. some workers have demonstrated practical applications. For instance, HPLC analvsis has been used to determine metals in steels and trade effluents'. I n short, the method promises much for simultaneous multi-element analysis. The object of this article, then, is to outline an undergraduate experiment that may be used to demonstrate this aspect of the use of HPLC in inorganic analysis. This experiment has been used as part of a practical course for environmental studies students. It has been designed to introduce them to HPLC, widen the scope of their knowledge of inorganic analysis, and tie in with taught material dealing with metals in the environment.
Retention Volumes and Capaclty Factors for the Metal Chelates Chelate
Retention volume (mL)
Capacity factor
Gopper Iron Cobalt
4.4 7.0 12.8
3.4 7.0
1.8
Materials Merol snlls. ANAI.AH-pade eoppdll) sulfnte, inm(ll1) nitrate, and eobalt~llichloride obtained from BDH Chemicals Ltd. were used. Ammonium tetramethylene dithioeorbomote. This was obtained from BDH Chemicals Ltd. Soluents. HPLC grade hexane was obtained from Fisons plc. HPLC-grade propanol and chloroform were obtained from BDH Chemicals Ltd. Buffersolution. A solution of pH 4.6 prepared from 0.1 Methanoic acid and 0.1 Msadium ethanoate. Preparation of Metal Chelates Solutions (1WO ppm) of the metal ions were initially prepared. These were suhsequently used to prepare mare dilute solutions (10 and 50 ppm). Aliquots (between 5 and 30 mL) of these solutions were placed in a separating funnel. Buffer solution was added followed by the addition of freshly prepared 1%ammonium tetramethylene dithiocarhamate solution. The resulting precipitate was extracted into chloroform (25 mL). The mixture was then allowed to stand for an hour after which the chloroform laver was run off. This was followed by a second treatment of the remaining aqueous layer. The two chloroform extracts were then eomhined. Separation Details The chromatographic separations were carried out on a Pye Unicam system comprising a PU 4010 pump, a 20-pL Rheodyne syringe
' Brinkman, U. A. Th.; De Vries, G.; Kuroda, R. J. Chrom. 1973,85, 1R7.
Moshier. R. W.; Sievers, R. E. "Gas Chromatography of Metal Chelates"; Pergammon: Oxford. 1965. Schwedt. G. Chromatographla 1979, 12, 613. Edward-lnatimi, E. 6.; Dalziel, J. A. W. Anal. Proc. 1980. 17, 40.
10
8
6
4 Tim /minutes
Figure 1. Chromatogram showing the separation of copper, iron, and cobalt chelates.
loading sample injector and a PU 4020 variable wavelength UV detector. The column used was a "Minisep" column (4.5 mm X 10 cm) that contained silica gel (particle size 5 @rn) The eluant used was a 1%solution of propanol in hexane. The following operating conditions were used throughout. The flow rate was set at 2 mllmin, the pressure was 75 bar. Detection was carried out at 254 nm. Discussion The table indicates typical values obtained for the retention volumes and capacity factors for the metal chelates. Figure 1 demonstrates the kind of separations achievable Volume 6 3
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Peak height / cm
Mass I ng Figure 2. Plot of mass of metal injected against peak height for Me copper and iron chelates. Figure 3. Plot of mass of metal chelate.
using this system. As the diagram clearly shows, good resolution of the peaks is attainahle thus making peak identification fairly unambiguous-an essential point for a satisfactorv ~* student exoerFment. Figures 2 and 3 indicate that there is a fairly good linear relationshin between ~ e a k heieht and mass of metal loaded onto the coiumn-each point &resenting the mean of 4 or 5 re~licates.I t is therefore clear that ctuantification, too, is possible. Work carried out bv the author indicates that the method is not universally suitable for the separation of all cations. For instance metals of environmental interest like lead, cadmium, and manganese are eluted with iron. However, separations of mercury, copper, nickel, iron, cobalt, and bismuth are routinely achievable5. From the point of view of implementation of the experi~
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injected against peak height for Me cobalt
ment students are supplied with five solutions of each metal chelate in chloroform. The concentrations vary from 1to 5 ppm. Additionally they are supplied with a mixture, in chloroform, of the copper and cobalt chelates (the concentration onrn). of each beine between 2 and 3 .. . Thev are asked to identify and quantify the components in the mixture. The oractical work mav be extended to student oreparation of the chelates, thouih this has been found ti be too timeconsuming for our purposes. As outlined the experiment maybe completed in an afternoon session.
Leharne. S. A,, unpublished work.
