Anthony P. Giauque, Chemistry and Biochemistry

Standard protocol for the analysis of transition metals by conventional chromatographic means calls for the complexation of the transition metal cations prior to their separation. Consequently, this addition of a complexing agent does not allow for conductimetric detection of transition metals due to the high conductivity of the complexing agent in the eluent. It was therefore my intent to develop a system in which the stationary phase would act as the complexing agent and allow the analytes to be detected conductimetrically-the universal means of detection in ion chromatography.

An experiment in which an aqueous standard of 6 transition metal cations (Co2+, Cu2+, Nj2+, Zn2+, Fe2+, and Fe3+-10 ppm each) was separated using an eluent containing a complexing agent, 2,6-pyridinedicarboxylic acid (PDCA), and then using an eluent void of this complexing agent. From the results of this experiment it was seen that without a complexing agent in the eluent, all transition metal cations co-elute rapidly, resulting in no separation. This is due to the fact that all transition metal cations are of approximately the same size and charge, causing them to interact similarly with the macrocycle adsorbed on the column. Complexation of the cations in the sample resulted in separation. This implies that the separation of transition metals is determined by the differing degrees of complexation between the complexing agent and the different cations. Finding the appropriate combination of complexing agent and column, therefore, was the key to separating transition metals efficiently. The ultimate goal was to find a complexing agent which was neutral and would allow for conductimetric detection.

Comparison of Columns

My preparation of the macrocycle-based system involved the use of two different macrocycles. The cryptand decyl-2.2.2 (D2.2.2) and the macrocycle tetradecyl-18-crown-6 (TD18C6) were adsorbed onto two different types of column resins of different size and porosity, known as MPIC and AS10. Preliminary analyses using the D2.2.2 on MPIC column for the separation of transition metals resulted in little separation. This was probably due to the macrocycle’s affinity for protons over transition metal complexes. Similar results were seen with D2.2.2 on AS 10 for the same reason. Therefore experiments involving the separations of cationic complexes on a D2.2.2 based column were abandoned and the TD 18C6 columns were found to yield satisfactory resolution of the transition metal complexes. The four analytes used were separated into four peaks which excelled in resolution over that achieved with a nonmacrocyclic column.

Comparison of Complexing Agents

At first, various charged complexing agents were employed to enhance the transition metals separations. Ethylenediaminetetraacetate (EDTA), oxalic acid, and PDCA were implemented as eluent additives. In addition, about 10 other possible complexing agents were considered. Due to the fact that the binding constants of these ligands with different transition metal cations were similar (as described in the literature), they were not used in the actual experiments. Of those which were investigated, PDCA gave the most satisfactory resolution to the separatory scheme, again because its binding constants with Co2+, Cu2+, Ni2+, Zn2+, Fe2+, and Fe3+ were more varied than the binding constants of the other complexing agents used.

Sulfer Crowns

Of late, sulfur crowns (crown ether analogs with sulfur atoms in place of oxygen atoms) have provided the possibility of an uncharged complexing agent. In fact, it was postulated that if they could be adsorbed onto a chromatographic column, they could separated transition metals by themselves without the need of a mobile phase complexing agent at all! Unfortunately, with no aliphatic moieties or oxygen atoms in these ligands, we have not been able to find a means of attaching them to our columns. We did, however, use these sulfur crowns as eluent additives, much in the same way we used the other complexing agents. As eluent additives, they yielded relatively poor results: the best of the four sulfur crowns we used separated the 6 analytes into 2 peaks. In an extension of this project, I plan to determine a method of adsorbing these sulfur crowns to a chromatographic column.

Conclusion

It has been found that the separation of transition metals depends upon two factors: (i) the column and, (ii) the complexing agent. Using the TD18C6 column and PDCA as the complexing agent we achieved our best separation which gives a resolution similar to that found in conventional chromatography, greater efficiency, and a different elution order. In order to detect transition metals conductimetrically, a neutral species must be found which will complex various transition metals to differing degrees. Sulfur crowns provide a possible candidate for such a neutral species. I wish to thank Dr. John D. Lamb for his support in this project. Portions of my research involving transition metal separations, and cation separations in general are ready to submit for publication in the Journal of Chromatography and possibly Analytical Chemistry.