Anion exchange chromatography of 99mTc(Sn)EHDP complexes: Determination of the charge of the components and influence of pH and ligand concentration

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The components of a 99mTc(Sn)EHDP complex mixture were separated by means of normal pressure and high-pressure anion exchange chromatography, on DEAE-Trisacryl and Aminex, respectively. Precautions were taken to prevent the dissociation of the complexes during chromatography. The charges of the components were determined according to the methods of Wilson and Pinkerton (1985) and Russell and Bischoff (1985). The values of the charges obtained with the two methods are not in agreement. The mean values of the charges obtained with the former method on DEAE-Trisacryl and Aminex disagree. Further, this method does not give correct results for simple ions [Cl −, SO 4 2−, Fe(CN) 6 3−]. On the other hand, the mean values of the charges obtained with the latter method on DEAE-Trisacryl and Aminex agree, and this method gives the correct charge of SO 4 2−. Thus, Russell and Bischoff's method, in which a reference ion is used, must be preferred. However, even with this method the accuracy of the data obtained is probably limited, due to the difficulty of making corrections for activity coefficients of highly-charged ions at the rather high electrolyte concentrations that must be used in the ion exchange method. So, we think that it is only warranted to conclude that the mean charge of the components of 99mTc(Sn)EHDP is about −6 at pH 7, and that the charges of the individual components are in the range of −4 to −9. The influence of pH and ligand concentration in the reaction mixture was determined with high pressure anion exchange chromatography. It was found that a decrease in the pH of the reaction mixture favours the production of complexes with a long retention time, which leads to a slightly higher mean charge. The ligand concentration of the reaction mixture scarcely influenced the relative concentrations of the components.