Determination of net ionic charge on Tc-99m DTPA and Tc-99m EDTA by a column ion-exchange method

The net charge on a stable complex ion is conveniently measured at tracer levels by studies of ion-exchange equilibrium. Previous applications to radiopharmaceuticals have used a batch equilibrium method, but such measurements are affected by any radiochemical impurities present. Since technetium pharmaceuticals are often heterogeneous, it is of value to have a technique that is directly applicable to a mixture of different species. Such a method is presented here: a column method to determine the net charge on the technetium-99m complex of diethylenetriaminepenta-acetic acid (DTPA), and also on the principal component in a mixture of species formed by reducing pertechnetate in the presence of ethylenediaminetetra-acetic acid (EDTA). The net charge was calculated from the effect of eluent concentration on retention time. The net charge on Tc-DTPA was found to be -2 at both pH 4.6 and 7.0. The net charge on the Tc-EDTA complex, measured at pH 7.0 only, was also -2.