Determination of ionisation constants of radiopharmaceuticals in mixed solvents by HPLC

The uptake of some ionogenic radiopharmaceuticals may be explained on the basis of a pH shift theory. An HPLC method for pK measurement has been developed for radiotracers. It has been tested with several amines and used to estimate the pK (pK in the mixed solvent 30% acetonitrile-water) of some sup(99m)Tc-PnAO complexes by observing the change in capacity factor k' with change in mobile phase pH. The non-ionogenic complexes Tc-PnAO and Tc-DE-PnAO showed constant k' values while the ionogenic complexes TC-MNE-PnAO and Tc-IP-PnAO had pKsup(s)sub(a) values of about 6.6 and 8.3. The method is reproducible and is applicable to nearly all ionogenic radiopharmaceuticals. (author)