Determination of the radiochemical purity of (99m) Tc medronate injection by thin layer chromatography on iTLC-SG: effect of medronate concentration on the value measured

When using iTLC-SG thin layer chromatography plates to measure radiochemical impurities in (99m) Tc medronate, falsely high values were obtained for (99m) Tc pertechnetate impurity. Preliminary investigations indicated that the mass of (99m) Tc medronate applied to the plate influences the value.The goal of this study was to determine if the concentration of medronate influences the value obtained for (99m) Tc pertechnetate impurity.(99m) Tc medronate was prepared at two concentrations: 4 mg/mL and 0.2 mg/mL. Impurity levels were measured using three stationary phases: dried and undried iTLC-SG and 54SFC paper. Two mobile phases were used: methyl ethyl ketone to detect (99m) Tc pertechnetate and sodium acetate 136 g/L to detect hydrolysed and colloidal (99m) Tc. Sample spot drying and volume were also investigated.With 4 mg/mL samples, the three stationary phases measured similar impurity levels (p 0.05). With the 0.2 mg/mL samples, higher levels of (99m) Tc pertechnetate were measured with iTLC-SG than with paper (p 0.05). Neither sample spot drying nor volume was found to affect impurity levels measured.When using iTLC-SG to measure the radiochemical purity of (99m) Tc medronate, an artefactually high level of (99m) Tc pertechnetate impurity is measured when the medronate concentration in the sample is low. The iTLC-SG stationary phase may be unsatisfactory for measuring the radiochemical purity of (99m) Tc medronate.