Determination of specific activity of radiohalide preparations (75Br, 77Br, 123I, 131I) by HPLC‐UV detection following chemical derivatization to 1‐halonaphthol‐2

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Abstract
Specific activity is an important parameter for receptor‐binding radiopharmaceuticals. We present here a method for determination of specific activities of radiobromide (75Br, 77Br) and radioiodide (123I, 131I) using naphthol‐2 and chloramin T as reagents for derivatization. Subsequent hplc determination of the mass of 1‐halo‐naphthols‐2 on a RP 18 column is highly sensitive due to their high UV extinction coefficient in the range of 232 nm (detection limit at 220 nm: 5 × 10−13 mol) and linear over 6 orders of magnitude. It can be rapidly and conveniently performed in parallel to the actual radiopharmaceutical preparation using only a small part of the radioactive halide (less than 1 mCi for all isotopes). Dilution factors (carrier atoms per radioactive atom) found for the radiohalide using this method were 21‐100 for 75Br, 2–58 for 77Br, 3–6 for 123I and 120‐130 for 131I, respectively.