A self-referencing radiation detector to test the radiochemical purity of a sample radiopharmaceutical solution. In some embodiments, the self-referencing radiation detector measures the radioactivity of a sample radiopharmaceutical solution before the sample radiopharmaceutical solution is passed through a high performance liquid chromatography column. The radiation detector then measures the radioactivity of each separated molecularly distinct species from the high performance liquid chromatography column. The radiochemical purity of the sample radiopharmaceutical solution is calculated by comparing the measured radioactivity of separated molecularly distinct species from said high performance liquid chromatography column to the measured radioactivity of the sample radiopharmaceutical solution before the sample radiopharmaceutical solution is passed through the high performance liquid chromatography column.