Characterization of Rare Earth Elements used for Radiolabeling Applications by ICP-QQQ

Radiolabeling refers to a technique where a compound or substance is labeled (or tagged) with a radioactive isotope of an element. The labeled material can then be used for controlled delivery of the radiation emitted by the active isotope, or detected and traced from the radioactivity of the isotopic label.

The use of radiolabeled materials is growing steadily, with the market for radiolabeled pharmaceutical compounds expected to be worth over 5 billion US dollars by 2024 (1). Elements that can form useful radioisotopes include the lanthanides – also known as rare earth elements (REEs). Radio-lanthanide compounds are used in pharmaceutical and imaging applications.

To meet the rising demand for radio-lanthanides, there is a critical need for analytical techniques to support the production of traceable, high purity, labeled lanthanides (2). Production scale chemical purification and labeling of radio-lanthanides is challenging though, because all the lanthanides have similar chemical properties and tend to behave as a consistent group. To prepare a pure radio-lanthanide, it is necessary to accurately characterize the chemical composition of the non-radioactive natural or isotopically enriched starting material used. Before a candidate starting material can be used for routine radionuclide production, each batch must be tested and validated to ensure that radionuclide yields will be of the desired quantity and quality (3). To carry out this level of quality assurance (QA), accurate analytical procedures are needed, particularly to determine the level of trace lanthanide contaminants in the pure lanthanide starting material. Since any impurities need to be removed, the QA data is also useful to guide the design of robust, reproducible chemical separation methods (4). Determining the type and quantity of impurities present in the starting material also helps manufacturers predict whether unwanted radioactive side-products will be produced during irradiation.

In this study, several pairs of trace REE analytes and adjacent mass neighboring REE matrix elements relevant to radio-lanthanide production were measured by ICP-QQQ. The samples included materials that are typically used as 3 irradiation targets for radio-lanthanide production. The use of ICP-QQQ for optimizing radiochemical separation of target lanthanides, and for measuring REE impurities in irradiated targets, was also demonstrated. The developed methods will enable end users to compile a REE impurity profile of materials of relevance to the production of radionuclides for labeling applications.

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