astatine. Wet The irradiated bismuth (or sometimes bismuth trioxide) target is first dissolved in, for example, concentrated nitric or perchloric acid. Following this first step, the acid can be distilled away to leave behind a white residue that contains both bismuth and the desired astatine product. This residue is then dissolved in a concentrated acid, such as hydrochloric acid. Astatine is extracted from this acid using an organic solvent such as butyl or isopropyl ether, diisopropylether (DIPE), or thiosemicarbazide. Using liquid-liquid extraction, the astatine product can be repeatedly washed with an acid, such as HCl, and extracted into the organic solvent layer. A separation yield of 93% using nitric acid has been reported, falling to 72% by the time purification procedures were completed (distillation of nitric acid, purging residual nitrogen oxides, and redissolving bismuth nitrate to enable liquid–liquid extraction). Wet methods involve "multiple radioactivity handling steps" and have not been considered well suited for isolating larger quantities of astatine. However, wet extraction methods are being examined for use in production of larger quantities of astatine-211, as it is thought that wet extraction methods can provide more consistency. They can enable the production of astatine in a specific oxidation state and may have greater applicability in experimental radiochemistry. Uses and precautions {| class="wikitable" |+ Several 211At-containing molecules and their experimental uses ! Agent ! Applications |- | [211At]astatine-tellurium colloids | Compartmental tumors |- | 6-[211At]astato-2-methyl-1,4-naphtaquinol diphosphate | Adenocarcinomas |- | 211At-labeled methylene blue | Melanomas |- | Meta-[211At]astatobenzyl guanidine | Neuroendocrine tumors |- | 5-[211At]astato-2'-deoxyuridine | Various |- | 211At-labeled biotin conjugates | Various pretargeting |- | 211At-labeled octreotide | Somatostatin receptor |- | 211At-labeled monoclonal antibodies and fragments | Various |- | 211At-labeled bisphosphonates | Bone metastases |} Newly formed astatine-211 is the subject of ongoing research in nuclear medicine. It must be used quickly as it decays with a half-life of 7.2 hours; this is long enough to permit multistep labeling strategies. Astatine-211 has potential for targeted alpha-particle therapy, since it decays either via emission of an alpha particle (to bismuth-207), or via electron capture (to an extremely short-lived nuclide, polonium-211, which undergoes further alpha decay), very quickly reaching its stable granddaughter lead-207. Polonium X-rays emitted as a result of the electron capture branch, in the range of 77–92 keV, enable the tracking of astatine in animals and patients. Although astatine-210 has a slightly longer half-life, it is wholly