## 4 Discussion
Arguments can be made that some of the radiolabeling difficulties could be due to having species of \({}^{211}\)At present that are not reactive or react differently than expected/desired. In this investigation we examined two radioHPLC methods for separation and identification of \({}^{211}\)At species so that we could use them for identification of the desired Na[211]At, and for quality control of the isolated \({}^{211}\)At. Both a C-18 reversed-phase separation method and an ion exchange separation method were evaluated. The ion exchange radioHPLC method has the advantage that it provides good separation of \({}^{211}\)At species when taking samples directly from the highly basic isolation solution, whereas the separations on reversed-phase were best conducted by injection of \({}^{211}\)At solutions that had been neutralized. Our radioHPLC studies identified At species we believe were astatide (At\({}^{-}\)) and astatate (AtO\({}_{3}\)) based on using known reactions to obtain these species. Minor amounts of other unidentified species were also seen, but we have demonstrated that the wet chemistry approach can provide high radiochemical purity astatide as a final product. In addition to knowing the nature of the species in the final product, it is of interest to know the nature of the astatine species obtained in each step of the isolation steps. Unfortunately, this has not been established and is difficult to ascertain. It is generally assumed that **step 1**, dissolution of the bismuth target in concentrated nitric acid provides At(0) in solution [33], but if that is the case it is surprising that no volatile astatine is released in this open-air vigorous dissolution. Irrespective of the initial At species in step 1, based on the fact that iodine (I\({}_{2}\)) is converted to iodate (IO\({}_{3}\)) under heating in concentrated HNO\({}_{3}\)[49] it seems highly likely that in **step 2** At(0) or other species present will be converted to astatate (AtO\({}_{3}\)). The fact that the HNO\({}_{3}\) distillation can be heated to dryness without release of volatile astatine species supports formation of a species such as astatate. [PERSON] suggested that the use of 8 M HCl to dissolve the residue from the HNO\({}_{3}\) distillation in **step 3** provides either AtCl\({}_{2}\)\({}^{-}\) or AtCl\({}_{4}\)\({}^{-}\)[30]. The hypothesis was based on the fact that ICI\({}_{2}\)\({}^{-}\) and ICI\({}_{4}\)\({}^{-}\) are known species and it was known that ICI\({}_{2}\) could be extracted into ethers from HCl solutions. In experiments conducted it was demonstrated that the species formed was efficiently extracted DIPE (as in **step 4**), but was not extracted into benzene or CCl\({}_{4}\). The later fact ruled out the formation of neutral species such as AtX (where X is a halide). [PERSON] and [PERSON] later reported on the stability constants of chloride complexes of At formed from varying concentrations of HCl with oxidants present and concluded that AtCl\({}_{2}\)\({}^{-}\) was the likely species formed [50]. Interhalogen species, including AtCl\({}_{2}\)\({}^{-}\), were also prepared by [PERSON] and [PERSON] via distillation methods [24]. Back-extraction with 4 M NaOH in **step 5** produces a species that has very different properties to the DIPE extracted species. Initial evaluation by reversed-phase and anion exchange radioHPLC can have a number of species present, but with time the minor species disappear leaving only one species, which appears to be astatide. It seems likely that the species in DIPE (perhaps AtCl\({}_{2}\)) is hydrolyzed with a strong base to intermediate species before obtaining astatide. It was not the purpose or focus of this investigation to study the At species formed, so the forgoing discussion on potential At species in each step of the isolation process is meant only for consideration. Definitive experiments to identify At species will have to be conducted as separate studies. As stated before, one of our goals was to optimize the wet chemistry isolation procedure.