## 4 Discussion  
This investigation of a wet chemistry approach for isolation of \({}^{211}\)At was prompted by difficulties in our production of \({}^{211}\)At brought about by a large increase in size of the bismuth target used. The increase in target size came about when designing and implementing an external target station for increasing our \({}^{211}\)At production levels [29]. It should be noted that our cyclotron situation is somewhat unique, as an external target was required rather than an internal target [43; 44; 45] because the cyclotron had to remain in daily operation for fast neutron therapy of cancer patients. In addition to circumventing difficulties with the size increase of target, we were also interested in determining whether the wet chemistry method could provide more consistent \({}^{211}\)At isolation yields than previously obtained with dry distillation. It is our belief that a robust isolation method will be required when conducting clinical studies. The wet chemistry approach described herein is not new. In 1949, [PERSON] _et al_. reported that bismuth targets could be dissolved in conc. HNO\({}_{3}\) with little loss of \({}^{211}\)At when the HNO\({}_{3}\) was distilled away [37]. Eight years later, [PERSON] described a similar procedure using extraction of \({}^{211}\)At into DIPE and back-extracting into NaOH [30]. In 1973, [PERSON] and [PERSON] reported extracting \({}^{211}\)At into DIPE, but used aqueous hydroxylamine to back-extract [31]. More recently [PERSON] _et al_. described a similar approach using DIPE or butylether for the extraction of \({}^{211}\)At [32], and [PERSON] _et al_. described an approach using the DIPE extract directly for radiolabeling of _m_-Me\({}_{3}\)Snbenzoate _N_-hydroxysuccinidyl ester [46]. [PERSON] _et al_. also described a similar wet chemistry approach to obtain \({}^{211}\)At on silver foils [33]. Interestingly, [PERSON] _et al_. reported on an investigation of the extraction mechanism when \({}^{211}\)At is extracted into DIPE [35]. Our goals in investigating the wet chemistry approach were: (1) to evaluate previous approaches and optimize conditions to efficiently isolate \({}^{211}\)At from the large amounts of bismuth (up to 3 g) in our targets; and (2) to demonstrate reproducibility of the optimal approach. The data presented herein support our achieving those goals. A secondary goal is to further refine the wet chemistry approach such that it can be automated. Isolation of \({}^{211}\)At by an automated wet chemistry isolation system is \begin{table}
\begin{tabular}{c c c c} \hline \({}^{211}\)At Isolation Runs & **Corrected Act.** & **Isolated Activity** & **Est. Attenuation* \& Decay** \\ **(most \(\sim\)0.75 h irrad.)** & **Produced (mCi) ** & **(mCi)** & **Corrected Recovery (\%) *** \\ \hline Non-optimized (\(n\) = 53) & 27.6 \(\pm\) 2.3 & 13.6 \(\pm\) 2.3 & 65 \(\pm\) 11 \\ Optimized (\(n\) = 55) & 26.1 \(\pm\) 2.7 & 15.9 \(\pm\) 2.7 & 78 \(\pm\) 11 \\ Preparative Run (2 h) & 72 & 43.2 & 73 \\ Preparative Run (4 h) & 134 & 72.0 & 65 \\ \hline \end{tabular}  
* An attenuation correction of \(\sim\)1.33 has been applied to the measured bismuth target value based on an average difference between the \({}^{211}\)At reading in DIPE (corrected for decay and losses) and the initial target reading;
** Estimated mCi of \({}^{211}\)At produced in bismuth target based on approximation that the reading was 75% of actual; *** Estimated recovery yields if decay and attenuation of \({}^{211}\)At emissions are factored into calculations.  
\end{table} 
Table 3: Wet chemistry \({}^{211}\)At isolation yields corrected for decay and attenuation *. currently under investigation. While all of the steps in the described isolation approach can be readily accomplished, the miscibility issues with DIPE separations requiring pre-saturation of reagents is not favorable for automation, so alternative separation approaches are being sought. The chemistry of radiolabeling with \({}^{211}\)At is not always straightforward [47, 48].