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FIGURE 2. 
-Ray spectra show 364.5-keV photopeak of 131I measured with HpGe well detector. Top and bottom panels show 131I photopeaks corresponding to radioactivity in 10,000 labeled (2.4 mBq per cell) and 10,000 unlabeled cells, respectively, obtained from cell sorter. Counts were accumulated for 1,000 s. Integral counts under peaks after background subtraction were 3,145 and 20 for labeled and unlabeled cells, respectively. It can be seen clearly that unlabeled cell population contains only 0.6% as much 131I as labeled cell population. This suggests at least a 99.4% sorting purity. Furthermore, high precision of sorting and activity measurement ensured that there was no significant migration of radioactivity from labeled cells to unlabeled cells.
