Fig. 2

Mass spectrum profile of the commercially available DOTA-tris (t-Bu ester). As can be seen, besides the main product peak at 573, DOTA (tetraxetan) peak at 405, monoalkylating product peak at 461, and dialkylating product peaks at 517 are clearly visible in the mass spectrum. In addition, the peak found at 635 possibly correlated with tetraalkylating product 629 (M + t-Bu)+