To determine whether Gd-daa3 is selective for Zn2 , researchers compared relaxivity measurements of Gd-daa3 in the presence of different cations. Varying amounts of XCl2 (where X is Zn, Ca, or Mg) were added to a 0.1 mM solution of Gd-daa3 plus 10 mM KCl/10 mM HEPES buffer at pH 7.4. What is the most appropriate control for this experiment