Abstract: | The selectivity of eight lariat crown ethers in the sym‐dibenzo‐16‐crown‐5 series toward alkali metal ions was studied with electrospray ionization mass spectrometry under different conditions. With the exception of 2g , which is equally selective toward Na+ and Li+, all other lariat crown ethers show the best selectivity toward Li+ in methanol. Factors that influence the selectivity include the water content, counterions, nature of the side arms, and the externally added cations. Iodide gives the best Na+ selectivity with RI > RBr > RCl. Increased water content profoundly increases the Na+ selectivity when the side arm is hydrophilic and the steric hindrance is small. Externally added cations (Cs+ and/or Rb+) enhance the Na+ selectivity by exchanging the smaller Li+ from the cavity. |