Cation complexation by crown ethers is an important area of study because many biological processes involve the interaction of an alkali or alkaline earth metal cation with a carrier or channel. These ionophores are often complex structures and assessment of interaction is difficult. Use of the simple crown ether structures facilitates these studies and offers other applications as well. The fast atom bombardment mass spectrometry (FAB/MS) technique allows the complexation phenomenon to be assessed largely in the absence of solvent and using relatively small quantities of ligands. Moreover, this technique gives information about complexation when other methods simply cannot be applied due to the complexity of the binding interaction.