Determination of binding selectivities by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry is used for the measurement of binding selectivities of four model hosts, 18-crown-6, dibenzo-18-crown-6, dicyclohexano-18-crown-6, and 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane, for both alkali and transition metal ions. Based on the intensities of the metal complexes observed in the mass spectra, the relative binding selectivities can be directly estimated. Results are compared to theoretical selectivities obtained by solving simultaneous equilibria equations that define the competing complexation reactions in solution. Good agreement is found in most cases, thus allowing rapid determination of binding selectivities of hosts in a variety of solvents.