Can Complexation Constants in Solution Be Measured by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry?

In matrix-assisted laser desorption/ionization (MALDI), ions are either created in the laser ablation plume or are already present in the condensed phase and liberated into the gas phase. The contribution of these two ion formation pathways varies from sample to sample. To study interactions between molecules in solution, it is therefore important to know to what extent the MALDI mass spectrum reflects the condensed phase chemistry. We investigated the complexation between a crown ether and metal ions in glycerol using a liquid/solid sample preparation (two-phase MALDI). Relative stability constants were determined for 18-crown-6 and sodium, potassium, rubidium, against cesium as the reference cation. Since different ions are not usually detected with equal efficiency, the relative sensitivities for each complex were first determined. The relative stability constants in glycerol were then measured and found to be in very good agreement with literature values. From these findings it was concluded that the two-phase MALDI mass spectra quantitatively reflect the glycerol solution chemistry of the alkali–crown complexes. This in turn suggests that, in this case, preformed ions are brought into the gas phase by the laser pulse, and that gas-phase reactions do not subsequently modify the ion distribution.