Can complexation constants in solution be measured by matrix- assisted laser desorption/ionization mass spectrometry?

Abstract

Laboratorium fur Organische Chemie, ETHZ, 8092 Zurich, Switzerland.E-mail: zenobi@org.chem.ethz.ch.In matrix-assisted laser desorption/ionization (MALDI), ions are either created in the laser ablation plume or are already present inthe condensed phase and liberated into the gas phase. The contribution of these two ion formation pathways varies from sample tosample. To study interactions between molecules in solution, it is therefore important to know to what extent the MALDI mass spec-trum reflects the condensed phase chemistry. We investigated the complexation between a crown ether and metal ions in glycerolusing a liquid/solid sample preparation (two-phase MALDI). Relative stability constants were determined for 18-crown-6 andsodium, potassium, rubidium, against cesium as the reference cation. Since different ions are not usually detected with equal effi-ciency, the relative sensitivities for each complex were first determined. The relative stability constants in glycerol were then mea-suredandfoundtobeinverygoodagreementwithliteraturevalues.Fromthesefindingsitwasconcludedthatthetwo-phaseMALDImass spectra quantitatively reflect the glycerol solution chemistry of the alkali–crown complexes. This in turn suggests that, in thiscase, preformed ions are brought into the gas phase by the laser pulse, and that gas-phase reactions do not subsequently modify theion distribution.Keywords: MALDI, mass spectrometry, non-covalent complexes, crown ethers, stability constants.