Efficient cationization by Cs + adduct ion formation in a supersonic beam

Abstract

We here propose Cs + adduct ion formation in a supersonic expansion as a universal and very efficient ionization method for the production of medium sized and large molecular ions. The method is combined with laser desorption and time-of-flight mass spectrometry. By laser desorption of a mixture of sample and alkali salt, atomic alkali ions and isolated sample molecules are initially formed and brought into the gas phase. Subsequent adduct ion formation is found to be greatly enhanced by supersonic co-expansion with rare gas. Among various alkali salts cesium salts produce the largest amount of atomic alkali ions. We attribute this to the low ionization potential of cesium and the facile charge separation in these salts. In contrast to all previous experiments investigating adduct ion formation, for Li +, Na +, K + and Cs + we find that the relative efficiency of adduct ion formation is not correlated with the binding energy. We explain this by an extremely efficient stabilization of short-lived collision complexes in the supersonic expansion. The efficiency of Cs + adduct formation is found to be sensitive to the number of polar groups acting as binding sites and their geometrical arrangement in the molecule. Such a multiple site binding in combination with a kinetic trapping effect explains the high efficiency of the cationization process found for large molecules: up to 90% of the laser-desorbed gramicidin D molecules were transformed to Cs + adduct ions. Due to efficient cooling and a strong reactivity enhancement in the supersonic expansion we also achieve efficient adduct formation for simple long-chain hydrocarbon molecules. Advantages of the new technique are: broad applicability, high ion yield, exclusive formation of singly charged ions and high mass resolution of m/ Δm = 2000.