Desorption of crown ether–alkali metal ion complexes in liquid secondary ion mass spectrometry (LSIMS)

Abstract

The liquid secondary ion mass spectra of crown ether solutions containing an equimolar mixture of alkali halides display large differences in the relative abundances of the crown ether–cation complexes formed, depending on various desorption yields. The results presented here show that from dilute solutions of samples, the less solvated ion species lead to better sensitivities. At higher concentrations, the matrix hydrophilicity plays an important role by discriminating between the mixed complexes in favour of those possessing both a surface activity and limited interactions with the solvent. The desorption of other ions is delayed by a slower desolvation, thus increasing the ion pairing by recombination with the anions present. This leads to an ion signal suppression in the case of strongly solvated complexes (lithium complexes) or anions (chlorides). Use of hydrophobic matrices limits considerably the solvation energy of the dissolved complexes thus reducing the discrimination between the complexes towards the desorption process. Provided that an appropriate choice of matrix, counteranion and other experimental parameters has been made, the desorption of crown ether-alkali cations does not suffer from any discriminatory effect.