DESORPTION OF CROWN-ETHER ALKALI-METAL ION COMPLEXES IN LIQUID SECONDARY-ION MASS-SPECTROMETRY (LSIMS)

The liquid secondary ion mass spectra of crown ether solutions contain ing an equimolar mixture of alkali halides display large differences i n the relative abundances of the crown ether-cation complexes formed, depending on various desorption yields. The results presented here sho w that from dilute solutions of samples, the less solvated ion species lead to better sensitivities. At higher concentrations, the matrix hy drophilicity plays an important role by discriminating between the mix ed complexes in favour of those possessing both a surface activity and limited interactions with the solvent. The desorption of other ions i s delayed by a slower desolvation, thus increasing the ion pairing by recombination with the anions present. This leads to an ion signal sup pression in the case of strongly solvated complexes (lithium complexes ) or anions (chlorides). Use of hydrophobic matrices limits considerab ly the solvation energy of the dissolved complexes thus reducing the d iscrimination between the complexes towards the desorption process. Pr ovided that an appropriate choice of matrix, counteranion and other ex perimental parameters has been made, the desorption of crown ether-alk ali cations does not suffer from any discriminatory effect.