Charge-remote molecular hydrogen removal in protonated and alkali-cationized long-chain fatty acid esters upon cesium ion bombardment

Abstract

The mass spectral behavior of cationized saturated fatty acid derivatives has been studied in order to gain insight into the loss of molecular hydrogen during cesium ion bombardment. It is shown that molecular hydrogen and hydrogen radical loss occurs for protonated and alkali (Li +/Na +)-cationized fatty acid methyl esters as well as for protonated acylcarnitines and that molecular hydrogen loss is dependent upon the acyl chain length. Investigation of ion structures by collision-induced dissociation tandem mass spectrometry indicates that in the case of alkali-cationization a hydrogen radical is lost from all positions of the saturated acyl chain, whereas in the case of protonated molecules a hydrogen radical is predominantly eliminated from the protonated ester group. For the primary reaction of the dehydrogenation chemistry, an ion-beam-induced excitation of a cationized molecule is proposed, yielding a diradical species that gives rise to an intermolecular reaction with a neutral analyte molecule. Subsequent cationization of the neutral [M–H 2] species formed in this primary reaction leads to the formation of the satellite ion at m/ z values 2 u lower compared to the cationized molecule.