Localization of double bonds in wax esters by high-performance liquid chromatography/atmospheric pressure chemical ionization mass spectrometry utilizing the fragmentation of acetonitrile-related adducts
Abstract
Unsaturated wax esters (WEs) provided molecular adducts with C(3)H(5)N ([M + 55](+center dot)) in APCI sources in the presence of acetonitrile. CID MS/MS of [M + 55](+center dot) yielded fragments allowing the localization of double bond(s) in the hydrocarbon chains of the WEs.
These fragments were formed by a cleavage on each side of the double bond. In methylene-interrupted polyunsaturated WEs, diagnostic fragments related to each double bond were detected; the most abundant were those corresponding to the cleavage of the C-C bond next to the first and the last double bond.
To differentiate between those fragments differing in their structure or origin, a simple nomenclature based on alpha and omega ions has been introduced. Fragmentation of the alpha-type ions (fragments containing an ester bond) provided information on the occurrence of a double bond in the acid or alcohol part of the WEs.
While no significant differences between the spectra of the WEs differing by cis/trans isomerism were found, the isomers were separated chromatographically. A data-dependent HPLC/APCI-MS(2) method for the comprehensive characterization of WEs in their complex mixtures has been developed and applied to natural mixtures of WEs isolated from jojoba oil and beeswax.
More than 50 WE molecular species were completely identified, including the information on the acid and alcohol chain length and the position of the double bonds.