Unsaturated wax esters (WEs) provided molecular adducts with C(3)H(5)N ([M + 55](+•)) in APCI sources in the presence of acetonitrile. CID MS/MS of [M + 55](+•) 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 α and ω ions has been introduced. Fragmentation of the α-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.

This article is available online at http://www.jlr.org Dietary WEs are an important source of very long chain fatty alcohols and acids that exert regulatory roles in the cholesterol metabolism ( 11 ). Naturally occurring WEs usually form complex mixtures composed of many molecular species. These mixtures contain straight-and branchedchain esters of various chain lengths and numbers of double bonds depending on the biochemical synthetic pathways in particular organisms. WEs are also produced industrially and used in large quantities in cosmetics, polishes, lubricants, surface coating, and other applications ( 12 ). Gas chromatography (GC) has frequently been used for analyzing WEs, often after their hydrolysis. The separation of intact WEs has been made possible by the introduction of high-temperature columns. GC coupled with electron ionization mass spectrometry (GC/EI-MS) offers high separation effi ciency, ease of use, and mass spectra allowing structure elucidation. Other analytical methods based on liquid chromatography/mass spectrometry or matrix-assisted laser desorption/ionization mass spectrometry ( 13-15 ) can also be utilized, especially in the case of thermally unstable or insuffi ciently volatile WEs.The research on the mass spectra of FA esters can be traced back to the early days of mass spectrometry. The EI spectra are explained perhaps in all of the textbooks on organic mass spectrometry, but the discussion is mostly limited to methyl or ethyl esters. The EI mass spectra of saturated straight-chain WEs has been studied for more than 40 years using both low-and high-resolution measurements and deuterium-labeling experiments (16)(17)(18)(19)(20)(21)(22). The spectra interpretation is usually straightforward in terms of the determination of the molecular weight and acid/alcohol chain length. However, the structure elucidation becomes much more complicated for branched or unsaturated esters, e.g., in case of samples of animal or human origin. Only a few reports closely dealing with the Abstract The interpretation of the electron ionization mass spectra of straight-chain and methyl-branched saturated and unsaturated wax esters (WEs) is discussed in this study based on the spectra of 154 standards. The most important fragments indicative of the structure of the acid and alcohol chains are identifi ed and summarized for WEs with various number of double bonds in the chains. Briefl y, most WEs provide acylium ions allowing structural characterization of the acid part, whereas the alcohol part gives corresponding alkyl radical cations. The elemental composition of selected important fragments is established from a highresolution accurate mass analysis. The ion abundances are discussed with respect to the length and unsaturation of the aliphatic chains. The interpretation of the spectra of branched or unsaturated WEs requires the recognition of small but important peaks that are diffi cult to discern among the other fragments. We demonstrate that such fragments are easily detected in differential mass spec...

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