Identification of fatty acid steryl esters in margarine and corn using direct flow injection ESI-MSn ion trap-mass spectrometry

• Steryl esters present in lipid extracts can be identified rapidly using flow injection ESI-MS as their ammoniated adducts.
• ESI-QIT MSn was successfully used for detailed structural elucidation of steryl esters.
• ESI-MS2 was used to distinguish among steryl ester isomers.
• ESI-MS3 allows positive identification of individual steryl hydrocarbon moieties based on their unique fragmentation fingerprint.

An approach for identification of steryl esters using flow injection ESI-MS2 and ESI-MS3 ion trap mass spectrometry has been developed. Sterols and other lipids extracted from samples of margarine and corn using hexane were subjected to solid phase extraction. The steryl ester fraction was eluted with hexane:diethyl ether (98:2, v/v). In ESI-MS experiments fatty acid steryl esters were easily detected as ammoniated adducts [M+NH4]+ using ammonium acetate as dopant. Steryl esters including molecular isomers could be identified using ESI-QIT MS2 by the facile ester cleavage whereby the charge resides with the steryl moiety. For positive confirmation of the sterol group, ESI-QIT MS3 was carried out on the intact steryl fragmentation cation. The resulting CID spectra of the steryl cation were found to be unique and similar to those from APCI-QIT MS2 CID of free sterol standards. All major steryl esters were identified in both margarine (12 esters) and corn (7 esters) extracts. Based on the ion intensity of the ESI-MS spectra the major steryl esters in margarine were: β-sitosteryl stearate, β-sitosteryl oleate, β-sitosteryl linoleate, campesteryl stearate and campesteryl linoleate. In corn they were: β-sitosteryl oleate, campesteryl stearate, campesteryl oleate and stigmasteryl linoleate. This method can be very useful for rapid and complete identification of fatty acid steryl esters extracted from other biological samples.

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