Structural Elucidations of derivatized Sterols using Electrospray Ionization/Atmospheric Pressure Chemical Ionization-Quadrupole Ion Trap-Mass spectrometry

Abstract

Free sterols are neutral molecules that are difficult to ionize by Electrospray Ionization (ESI) or Atmospheric Pressure Chemical Ionization (APCI). Therefore, in order to increase their ionization efficiency, sterols were converted into their corresponding picolinyl esters. In this study we examined the possibility of analyzing picolinyl ester of sterols derivatives using flow injection ESI-quadrupole ion trap (QIT) MS and APCI-QIT MS and we investigated their fragmentation pathways using low energy collision induced dissociation-tandem mass spectrometry (CID-MS2). This study also aimed to examine the possibility of using ESI-QIT MS3 to identify sterol isomers. The picolinyl esters readily formed protonated molecular ions ([M+H]+) in ESI and APCI sources except for the picolinyl ester of 7-dehydrocholesterol which was detected as the radical cation ion [M].+ using APCI-QIT MS. The ester bonds of picolinyl esters cleaved during CID MS2, resulting in diagnostic fragments corresponding to steryl cation moieties [M+H-C6H5NO2]+. The CID MS3 of [M+H]+ → [M+H-C6H5NO2]+ of picolinyl esters was found to be useful for structural elucidation and for distinguishing among steryl isomers.