Ion mobility-mass spectrometry separation of steroid structural isomers and epimers

Abstract

Drift tube ion mobility spectrometry (DTIMS) coupled with mass spectrometry was evaluated for its capabilities in rapid separation of endogenous isomeric steroids. These compounds, which included eight isomer groups, were investigated as protonated and sodiated species and collision cross sections were measured for all ionization species of each steroid. Pregnenolone (CCSN2 176.7 A2) and 5α-dihydroprogesterone (CCSN2 191.4 A2) could be separated as protonated species, and aldosterone (CCSN2 197.7 A2) and cortisone (CCSN2 211.7 A2) could be separated as sodiated monomers. However, the sodiated dimers of the remaining isomers yielded increased separation, resulting in baseline resolution. Specific structural differences including ring conformation and the chirality of hydroxyl groups were compared to evaluate their relative effects on collision cross section in isomers. These results indicated that C5 ring conformation isomers androsterone and etiocholanolone, which both contain a C3 α-hydroxyl group, yielded similar dimer CCS. Yet these compounds were well resolved from their respective β-hydroxyl epimers, trans-androsterone and epietiocholanolone. Alternative drift gases were evaluated, and carbon dioxide drift gas offered slight improvement in isomer resolution well, including allowing separation of testosterone (CCSCO2 330.0 A2), dehydroepiandrosterone (CCSCO2 312.6 A2), and epitestosterone (CCSCO2 305.6 A2). Finally, different metal cation adducts, including alkali, alkaline earth, and first row transition metal adducts were analyzed, and several of these species provided improved resolution between steroid epimers. Overall, this study shows that drift tube ion mobility is a promising tool for improved separation of isomeric steroids.