High Resolution Mass Spectrometry Fragmentation Rules of Etomidate and Its Structural Analogues.

Objectives: To explore the characteristic fragment ions, ion abundance ratios and mass spectrometry fragmentation rules of etomidate and its structural analogues by using gas chromatography-quadrupole/orbitrap mass spectrometry (GC-Q/Orbitrap MS) and liquid chromatography-linear ion trap quadrupole-orbitrap mass spectrometry (LC-LTQ-Orbitrap MS) techniques, providing important data support for the identification and prediction of etomidate structural analogues.

Methods: GC-Q/Orbitrap MS and LC-LTQ-Orbitrap MS were used to analyze metomidate, etomidate, isopropoxate and propoxate, to obtain their GC high resolution mass spectra and LC high resolution mass spectra.

Results: Under the electron impact (EI) ion source mode, etomidate, metomidate and the other two analogues all produced their molecular ions and seven identical fragment ions (m/z 77.038 6, 79.054 2, 95.024 0, 105.069 9, 143.073 0, 172.099 5 and 199.086 6), among which isopropoxate and propoxate also produced characteristic fragment ions m/z 216.089 3. In the collision cell of the electrospray ionization (ESI) source mode, etomidate, metomidate and the other two analogues all produced three identical fragment ions (m/z 95.024 0, 105.069 9 and 113.034 6). Meanwhile, each substance produced one characteristic fragment ion (metomidate: m/z 127.050 2; etomidate: m/z 141.065 9; isopropoxate and propoxate: m/z 155.081 5).

Conclusions: Common fragment ions exist among etomidate and its structural analogues, and characteristic ion fragments are generated based on the different carbon numbers of their side chains. The structure of side chains can affect the abundance ratio of each fragment ion, providing a basis for the structural identification and prediction of such compounds.