Drug Impurities Analysis using Electron Ionization LC-MS with Cold EI

Svetlana Tsizin, Chemistry, Tel Aviv University, Tel Aviv, Israel (svet3@mail.tau.ac.il)
Tal Alon, Chemistry, Tel Aviv University, Tel Aviv, Israel
Alexander Fialkov, Chemistry, Tel Aviv University, Tel Aviv, Israel
Aviv Amirav, Chemistry, Tel Aviv University, Tel Aviv, Israel

When active pharmaceutical ingredients (API) are analyzed for the presence of impurities, the general requirements are that every impurity found should be at concentration below 0.1% of the API or it must be either cleaned or identified and toxicologically characterized which is an expensive and time consuming procedure. However, the ionization yields of Electrospray and/or APCI in LC-MS are highly non-uniform and compound dependent, and as a result when an impurity is found in a given LC-MS mass chromatogram its actual concentration is unknown. In addition, several types of impurities such as non-polar compounds are not detected by ESI-LC-MS plus those impurities that elute near the API suffer from ion suppression effects.

We developed a unique LC-MS system that uses electron ionization (EI) of vibrationally cold molecules in supersonic molecular beams (SMB). The LC effluents are pneumatically sprayed inside a heated liner that is connected to the supersonic nozzle via a capillary flow restrictor to suppress cluster formation. Ones expended through this nozzle the sample compounds are vibrationally cooled prior to their ionization. The results of electron ionization at this stage create Cold EI mass spectra that feature highly abundant molecular ions with extensive fragment information for effective library identification. Also, the use of the fly-through ion source eliminates any ion source degradation that is otherwise present. This EI-LC-MS-SMB system uniquely enables library based identification including at the isomer level and it is free of any ion suppression effects that plague ESI and/or APCI.  

We explored the use of our EI-LC-MS for drug impurities analysis and we demonstrated Milestone sensitivity of S/N>10,000 for Ibuprofen and imipramine API drugs. LC-MS with Cold EI has uniform response, no ion suppression, ability to ionize also non-polar compounds and provide valuable fragment information for impurities identification thus this application seems ideal for Cold EI.