Electron Ionization LC-MS with Cold EI and its Recent Applications

Svetlana Tsizin, Chemistry, Tel Aviv University, Tel Aviv, Israel
Boaz Seemann, Chemistry, Tel Aviv University, Tel Aviv, Israel
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

Electron Ionization (EI) LC-MS is highly desirable as it enables automated library based identification and is free of any ion suppression effects that plague ESI and/or APCI. Thus, bringing back EI to LC-MS is highly valuable if a robust, simple to operate and low cost EI-LC-MS system can be designed and built.

We developed such an EI-LC-MS instrument that can be assembled on Varian 1200 or Agilent 5977 MSD , both of which have quadrupole mass analyzer. Moreover, the system is based on Cold EI method, namely EI of vibrationally cold molecules in supersonic molecular beams (SMB). The LC effluents are pneumatically sprayed inside a heated liner, thus go through thermal vaporization. The vaporization chamber is connected to the supersonic nozzle via a capillary flow restrictor to suppress cluster formation. After passing through the supersonic nozzle, the supersonic molecular beams are ionized in a fly-through ion source and analyzed by a quadrupole mass analyzer.

Cold EI results in enhanced molecular ions which are often weak or missing in standard EI, yet it is fully compatible with library identification and it uniquely provides: 

1. Library identification with compounds names and structures

2. Extensive fragment MS information

3. Our TAMI molecule identifier software enables the elucidation of elemental formula

4. The fly-through ion source eliminates any ion source degradation

5. Non-polar compounds analysis is enabled

6. Cold EI provides uniform, compound independent ionization efficiency

7. Cold EI does not suffer from any ion suppression or enhancement effects 

Our system was explored in the analysis of cannabinoids in Cannabis flowers, in the analysis of anti-oxidant compounds and cholesterol, free fatty acids and lipids that are highly important for blood analysis applications.