Electron Ionization LC-MS with Cold EI – Why is it Better Than APCI-LC-MS

Electron Ionization (EI) is a highly desirable ionization method for LC-MS (EI-LC-MS) as it enables library-based identification including at the isomer level, can serve for the analysis of non-polar compounds, provides uniform compound independent response and it is free of any ion suppression effects.
We developed EI-LC-MS with Cold EI based on an Agilent 5977 quadruple MS. Cold EI is EI of vibrationally cold molecules in supersonic molecular beams (SMB) in a fly-through ion source. Cold EI provides enhanced molecular ions, significantly extended range of compounds amenable for analysis in comparison with standard EI, lower limits of detection and uniform response.
In EI-LC-MS with Cold EI the LC effluents are pneumatically sprayed (as in APCI), then fast thermally vaporized. The vaporization chamber is connected to the supersonic nozzle via a capillary flow restrictor to suppress cluster formation. The molecules in the SMB are then ionized in a fly-through ion source.
Recently we combined our GC-MS and LC-MS with Cold EI systems in one Agilent 5975 MS system. The unified LC-MS and GC-MS use the same fly-through ion source and mode changing is automated by the mouse click without any hardware change. We consider Cold EI as a superior LC-MS ion source than APCI or APPI in view of its following benefits:
1. Library based identification with names & structures at the isomer level.
2. Enhanced molecular ions and MS information with Cold EI than standard EI.
3. Library identification means low-cost single quadrupole can be used.
4. No ion suppression or enhancement effects, thus faster LC-MS is enabled.
5. Uniform semi-quantitative ionization yields for improved quantification.
6. Broad range of compounds like with APCI plus non-polars.
7. Good sensitivity (S/N), and superior linearity.
8. GC-MS and EI-LC-MS can be combined in one MS system.