Electron Ionization LC-MS: What Is It and Why Use It?

Electron ionization (EI) is a unique and advantageous ionization technique typically employed in gas chromatography-mass spectrometry (GC-MS). Here are presented two different approaches to benefit from EI in liquid chromatography-mass spectrometry (LC-MS). The first EI-based interface exploits EI of vibrationally cold molecules in supersonic molecular beams (SMBs) (hence named Cold EI). This system and its operational methods are centred on pneumatic spray formation of the liquid flow in a heated vaporization chamber, full thermal vaporization of the sample compounds and their subsequent EI as vibrationally cold molecules in the SMB after their supersonic expansion from a supersonic nozzle. Cold EI mass spectrum exhibits the standard EI fragments with enhanced molecular ion. The second EI-based interface is called Liquid-EI (LEI), and it is based on the nebulization of the liquid phase carrying the analytes inside a specifically designed region where the vaporization of solutes occurs at atmospheric pressure before entering the high-vacuum ion source. LEI guarantees more flexibility towards low volatile and thermolabile molecules analysis in respect to previous versions of this approach. The fine control of the vaporization temperature and the inertness of the new vaporization surface can enhance the overall interface performances. LEI generates typical EI mass spectra for the undoubted identification of targeted and nontargeted analytes. The two systems feature broad linear dynamic range, uniform response both for polar and nonpolar compounds and lack of matrix ion suppression effects that plague electrospray ionization and atmospheric pressure chemical ionization. These features are shown, described and demonstrated in different examples and applications.