Electron ionization can significantly benefit LC-MS through the provision of automated library identification and extensive fragment information which is ideally suitable for LC-MS identification of unknown compounds. Thus, bringing back EI to LC-MS is highly valuable if a reliable and robust EI interface, with standard flow rates acceptance can be developed. We developed a novel EI-LC-MS approach, based on interfacing LC and MS with supersonic molecular beams (SMB) and sample ionization with electrons as vibrationally cold compounds in the SMB. The output of the LC is vaporized and expands into the vacuum system from a 300 (mu)m supersonic nozzle at about 0.1 Bar nozzle backing pressure to suppress cluster formation while obtaining efficient vibrational cooling. Sample vaporization is based on pneumatic spray formation followed by fast, thermal vaporization of the sample compounds prior to their supersonic expansion. The new design prevents the robustness problems of previous thermally assisted spray models, arising from frequent clogging of the solvent delivery tube whose replacement required venting of the MS vacuum system.
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