Electron Ionization (EI) Electron Ionization (EI) Electron - PowerPoint PPT Presentation

Electron Ionization (EI) Electron Ionization (EI) Electron Ionization (EI) Electron Ionization (EI)

Electron Ionization (EI) Electron Ionization (EI) ���������������������������������������������������

Electron Ionization (EI) Electron Ionization (EI) � Electron beam from a tungsten filament to knock off an Electron beam from a tungsten filament to knock off an � electron from analyte to form an ion electron from analyte to form an ion � Electrons are usually accelerated by a potential of 70eV Electrons are usually accelerated by a potential of 70eV � � The paths of the electrons and the molecules are at right � The paths of the electrons and the molecules are at right The paths of the electrons and the molecules are at right The paths of the electrons and the molecules are at right � � angles and they intersect at the source where collisions angles and they intersect at the source where collisions and ionizations occur and ionizations occur � Excess energy results in “daughter ions” Excess energy results in “daughter ions” � � Hard ionization technique Hard ionization technique � � “Daughter ions” can be used to identify target “Daughter ions” can be used to identify target � compounds compounds

Electron Ionization (EI) (contd…) Electron Ionization (EI) (contd…) � Can be performed in direct probe as well as Can be performed in direct probe as well as � interfaced interfaced � Sample can be solid, liquid or gas Sample can be solid, liquid or gas — has to be has to be � volatile volatile � High sensitivity can be obtained High sensitivity can be obtained �

EI Fragmentation of MeOH EI Fragmentation of MeOH

EI Mass Spectrum of MeOH EI Mass Spectrum of MeOH

Typical Reactions In EI Source Typical Reactions In EI Source

PLUS and MINUS of EI PLUS and MINUS of EI

Advantages of EI Advantages of EI � Reproducible method Reproducible method � � Libraries of EI spectra Libraries of EI spectra � � High Ionization High Ionization help ID help ID � Efficiency Efficiency � Improved sensitivity by Improved sensitivity by � � Ionization is non � Ionization is non Ionization is non- Ionization is non- � � having no suppression having no suppression having no suppression having no suppression selective selective � All vaporized molecules All vaporized molecules � � Extensive fragmentation Extensive fragmentation � can be ionized (non polar can be ionized (non polar occurs occurs and insoluble) and insoluble) � Interface to GC possible Interface to GC possible � � Molecular structural Molecular structural � information information

Disadvantages of EI Disadvantages of EI � Only +ve ions are formed Only +ve ions are formed � Not ideal for some Not ideal for some � � classes of compounds classes of compounds � Sample has to be volatile Sample has to be volatile � � Rearrangement process Rearrangement process � � Large internal energy Large internal energy � complicates spectra complicates spectra complicates spectra complicates spectra method method method method � Limits to 600Da or less Limits to 600Da or less � � No interface to LC No interface to LC � MW MW � Solid probes need skilled Solid probes need skilled � � Limits value in MW Limits value in MW � operator operator determination due to determination due to extensive fragmentation extensive fragmentation