Field Ionization (FI) versus Field Desorption (FD)
! The Theory
A high electric voltage (potential) that gives rise to lines of equipotential result in an electric field crowd around the needle tip. The electric field is most intense at the surface (point) of the tip and is where ionization occurs.
FI : sample is heated in a vacuum so as to volatilize it onto an ionization surface. FI is suited for use with volatile, thermally stable compounds. FI sources are arranged to function also as FD sources
equipotental lines of the electric field
positive tip electrode
to mass analyzer
FD : the sample is placed directly onto the surface (dipping emitter in an analyte solution) before ionization but FD is needed for non- volatile and/or thermally labile substances.
FI spectrum for D-glucose
FD spectrum for D-glucose