Thermodesorption Kinetics of Nikethamide on Oxidized Tungsten Surfaces

Abstract

The analeptic substance nikethamide was studied under both stationary and non-stationary processes using the surface ionization method. The composition of ions formed during the thermal ionization of nikethamide molecules from a heated oxidized tungsten surface, as well as their temperature dependences, was determined under stationary surface ionization conditions. A voltage modulation system was developed and implemented in a magnetic mass spectrometer to investigate the kinetic characteristics of nikethamide molecules under non-stationary surface ionization conditions. The kinetic characteristics of the thermal desorption of the deprotonated ion [M–H]+ (m/z 177) and the fragment (radical) ion [M–R]+ (m/z 161) of nikethamide were determined using the voltage modulation method. These characteristics include desorption rate constants, ionization coefficients, pre-exponential factors, and activation energies. The activation energies for the corresponding neutral species were also evaluated. In addition, the ionization potentials of these ions were determined. The obtained ionization energies are lower than that of the parent molecule, confirming consistency with previously established surface ionization regularities. Moreover, the calculated ionization coefficients for the ions at m/z 177 and 161 were 0.96 and 0.97, respectively, indicating that the deprotonated ion at m/z 177 and the fragment ion at m/z 161 are formed with nearly complete efficiency during thermal ionization.