Ion velocities of laser desorbed ions passing through quadrupole electric fields

This paper is a study of the effects of quadrupole electric fields on the velocities of ions generated by laser desorption ionization (LDI). We used a charge detector to measure ion flight time in the absence of electric and magnetic fields to obtain the initial velocity distribution of ions generated in the LDI process. It was found that the measured ion velocity of fullerene and matrix ions was proportional to the energy of the laser pulse. Then we measured velocity distributions of fullerene ions in rf-only quadrupoles. It was observed that the electric field in the fringing field region was proportional to pseudo-potential well depth (Dz) and that ions gained velocity in the fringing field region and speeded up as the Dz was increased due to transfer of rotational energy into translational energy in the high-order field region. Initial velocity of ions passing through different stages of quadruple was increased at every stage. In addition, the mass spectrometric analysis showed when the rf amplitude was low, the quadrupole field allowed stable motion of low mass ions; when the rf amplitude was increased, effective potential well depth was reduced. This is evident that the gradient of ion velocity drops as the difference in Dz increases for C60 and high mass Cn cluster ions.