Debye-shielding mechanism for trapping ions formed by laser desorption Fourier transform ion cyclotron resonance mass spectrometry

Abstract

The trapping of metal ions in laser desorption/ionization (LDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is attributed to an electrostatic shielding mechanism promoted by the sustained quasi-neutral plasma behavior of the desorbed particle plume in strong magnetic fields. This shielding process allows low energy ions to penetrate the applied trapping potentials at the trapped-ion cell, thus resulting in the introduction of ions with energies less than the effective depth of the trapping potential well. Subsequent deshielding of these ions while in the cell exposes them to the trapping field and results in their retention. Data from time-of-flight (TOF) studies indicate that large spatially and temporally overlapped populations of high energy ions and low energy electrons are generated by LDI of a variety of metal targets when laser power density exceeds 10 7–10 8W cm −2. The charge density in the desorbed plasma is shown to increase during flight along converging magnetic field lines but to dissipate rapidly on exiting the field. Retarding potential studies in the magnetic field performed with both TOF and FT-ICR detection indicate that the Debye shielding exhibited by these quasi-neutral populations is sufficient in some cases to allow ions with energies on the order of 1 eV to penetrate retarding potentials as high as 500 V. Further indication that such effects are present in the LDI FT-ICR experiment is given by TOF kinetic energy analysis of ions acquired in the trapped-ion cell from LDI and then dumped to an external detector. This analysis indicates that the average kinetic energy of such ions is typically only 60% of the applied trapping potential.