A laser ablation electron impact ionization time-of-flight mass spectrometer for analysis of condensed materials

Laser desorption/ionization (LDI) mass spectrometry suffers from poor reproducibility and a strong bias in representing the chemical composition of the sample in the mass spectra. In regard to develop laser mass spectrometry towards a quantitative analysis technique desorption and ionization processes should be separated. Here we describe a time-of-flight mass spectrometer (TOF-MS) with laser ablation and subsequent electron impact ionization (LA-EI-MS). Sample material is ablated by an Nd-YAG laser pulse (lambda=355 nm, E=0.5-35 mJ, tau=9 ns), which produces ions and neutrals in the ablation plume. After expansion, parts of the plume enter the EI ion source where neutrals are ionized by electron impact (E-el=70 eV). Ions are extracted from the ion source into the flight tube of the TOF-MS by applying a pulsed repeller potential. Depending on delay times between laser pulse and repeller pulse either neutrals or ions emitted from the sample were analyzed. Different mass spectra of the same sample are obtained in these two analyzing modes under the same irradiation conditions. With a relative standard deviation of 10% to 15%, reproducibility of mass spectra is significantly better in LA-EI-MS than in LDI-MS. (C) 2002 American Institute of Physics.