The application of ultra-short light pulses for the analysis of quickly relaxing organic molecules by means of laser mass spectrometry

The combination of resonance enhanced multiphoton ionization (REMPI) and ti me-of-flight mass spectrometry, commonly referred to as laser mass spectrom etry, has proven to be a powerful analytical tool with advantageous feature s for certain problems. However, these advantages are only achievable as lo ng as the resonant intermediate level(s) being involved in the photo ioniza tion step are long-lived on the time scale of the ionizing laser pulse. A w ay to overcome this problem is to apply intense ultra short laser pulses in order to finish the ionization process before relaxation in the intermedia te state(s) can occur. In this paper the differences between conventional n anosecond and sub-picosecond multiphoton ionization of polyatomic organic m olecules and the consequences on the laser mass spectrometry of these compo unds are discussed. As examples for groups of substances for which sub-pico second laser mass spectrometry delivers favorable results metal organic com pounds, chemical warfare agents and biomolecules were chosen. It is shown t hat in contrast to nanosecond-REMPI the use of ultra-short laser pulses res ults in well interpretable mass spectra of these substances. In order to de monstrate that REMPI with ultra-short laser pulses may be employed for the detection of quickly relaxing compounds in environmental samples the quanti tative analysis of a soil sample spiked with the warfare agent adamsite was performed. (C) 1999 Elsevier Science B.V.