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

Abstract

The combination of resonance enhanced multiphoton ionization (REMPI) and time-of-flight mass spectrometry, commonly referred to as laser mass spectrometry, has proven to be a powerful analytical tool with advantageous features for certain problems. However, these advantages are only achievable as long as the resonant intermediate level(s) being involved in the photo ionization step are long-lived on the time scale of the ionizing laser pulse. A way to overcome this problem is to apply intense ultra short laser pulses in order to finish the ionization process before relaxation in the intermediate state(s) can occur. In this paper the differences between conventional nanosecond and sub-picosecond multiphoton ionization of polyatomic organic molecules and the consequences on the laser mass spectrometry of these compounds are discussed. As examples for groups of substances for which sub-picosecond laser mass spectrometry delivers favorable results metal organic compounds, chemical warfare agents and biomolecules were chosen. It is shown that in contrast to nanosecond-REMPI the use of ultra-short laser pulses results in well interpretable mass spectra of these substances. In order to demonstrate that REMPI with ultra-short laser pulses may be employed for the detection of quickly relaxing compounds in environmental samples the quantitative analysis of a soil sample spiked with the warfare agent adamsite was performed. Keywords: Laser mass spectrometry; Ultra-short laser pulses; Analytical applications; Organic substances; Ion fragmentation; Laser desorption.