LASER-ABLATION OF ORGANIC-MOLECULES FROM FROZEN MATRICES

The technique of frozen matrix-assisted laser ablation coupled with re sonance-enhanced multiphoton ionization and reflectron time-of-flight mass spectrometry was used to detect intact organic molecules directly from solutions. When frozen at the temperature of liquid nitrogen, th e matrices of interest were ablated by a pulsed CO2 single-mode laser. The analyte molecules emerging from the ablated plume were then ioniz ed by a tunable XeCl excimer laser-pumped dye laser and analysed with a gridless reflectron time-of-flight mass spectrometer. The ablation p rocess from an ice matrix was studied with the amino acids tryptophan and tyrosine dissolved in an aqueous ethanol solution to a concentrati on level of 5 x 10(-4) M. It was found that fragmentation of the analy te molecules is strongly dependent on the ablating laser fluence and t hat there is a laser fluence range just above the ablation threshold w here the decomposition is negligible. The different fragmentation mech anisms are discussed and a cavitation under the liquid surface, causin g the sonoluminescence signal from an ice matrix, was shown to be resp onsible for the decomposition of the analyte molecules ablated by a lo w photon energy IR laser. Under the appropriate conditions, the analyt e molecules were found to have a low rotational temperature of about 1 50 K resulting from the jet-like cooling in multiple collisions with m atrix molecules.