Abstract

In our work resonant two-photon ionization (R2PI) in supersonic jets is used as a means of optically selecting ions for mass spectrometry. The use of supersonic beam injection provides internally ultracold molecules with sharp spectral features for chemical analysis for molecules whose spectra would otherwise exhibit broad, unresolvable spectral contours at room temperature. The use of high-powered tunable monochromatic laser sources can be used to probe this sharp structure by using laser R2PI spectroscopy. This method has been used for analysis of small biological species based on the combination of wavelength selectivity and the high efficiency for producing molecular ions by the R2PI technique. One method investigated for use in solubilizing nonvolatile and thermally labile molecules involves expansions of supercritical fluids of CO₂ and N₂O at up to 400-atm backpressure in to a mass spectrometer at 10^-5 Torr. The expansion of this ultradense jet is shown to provide cold spectra for molecules which would normally require significant heating for volatilization into the gas phase. A second method used in our work for volitilization of nonvolatiles uses laser desorption to rapidly heat molecules off a surface before they can decompose. The resulting desorbed cloud is then swept into a supersonic jet and into a TOFMS for analysis by R2PI. The parameters required to volatilize various compounds and the spectroscopic and mass spectrometric results for several small biological systems including amino acids, drugs, catecholamines, and vitamins are discussed.

© 1986 Optical Society of America