Abstract

Supercritical fluids of CO₂ and N₂O are used as a means of solubilizing nonvolatile polynuclear aromatic hydrocarbons and small thermally labile biologicals for expansion into supersonic beams for mass spectrometry. The resulting expansion into vacuum results in internally ultracold molecules with sharp spectral features for unique identification in UV-VIS absorption spectroscopy. In this work laser resonant two-photon ionization is used as a means of selectively producing ions for detection in mass spectrometry where the first photon excites the molecule to a real resonant state and the second photon ionizes the molecule. Although ions are produced for mass spectrometry, the ionization cross section reflects the S_O → S₁ transition so that ionization spectroscopy can be used as a means of identifying molecules in a mass spectrometer. We have applied this method to detection of several PNAH's where supercritical fluid expansion was used to solubilize these molecules essentially at room temperature, i.e. without heating. In addition, small biological and pharmaceutical analogs have been examined and the effect of the supercritical fluid solvent is studied. Most recently supercritical ammonia has been investigated as a highly polar solvent for dissolving indoleamines, catecholamines and other polar biological species. The key to this experiment is a novel compact molecular beam apparatus for supercritical fluid injection which takes advantage of (a) efficient liquid N₂ cryopumping of CO₂ and (b) a special high-pressure pulsed valve. This valve is capable of operation up to 400 atm backpressure at 250°C. The combination of pulsed valve injection with cryopumping allows the use of a 150-200 μ orifice for high on-axis density and thus sensitivity. The resulting chamber pressure at 400 atm reservoir pressure is <2×10⁻⁵ torr so that the ions produced by R2PI can be detected and mass analyzed in a time-of-flight mass spectrometer. The future potential of this method is discussed and compared to the use of the laser desorption method for volatilizing labile compounds into jet expansions.

© 1987 Optical Society of America