A novel apparatus was designed for the introduction of supersonic jets of supercritical CO2, N2O and high-pressure hot liquids such as methanol and water. The key features of this experiment are a pulsed injection source, which was designed with efficient liquid nitrogen cryopumping to operate at pressures of up to 380 atm and nearly 200°C. The reduction in duty cycle provided by a pulsed source has allowed the use of a 200 μm orifice, which significantly increases the on-axis density in the jet compared to continuous expansions where a 10 μm orifice is customarily used. Resonant two-photon ionization is then used to softly ionize the molecules in the beam for mass analysis and detection in a time-of-flight mass spectrometer.