This disclosure is concerned with method and apparatus for vaporizing liquid solutions in order to detect, quantitate, and/or determine physical or chemical properties of samples present in liquid solution. Mixtures may be separated by an on-line liquid chromatographic column and the methods used for detection, quantitation, indentification, and/or determination of chemical and physical properties include mass spectrometry, photoionization, flame ionization, electron capture, optical photemetry, including UV, visible, and IR regions of the spectrum, light scattering, light emission, atomic absorption, and any other technique suitable for detecting or analyzing molecules or particles in a gaseous or vacuum environment. The method and apparatus involves controlled partial vaporization of the solution. Methods are disclosed for controlling the degree of partial vaporization and the temperature at which this vaporization occurs, and for maintaining this degree of vaporization essentially constant even though the solvent flow rate and/or composition may vary in either a controlled or an uncontrolled fasion. This "thermospray" method and apparatus allows the solvent to be substantially vaporized to produce a supersonic free jet containing a fraction of unvaporized solvent as liquid droplets entrained in the jet. Solutes which are less volatile than the solvent are preferentially contained in the droplets. Methods are disclosed for controlling the temperature at which the process occurs in order to prevent unwanted chemical modification of the solutes (for example, pyrolysis) and to prevent premature vaporization of the solutes.