Atmospheric pressure photoionization (APPI): a new ionization method for liquid chromatography-mass spectrometry

There is provided a method of, and apparatus for, analyzing a sample of an analyte provided as a sample solution comprising a solvent and an analyte. A dopant is provided, either separately or as the solvent of the sample solution. The sample solution is formed into a spray, for example in a nebulizer, and the solvent evaporated. The sample stream is irradiated in a region at atmospheric pressure, either in the liquid state prior to formation of a spray, or in the liquid state after formation of a droplet spray, or in the vapour state after evaporation of the sprayed droplets, to ionize the dopant. Then, subsequent collisions between the ionized dopant and the analyte, either directly or indirectly, result in ionization of the analyte. Analyte ions are passed from the atmospheric pressure ionization region into a mass analyzer for mass analysis. This technique has been found to give much enhanced ionization for some substances, as compared to atmospheric pressure chemical ionization.