Atmospheric Pressure Photoionization:  An Ionization Method for Liquid Chromatography−Mass Spectrometry

Atmospheric pressure photoionization (APPI) has been successfully demonstrated to provide high sensitivity to LC−MS analysis. A vacuum-ultraviolet lamp designed for photoionization detection in gas chromatography is used as a source of 10-eV photons. The mixture of samples and solvent eluting from an HPLC is fully evaporated prior to introduction into the photoionization region. In the new method, large quantities of an ionizable dopant are added to the vapor generated from the LC eluant, allowing for a great abundance of dopant photoions to be produced. Because the ion source is at atmospheric pressure, and the collision rate is high, the dopant photoions react to completion with solvent and analyte molecules present in the ion source. Using APPI, at an LC flow rate of 200 μL/min, it is possible to obtain analyte signal intensities 8 times as high as those obtainable with a commercially available corona discharge-atmospheric pressure chemical ionization source.