The liquid-microjunction surface sampling probe uses solvent extraction to isolate material from a sample zone and transport it in solution to the ion source of a mass spectrometer [104,112]. It is used predominantly with electrospray and atmospheric pressure chemical ionization sources. The sample extraction and ionization processes, however, are separate events, and in principle, the approach could be used with any ionization source compatible with the extraction solvent. The sampling head consists of a coaxial arrangement of two capillary tubes, with the annular space between the inner and outer tubes used for solvent delivery and the inner tube to draw liquid from the surface and transfer it to the ionization source. The sampling probe is held at a short distanceabove the layer and forms a stable liquid film, or junction, between the surface and the extraction head, with the rate of solvent in fused to the layer balanced by the solvent extracted from the film.